- Support concurrency iterate rows and columns

- Rename exported field `File.XLSX` to `File.Pkg`
- Exported error message

38 files changed, 377 insertions, 262 deletions

diff --git a/adjust_test.go b/adjust_test.go
index 0d63ed6..c4af38b 100644
--- a/adjust_test.go
+++ b/adjust_test.go
@@ -73,20 +73,10 @@ func TestAdjustAutoFilter(t *testing.T) {
 func TestAdjustHelper(t *testing.T) {
 	f := NewFile()
 	f.NewSheet("Sheet2")
-	f.Sheet["xl/worksheets/sheet1.xml"] = &xlsxWorksheet{
-		MergeCells: &xlsxMergeCells{
-			Cells: []*xlsxMergeCell{
-				{
-					Ref: "A:B1",
-				},
-			},
-		},
-	}
-	f.Sheet["xl/worksheets/sheet2.xml"] = &xlsxWorksheet{
-		AutoFilter: &xlsxAutoFilter{
-			Ref: "A1:B",
-		},
-	}
+	f.Sheet.Store("xl/worksheets/sheet1.xml", &xlsxWorksheet{
+		MergeCells: &xlsxMergeCells{Cells: []*xlsxMergeCell{{Ref: "A:B1"}}}})
+	f.Sheet.Store("xl/worksheets/sheet2.xml", &xlsxWorksheet{
+		AutoFilter: &xlsxAutoFilter{Ref: "A1:B"}})
 	// testing adjustHelper with illegal cell coordinates.
 	assert.EqualError(t, f.adjustHelper("Sheet1", rows, 0, 0), `cannot convert cell "A" to coordinates: invalid cell name "A"`)
 	assert.EqualError(t, f.adjustHelper("Sheet2", rows, 0, 0), `cannot convert cell "B" to coordinates: invalid cell name "B"`)
diff --git a/calcchain.go b/calcchain.go
index ea3080f..1b99a04 100644
--- a/calcchain.go
+++ b/calcchain.go
@@ -54,7 +54,7 @@ func (f *File) deleteCalcChain(index int, axis string) {
 	}
 	if len(calc.C) == 0 {
 		f.CalcChain = nil
-		delete(f.XLSX, "xl/calcChain.xml")
+		f.Pkg.Delete("xl/calcChain.xml")
 		content := f.contentTypesReader()
 		for k, v := range content.Overrides {
 			if v.PartName == "/xl/calcChain.xml" {
diff --git a/calcchain_test.go b/calcchain_test.go
index 842dde1..4956f60 100644
--- a/calcchain_test.go
+++ b/calcchain_test.go
@@ -5,7 +5,7 @@ import "testing"
 func TestCalcChainReader(t *testing.T) {
 	f := NewFile()
 	f.CalcChain = nil
-	f.XLSX["xl/calcChain.xml"] = MacintoshCyrillicCharset
+	f.Pkg.Store("xl/calcChain.xml", MacintoshCyrillicCharset)
 	f.calcChainReader()
 }
 
diff --git a/cell.go b/cell.go
index 4dec093..1d08c8a 100644
--- a/cell.go
+++ b/cell.go
@@ -13,7 +13,6 @@ package excelize
 
 import (
 	"encoding/xml"
-	"errors"
 	"fmt"
 	"reflect"
 	"strconv"
@@ -187,6 +186,8 @@ func (f *File) SetCellInt(sheet, axis string, value int) error {
 	if err != nil {
 		return err
 	}
+	ws.Lock()
+	defer ws.Unlock()
 	cellData.S = f.prepareCellStyle(ws, col, cellData.S)
 	cellData.T, cellData.V = setCellInt(value)
 	return err
@@ -262,6 +263,8 @@ func (f *File) SetCellStr(sheet, axis, value string) error {
 	if err != nil {
 		return err
 	}
+	ws.Lock()
+	defer ws.Unlock()
 	cellData.S = f.prepareCellStyle(ws, col, cellData.S)
 	cellData.T, cellData.V = f.setCellString(value)
 	return err
@@ -742,7 +745,7 @@ func (f *File) SetSheetRow(sheet, axis string, slice interface{}) error {
 	// Make sure 'slice' is a Ptr to Slice
 	v := reflect.ValueOf(slice)
 	if v.Kind() != reflect.Ptr || v.Elem().Kind() != reflect.Slice {
-		return errors.New("pointer to slice expected")
+		return ErrParameterInvalid
 	}
 	v = v.Elem()
 
@@ -762,8 +765,6 @@ func (f *File) SetSheetRow(sheet, axis string, slice interface{}) error {
 
 // getCellInfo does common preparation for all SetCell* methods.
 func (f *File) prepareCell(ws *xlsxWorksheet, sheet, cell string) (*xlsxC, int, int, error) {
-	ws.Lock()
-	defer ws.Unlock()
 	var err error
 	cell, err = f.mergeCellsParser(ws, cell)
 	if err != nil {
@@ -775,7 +776,8 @@ func (f *File) prepareCell(ws *xlsxWorksheet, sheet, cell string) (*xlsxC, int,
 	}
 
 	prepareSheetXML(ws, col, row)
-
+	ws.Lock()
+	defer ws.Unlock()
 	return &ws.SheetData.Row[row-1].C[col-1], col, row, err
 }
 
diff --git a/cell_test.go b/cell_test.go
index 2282e55..e289983 100644
--- a/cell_test.go
+++ b/cell_test.go
@@ -30,6 +30,20 @@ func TestConcurrency(t *testing.T) {
 			assert.Equal(t, "", name)
 			assert.Nil(t, raw)
 			assert.NoError(t, err)
+			// Concurrency iterate rows
+			rows, err := f.Rows("Sheet1")
+			assert.NoError(t, err)
+			for rows.Next() {
+				_, err := rows.Columns()
+				assert.NoError(t, err)
+			}
+			// Concurrency iterate columns
+			cols, err := f.Cols("Sheet1")
+			assert.NoError(t, err)
+			for rows.Next() {
+				_, err := cols.Rows()
+				assert.NoError(t, err)
+			}
 
 			wg.Done()
 		}(i, t)
@@ -149,8 +163,8 @@ func TestGetCellValue(t *testing.T) {
 	// Test get cell value without r attribute of the row.
 	f := NewFile()
 	sheetData := `<worksheet xmlns="http://schemas.openxmlformats.org/spreadsheetml/2006/main"><sheetData>%s</sheetData></worksheet>`
-	delete(f.Sheet, "xl/worksheets/sheet1.xml")
-	f.XLSX["xl/worksheets/sheet1.xml"] = []byte(fmt.Sprintf(sheetData, `<row r="3"><c t="str"><v>A3</v></c></row><row><c t="str"><v>A4</v></c><c t="str"><v>B4</v></c></row><row r="7"><c t="str"><v>A7</v></c><c t="str"><v>B7</v></c></row><row><c t="str"><v>A8</v></c><c t="str"><v>B8</v></c></row>`))
+	f.Sheet.Delete("xl/worksheets/sheet1.xml")
+	f.Pkg.Store("xl/worksheets/sheet1.xml", []byte(fmt.Sprintf(sheetData, `<row r="3"><c t="str"><v>A3</v></c></row><row><c t="str"><v>A4</v></c><c t="str"><v>B4</v></c></row><row r="7"><c t="str"><v>A7</v></c><c t="str"><v>B7</v></c></row><row><c t="str"><v>A8</v></c><c t="str"><v>B8</v></c></row>`)))
 	f.checked = nil
 	cells := []string{"A3", "A4", "B4", "A7", "B7"}
 	rows, err := f.GetRows("Sheet1")
@@ -164,20 +178,20 @@ func TestGetCellValue(t *testing.T) {
 	cols, err := f.GetCols("Sheet1")
 	assert.Equal(t, [][]string{{"", "", "A3", "A4", "", "", "A7", "A8"}, {"", "", "", "B4", "", "", "B7", "B8"}}, cols)
 	assert.NoError(t, err)
-	delete(f.Sheet, "xl/worksheets/sheet1.xml")
-	f.XLSX["xl/worksheets/sheet1.xml"] = []byte(fmt.Sprintf(sheetData, `<row r="2"><c r="A2" t="str"><v>A2</v></c></row><row r="2"><c r="B2" t="str"><v>B2</v></c></row>`))
+	f.Sheet.Delete("xl/worksheets/sheet1.xml")
+	f.Pkg.Store("xl/worksheets/sheet1.xml", []byte(fmt.Sprintf(sheetData, `<row r="2"><c r="A2" t="str"><v>A2</v></c></row><row r="2"><c r="B2" t="str"><v>B2</v></c></row>`)))
 	f.checked = nil
 	cell, err := f.GetCellValue("Sheet1", "A2")
 	assert.Equal(t, "A2", cell)
 	assert.NoError(t, err)
-	delete(f.Sheet, "xl/worksheets/sheet1.xml")
-	f.XLSX["xl/worksheets/sheet1.xml"] = []byte(fmt.Sprintf(sheetData, `<row r="2"><c r="A2" t="str"><v>A2</v></c></row><row r="2"><c r="B2" t="str"><v>B2</v></c></row>`))
+	f.Sheet.Delete("xl/worksheets/sheet1.xml")
+	f.Pkg.Store("xl/worksheets/sheet1.xml", []byte(fmt.Sprintf(sheetData, `<row r="2"><c r="A2" t="str"><v>A2</v></c></row><row r="2"><c r="B2" t="str"><v>B2</v></c></row>`)))
 	f.checked = nil
 	rows, err = f.GetRows("Sheet1")
 	assert.Equal(t, [][]string{nil, {"A2", "B2"}}, rows)
 	assert.NoError(t, err)
-	delete(f.Sheet, "xl/worksheets/sheet1.xml")
-	f.XLSX["xl/worksheets/sheet1.xml"] = []byte(fmt.Sprintf(sheetData, `<row r="1"><c r="A1" t="str"><v>A1</v></c></row><row r="1"><c r="B1" t="str"><v>B1</v></c></row>`))
+	f.Sheet.Delete("xl/worksheets/sheet1.xml")
+	f.Pkg.Store("xl/worksheets/sheet1.xml", []byte(fmt.Sprintf(sheetData, `<row r="1"><c r="A1" t="str"><v>A1</v></c></row><row r="1"><c r="B1" t="str"><v>B1</v></c></row>`)))
 	f.checked = nil
 	rows, err = f.GetRows("Sheet1")
 	assert.Equal(t, [][]string{{"A1", "B1"}}, rows)
@@ -264,17 +278,23 @@ func TestGetCellRichText(t *testing.T) {
 	assert.True(t, reflect.DeepEqual(runsSource[1].Font, runs[1].Font), "should get the same font")
 
 	// Test get cell rich text when string item index overflow
-	f.Sheet["xl/worksheets/sheet1.xml"].SheetData.Row[0].C[0].V = "2"
+	ws, ok := f.Sheet.Load("xl/worksheets/sheet1.xml")
+	assert.True(t, ok)
+	ws.(*xlsxWorksheet).SheetData.Row[0].C[0].V = "2"
 	runs, err = f.GetCellRichText("Sheet1", "A1")
 	assert.NoError(t, err)
 	assert.Equal(t, 0, len(runs))
 	// Test get cell rich text when string item index is negative
-	f.Sheet["xl/worksheets/sheet1.xml"].SheetData.Row[0].C[0].V = "-1"
+	ws, ok = f.Sheet.Load("xl/worksheets/sheet1.xml")
+	assert.True(t, ok)
+	ws.(*xlsxWorksheet).SheetData.Row[0].C[0].V = "-1"
 	runs, err = f.GetCellRichText("Sheet1", "A1")
 	assert.NoError(t, err)
 	assert.Equal(t, 0, len(runs))
 	// Test get cell rich text on invalid string item index
-	f.Sheet["xl/worksheets/sheet1.xml"].SheetData.Row[0].C[0].V = "x"
+	ws, ok = f.Sheet.Load("xl/worksheets/sheet1.xml")
+	assert.True(t, ok)
+	ws.(*xlsxWorksheet).SheetData.Row[0].C[0].V = "x"
 	_, err = f.GetCellRichText("Sheet1", "A1")
 	assert.EqualError(t, err, "strconv.Atoi: parsing \"x\": invalid syntax")
 	// Test set cell rich text on not exists worksheet
diff --git a/chart.go b/chart.go
index 9bc4b20..52fd543 100644
--- a/chart.go
+++ b/chart.go
@@ -14,7 +14,6 @@ package excelize
 import (
 	"encoding/json"
 	"encoding/xml"
-	"errors"
 	"fmt"
 	"strconv"
 	"strings"
@@ -945,7 +944,7 @@ func (f *File) AddChartSheet(sheet, format string, combo ...string) error {
 	sheetID++
 	path := "xl/chartsheets/sheet" + strconv.Itoa(sheetID) + ".xml"
 	f.sheetMap[trimSheetName(sheet)] = path
-	f.Sheet[path] = nil
+	f.Sheet.Store(path, nil)
 	drawingID := f.countDrawings() + 1
 	chartID := f.countCharts() + 1
 	drawingXML := "xl/drawings/drawing" + strconv.Itoa(drawingID) + ".xml"
@@ -981,12 +980,12 @@ func (f *File) getFormatChart(format string, combo []string) (*formatChart, []*f
 			return formatSet, comboCharts, err
 		}
 		if _, ok := chartValAxNumFmtFormatCode[comboChart.Type]; !ok {
-			return formatSet, comboCharts, errors.New("unsupported chart type " + comboChart.Type)
+			return formatSet, comboCharts, newUnsupportChartType(comboChart.Type)
 		}
 		comboCharts = append(comboCharts, comboChart)
 	}
 	if _, ok := chartValAxNumFmtFormatCode[formatSet.Type]; !ok {
-		return formatSet, comboCharts, errors.New("unsupported chart type " + formatSet.Type)
+		return formatSet, comboCharts, newUnsupportChartType(formatSet.Type)
 	}
 	return formatSet, comboCharts, err
 }
@@ -1015,11 +1014,12 @@ func (f *File) DeleteChart(sheet, cell string) (err error) {
 // folder xl/charts.
 func (f *File) countCharts() int {
 	count := 0
-	for k := range f.XLSX {
-		if strings.Contains(k, "xl/charts/chart") {
+	f.Pkg.Range(func(k, v interface{}) bool {
+		if strings.Contains(k.(string), "xl/charts/chart") {
 			count++
 		}
-	}
+		return true
+	})
 	return count
 }
 
diff --git a/chart_test.go b/chart_test.go
index 657230b..8957b93 100644
--- a/chart_test.go
+++ b/chart_test.go
@@ -65,10 +65,10 @@ func TestChartSize(t *testing.T) {
 		anchor  decodeTwoCellAnchor
 	)
 
-	content, ok := newFile.XLSX["xl/drawings/drawing1.xml"]
+	content, ok := newFile.Pkg.Load("xl/drawings/drawing1.xml")
 	assert.True(t, ok, "Can't open the chart")
 
-	err = xml.Unmarshal([]byte(content), &workdir)
+	err = xml.Unmarshal(content.([]byte), &workdir)
 	if !assert.NoError(t, err) {
 		t.FailNow()
 	}
@@ -340,11 +340,15 @@ func TestChartWithLogarithmicBase(t *testing.T) {
 	type xmlChartContent []byte
 	xmlCharts := make([]xmlChartContent, expectedChartsCount)
 	expectedChartsLogBase := []float64{0, 10.5, 0, 2, 0, 1000}
-	var ok bool
-
+	var (
+		drawingML interface{}
+		ok        bool
+	)
 	for i := 0; i < expectedChartsCount; i++ {
 		chartPath := fmt.Sprintf("xl/charts/chart%d.xml", i+1)
-		xmlCharts[i], ok = newFile.XLSX[chartPath]
+		if drawingML, ok = newFile.Pkg.Load(chartPath); ok {
+			xmlCharts[i] = drawingML.([]byte)
+		}
 		assert.True(t, ok, "Can't open the %s", chartPath)
 
 		err = xml.Unmarshal([]byte(xmlCharts[i]), &chartSpaces[i])
diff --git a/col.go b/col.go
index 2fd90b2..91ca3da 100644
--- a/col.go
+++ b/col.go
@@ -199,8 +199,11 @@ func (f *File) Cols(sheet string) (*Cols, error) {
 	if !ok {
 		return nil, ErrSheetNotExist{sheet}
 	}
-	if f.Sheet[name] != nil {
-		output, _ := xml.Marshal(f.Sheet[name])
+	if ws, ok := f.Sheet.Load(name); ok && ws != nil {
+		worksheet := ws.(*xlsxWorksheet)
+		worksheet.Lock()
+		defer worksheet.Unlock()
+		output, _ := xml.Marshal(worksheet)
 		f.saveFileList(name, f.replaceNameSpaceBytes(name, output))
 	}
 	var colIterator columnXMLIterator
diff --git a/col_test.go b/col_test.go
index 6ab5e57..8159a11 100644
--- a/col_test.go
+++ b/col_test.go
@@ -48,11 +48,11 @@ func TestCols(t *testing.T) {
 	_, err = f.Rows("Sheet1")
 	assert.NoError(t, err)
 
-	f.Sheet["xl/worksheets/sheet1.xml"] = &xlsxWorksheet{
+	f.Sheet.Store("xl/worksheets/sheet1.xml", &xlsxWorksheet{
 		Dimension: &xlsxDimension{
 			Ref: "C2:C4",
 		},
-	}
+	})
 	_, err = f.Rows("Sheet1")
 	assert.NoError(t, err)
 }
@@ -110,15 +110,15 @@ func TestGetColsError(t *testing.T) {
 	assert.EqualError(t, err, "sheet SheetN is not exist")
 
 	f = NewFile()
-	delete(f.Sheet, "xl/worksheets/sheet1.xml")
-	f.XLSX["xl/worksheets/sheet1.xml"] = []byte(`<worksheet><sheetData><row r="A"><c r="2" t="str"><v>B</v></c></row></sheetData></worksheet>`)
+	f.Sheet.Delete("xl/worksheets/sheet1.xml")
+	f.Pkg.Store("xl/worksheets/sheet1.xml", []byte(`<worksheet><sheetData><row r="A"><c r="2" t="str"><v>B</v></c></row></sheetData></worksheet>`))
 	f.checked = nil
 	_, err = f.GetCols("Sheet1")
 	assert.EqualError(t, err, `strconv.Atoi: parsing "A": invalid syntax`)
 
 	f = NewFile()
-	delete(f.Sheet, "xl/worksheets/sheet1.xml")
-	f.XLSX["xl/worksheets/sheet1.xml"] = []byte(`<worksheet><sheetData><row r="2"><c r="A" t="str"><v>B</v></c></row></sheetData></worksheet>`)
+	f.Sheet.Delete("xl/worksheets/sheet1.xml")
+	f.Pkg.Store("xl/worksheets/sheet1.xml", []byte(`<worksheet><sheetData><row r="2"><c r="A" t="str"><v>B</v></c></row></sheetData></worksheet>`))
 	f.checked = nil
 	_, err = f.GetCols("Sheet1")
 	assert.EqualError(t, err, `cannot convert cell "A" to coordinates: invalid cell name "A"`)
@@ -142,14 +142,14 @@ func TestColsRows(t *testing.T) {
 	assert.NoError(t, err)
 
 	assert.NoError(t, f.SetCellValue("Sheet1", "A1", 1))
-	f.Sheet["xl/worksheets/sheet1.xml"] = &xlsxWorksheet{
+	f.Sheet.Store("xl/worksheets/sheet1.xml", &xlsxWorksheet{
 		Dimension: &xlsxDimension{
 			Ref: "A1:A1",
 		},
-	}
+	})
 
 	f = NewFile()
-	f.XLSX["xl/worksheets/sheet1.xml"] = nil
+	f.Pkg.Store("xl/worksheets/sheet1.xml", nil)
 	_, err = f.Cols("Sheet1")
 	if !assert.NoError(t, err) {
 		t.FailNow()
diff --git a/comment.go b/comment.go
index b05f308..306826d 100644
--- a/comment.go
+++ b/comment.go
@@ -299,11 +299,12 @@ func (f *File) addComment(commentsXML, cell string, formatSet *formatComment) {
 // the folder xl.
 func (f *File) countComments() int {
 	c1, c2 := 0, 0
-	for k := range f.XLSX {
-		if strings.Contains(k, "xl/comments") {
+	f.Pkg.Range(func(k, v interface{}) bool {
+		if strings.Contains(k.(string), "xl/comments") {
 			c1++
 		}
-	}
+		return true
+	})
 	for rel := range f.Comments {
 		if strings.Contains(rel, "xl/comments") {
 			c2++
@@ -321,10 +322,10 @@ func (f *File) decodeVMLDrawingReader(path string) *decodeVmlDrawing {
 	var err error
 
 	if f.DecodeVMLDrawing[path] == nil {
-		c, ok := f.XLSX[path]
-		if ok {
+		c, ok := f.Pkg.Load(path)
+		if ok && c != nil {
 			f.DecodeVMLDrawing[path] = new(decodeVmlDrawing)
-			if err = f.xmlNewDecoder(bytes.NewReader(namespaceStrictToTransitional(c))).
+			if err = f.xmlNewDecoder(bytes.NewReader(namespaceStrictToTransitional(c.([]byte)))).
 				Decode(f.DecodeVMLDrawing[path]); err != nil && err != io.EOF {
 				log.Printf("xml decode error: %s", err)
 			}
@@ -339,7 +340,7 @@ func (f *File) vmlDrawingWriter() {
 	for path, vml := range f.VMLDrawing {
 		if vml != nil {
 			v, _ := xml.Marshal(vml)
-			f.XLSX[path] = v
+			f.Pkg.Store(path, v)
 		}
 	}
 }
@@ -348,12 +349,11 @@ func (f *File) vmlDrawingWriter() {
 // after deserialization of xl/comments%d.xml.
 func (f *File) commentsReader(path string) *xlsxComments {
 	var err error
-
 	if f.Comments[path] == nil {
-		content, ok := f.XLSX[path]
-		if ok {
+		content, ok := f.Pkg.Load(path)
+		if ok && content != nil {
 			f.Comments[path] = new(xlsxComments)
-			if err = f.xmlNewDecoder(bytes.NewReader(namespaceStrictToTransitional(content))).
+			if err = f.xmlNewDecoder(bytes.NewReader(namespaceStrictToTransitional(content.([]byte)))).
 				Decode(f.Comments[path]); err != nil && err != io.EOF {
 				log.Printf("xml decode error: %s", err)
 			}
diff --git a/comment_test.go b/comment_test.go
index 19b705f..f1b60dc 100644
--- a/comment_test.go
+++ b/comment_test.go
@@ -36,7 +36,7 @@ func TestAddComments(t *testing.T) {
 	}
 
 	f.Comments["xl/comments2.xml"] = nil
-	f.XLSX["xl/comments2.xml"] = []byte(`<?xml version="1.0" encoding="UTF-8" standalone="yes"?><comments xmlns="http://schemas.openxmlformats.org/spreadsheetml/2006/main"><authors><author>Excelize: </author></authors><commentList><comment ref="B7" authorId="0"><text><t>Excelize: </t></text></comment></commentList></comments>`)
+	f.Pkg.Store("xl/comments2.xml", []byte(`<?xml version="1.0" encoding="UTF-8" standalone="yes"?><comments xmlns="http://schemas.openxmlformats.org/spreadsheetml/2006/main"><authors><author>Excelize: </author></authors><commentList><comment ref="B7" authorId="0"><text><t>Excelize: </t></text></comment></commentList></comments>`))
 	comments := f.GetComments()
 	assert.EqualValues(t, 2, len(comments["Sheet1"]))
 	assert.EqualValues(t, 1, len(comments["Sheet2"]))
@@ -46,14 +46,14 @@ func TestAddComments(t *testing.T) {
 func TestDecodeVMLDrawingReader(t *testing.T) {
 	f := NewFile()
 	path := "xl/drawings/vmlDrawing1.xml"
-	f.XLSX[path] = MacintoshCyrillicCharset
+	f.Pkg.Store(path, MacintoshCyrillicCharset)
 	f.decodeVMLDrawingReader(path)
 }
 
 func TestCommentsReader(t *testing.T) {
 	f := NewFile()
 	path := "xl/comments1.xml"
-	f.XLSX[path] = MacintoshCyrillicCharset
+	f.Pkg.Store(path, MacintoshCyrillicCharset)
 	f.commentsReader(path)
 }
 
diff --git a/crypt.go b/crypt.go
index 88abd0e..a0096c9 100644
--- a/crypt.go
+++ b/crypt.go
@@ -21,7 +21,6 @@ import (
 	"encoding/base64"
 	"encoding/binary"
 	"encoding/xml"
-	"errors"
 	"hash"
 	"math/rand"
 	"reflect"
@@ -145,7 +144,7 @@ func Decrypt(raw []byte, opt *Options) (packageBuf []byte, err error) {
 	case "standard":
 		return standardDecrypt(encryptionInfoBuf, encryptedPackageBuf, opt)
 	default:
-		err = errors.New("unsupport encryption mechanism")
+		err = ErrUnsupportEncryptMechanism
 	}
 	return
 }
@@ -265,7 +264,7 @@ func Encrypt(raw []byte, opt *Options) (packageBuf []byte, err error) {
 	}
 	// TODO: Create a new CFB.
 	_, _ = encryptedPackage, encryptionInfoBuffer
-	err = errors.New("not support encryption currently")
+	err = ErrEncrypt
 	return
 }
 
@@ -293,7 +292,7 @@ func extractPart(doc *mscfb.Reader) (encryptionInfoBuf, encryptedPackageBuf []by
 // encryptionMechanism parse password-protected documents created mechanism.
 func encryptionMechanism(buffer []byte) (mechanism string, err error) {
 	if len(buffer) < 4 {
-		err = errors.New("unknown encryption mechanism")
+		err = ErrUnknownEncryptMechanism
 		return
 	}
 	versionMajor, versionMinor := binary.LittleEndian.Uint16(buffer[0:2]), binary.LittleEndian.Uint16(buffer[2:4])
@@ -306,7 +305,7 @@ func encryptionMechanism(buffer []byte) (mechanism string, err error) {
 	} else if (versionMajor == 3 || versionMajor == 4) && versionMinor == 3 {
 		mechanism = "extensible"
 	}
-	err = errors.New("unsupport encryption mechanism")
+	err = ErrUnsupportEncryptMechanism
 	return
 }
 
@@ -470,7 +469,6 @@ func convertPasswdToKey(passwd string, blockKey []byte, encryption Encryption) (
 	if len(key) < keyBytes {
 		tmp := make([]byte, 0x36)
 		key = append(key, tmp...)
-		key = tmp
 	} else if len(key) > keyBytes {
 		key = key[:keyBytes]
 	}
@@ -599,7 +597,6 @@ func createIV(blockKey interface{}, encryption Encryption) ([]byte, error) {
 	if len(iv) < encryptedKey.BlockSize {
 		tmp := make([]byte, 0x36)
 		iv = append(iv, tmp...)
-		iv = tmp
 	} else if len(iv) > encryptedKey.BlockSize {
 		iv = iv[0:encryptedKey.BlockSize]
 	}
diff --git a/docProps_test.go b/docProps_test.go
index 0cb6f71..40ae2dc 100644
--- a/docProps_test.go
+++ b/docProps_test.go
@@ -42,12 +42,12 @@ func TestSetDocProps(t *testing.T) {
 		Version:        "1.0.0",
 	}))
 	assert.NoError(t, f.SaveAs(filepath.Join("test", "TestSetDocProps.xlsx")))
-	f.XLSX["docProps/core.xml"] = nil
+	f.Pkg.Store("docProps/core.xml", nil)
 	assert.NoError(t, f.SetDocProps(&DocProperties{}))
 
 	// Test unsupported charset
 	f = NewFile()
-	f.XLSX["docProps/core.xml"] = MacintoshCyrillicCharset
+	f.Pkg.Store("docProps/core.xml", MacintoshCyrillicCharset)
 	assert.EqualError(t, f.SetDocProps(&DocProperties{}), "xml decode error: XML syntax error on line 1: invalid UTF-8")
 }
 
@@ -59,13 +59,13 @@ func TestGetDocProps(t *testing.T) {
 	props, err := f.GetDocProps()
 	assert.NoError(t, err)
 	assert.Equal(t, props.Creator, "Microsoft Office User")
-	f.XLSX["docProps/core.xml"] = nil
+	f.Pkg.Store("docProps/core.xml", nil)
 	_, err = f.GetDocProps()
 	assert.NoError(t, err)
 
 	// Test unsupported charset
 	f = NewFile()
-	f.XLSX["docProps/core.xml"] = MacintoshCyrillicCharset
+	f.Pkg.Store("docProps/core.xml", MacintoshCyrillicCharset)
 	_, err = f.GetDocProps()
 	assert.EqualError(t, err, "xml decode error: XML syntax error on line 1: invalid UTF-8")
 }
diff --git a/drawing.go b/drawing.go
index 58e2669..181bb43 100644
--- a/drawing.go
+++ b/drawing.go
@@ -1151,7 +1151,7 @@ func (f *File) drawingParser(path string) (*xlsxWsDr, int) {
 		content := xlsxWsDr{}
 		content.A = NameSpaceDrawingML.Value
 		content.Xdr = NameSpaceDrawingMLSpreadSheet.Value
-		if _, ok = f.XLSX[path]; ok { // Append Model
+		if _, ok = f.Pkg.Load(path); ok { // Append Model
 			decodeWsDr := decodeWsDr{}
 			if err = f.xmlNewDecoder(bytes.NewReader(namespaceStrictToTransitional(f.readXML(path)))).
 				Decode(&decodeWsDr); err != nil && err != io.EOF {
diff --git a/drawing_test.go b/drawing_test.go
index 3c0b619..f2413cf 100644
--- a/drawing_test.go
+++ b/drawing_test.go
@@ -19,10 +19,10 @@ import (
 func TestDrawingParser(t *testing.T) {
 	f := File{
 		Drawings: sync.Map{},
-		XLSX: map[string][]byte{
-			"charset": MacintoshCyrillicCharset,
-			"wsDr":    []byte(`<?xml version="1.0" encoding="UTF-8" standalone="yes"?><xdr:wsDr xmlns:xdr="http://schemas.openxmlformats.org/drawingml/2006/spreadsheetDrawing"><xdr:oneCellAnchor><xdr:graphicFrame/></xdr:oneCellAnchor></xdr:wsDr>`)},
+		Pkg:      sync.Map{},
 	}
+	f.Pkg.Store("charset", MacintoshCyrillicCharset)
+	f.Pkg.Store("wsDr", []byte(`<?xml version="1.0" encoding="UTF-8" standalone="yes"?><xdr:wsDr xmlns:xdr="http://schemas.openxmlformats.org/drawingml/2006/spreadsheetDrawing"><xdr:oneCellAnchor><xdr:graphicFrame/></xdr:oneCellAnchor></xdr:wsDr>`))
 	// Test with one cell anchor
 	f.drawingParser("wsDr")
 	// Test with unsupported charset
diff --git a/errors.go b/errors.go
index a0c61c8..4931198 100644
--- a/errors.go
+++ b/errors.go
@@ -32,6 +32,10 @@ func newInvalidExcelDateError(dateValue float64) error {
 	return fmt.Errorf("invalid date value %f, negative values are not supported supported", dateValue)
 }
 
+func newUnsupportChartType(chartType string) error {
+	return fmt.Errorf("unsupported chart type %s", chartType)
+}
+
 var (
 	// ErrStreamSetColWidth defined the error message on set column width in
 	// stream writing mode.
@@ -71,4 +75,34 @@ var (
 	// ErrMaxFileNameLength defined the error message on receive the file name
 	// length overflow.
 	ErrMaxFileNameLength = errors.New("file name length exceeds maximum limit")
+	// ErrEncrypt defined the error message on encryption spreadsheet.
+	ErrEncrypt = errors.New("not support encryption currently")
+	// ErrUnknownEncryptMechanism defined the error message on unsupport
+	// encryption mechanism.
+	ErrUnknownEncryptMechanism = errors.New("unknown encryption mechanism")
+	// ErrUnsupportEncryptMechanism defined the error message on unsupport
+	// encryption mechanism.
+	ErrUnsupportEncryptMechanism = errors.New("unsupport encryption mechanism")
+	// ErrParameterRequired defined the error message on receive the empty
+	// parameter.
+	ErrParameterRequired = errors.New("parameter is required")
+	// ErrParameterInvalid defined the error message on receive the invalid
+	// parameter.
+	ErrParameterInvalid = errors.New("parameter is invalid")
+	// ErrDefinedNameScope defined the error message on not found defined name
+	// in the given scope.
+	ErrDefinedNameScope = errors.New("no defined name on the scope")
+	// ErrDefinedNameduplicate defined the error message on the same name
+	// already exists on the scope.
+	ErrDefinedNameduplicate = errors.New("the same name already exists on the scope")
+	// ErrFontLength defined the error message on the length of the font
+	// family name overflow.
+	ErrFontLength = errors.New("the length of the font family name must be smaller than or equal to 31")
+	// ErrFontSize defined the error message on the size of the font is invalid.
+	ErrFontSize = errors.New("font size must be between 1 and 409 points")
+	// ErrSheetIdx defined the error message on receive the invalid worksheet
+	// index.
+	ErrSheetIdx = errors.New("invalid worksheet index")
+	// ErrGroupSheets defined the error message on group sheets.
+	ErrGroupSheets = errors.New("group worksheet must contain an active worksheet")
 )
diff --git a/excelize.go b/excelize.go
index 66cfd00..bdb7120 100644
--- a/excelize.go
+++ b/excelize.go
@@ -43,7 +43,7 @@ type File struct {
 	Path             string
 	SharedStrings    *xlsxSST
 	sharedStringsMap map[string]int
-	Sheet            map[string]*xlsxWorksheet
+	Sheet            sync.Map // map[string]*xlsxWorksheet
 	SheetCount       int
 	Styles           *xlsxStyleSheet
 	Theme            *xlsxTheme
@@ -51,7 +51,7 @@ type File struct {
 	VMLDrawing       map[string]*vmlDrawing
 	WorkBook         *xlsxWorkbook
 	Relationships    sync.Map
-	XLSX             map[string][]byte
+	Pkg              sync.Map
 	CharsetReader    charsetTranscoderFn
 }
 
@@ -95,7 +95,7 @@ func newFile() *File {
 		Comments:         make(map[string]*xlsxComments),
 		Drawings:         sync.Map{},
 		sharedStringsMap: make(map[string]int),
-		Sheet:            make(map[string]*xlsxWorksheet),
+		Sheet:            sync.Map{},
 		DecodeVMLDrawing: make(map[string]*decodeVmlDrawing),
 		VMLDrawing:       make(map[string]*vmlDrawing),
 		Relationships:    sync.Map{},
@@ -129,7 +129,10 @@ func OpenReader(r io.Reader, opt ...Options) (*File, error) {
 	if err != nil {
 		return nil, err
 	}
-	f.SheetCount, f.XLSX = sheetCount, file
+	f.SheetCount = sheetCount
+	for k, v := range file {
+		f.Pkg.Store(k, v)
+	}
 	f.CalcChain = f.calcChainReader()
 	f.sheetMap = f.getSheetMap()
 	f.Styles = f.stylesReader()
@@ -172,40 +175,40 @@ func (f *File) workSheetReader(sheet string) (ws *xlsxWorksheet, err error) {
 		name string
 		ok   bool
 	)
-
 	if name, ok = f.sheetMap[trimSheetName(sheet)]; !ok {
 		err = fmt.Errorf("sheet %s is not exist", sheet)
 		return
 	}
-	if ws = f.Sheet[name]; f.Sheet[name] == nil {
-		if strings.HasPrefix(name, "xl/chartsheets") {
-			err = fmt.Errorf("sheet %s is chart sheet", sheet)
-			return
-		}
-		ws = new(xlsxWorksheet)
-		if _, ok := f.xmlAttr[name]; !ok {
-			d := f.xmlNewDecoder(bytes.NewReader(namespaceStrictToTransitional(f.readXML(name))))
-			f.xmlAttr[name] = append(f.xmlAttr[name], getRootElement(d)...)
-		}
-		if err = f.xmlNewDecoder(bytes.NewReader(namespaceStrictToTransitional(f.readXML(name)))).
-			Decode(ws); err != nil && err != io.EOF {
-			err = fmt.Errorf("xml decode error: %s", err)
+	if worksheet, ok := f.Sheet.Load(name); ok && worksheet != nil {
+		ws = worksheet.(*xlsxWorksheet)
+		return
+	}
+	if strings.HasPrefix(name, "xl/chartsheets") {
+		err = fmt.Errorf("sheet %s is chart sheet", sheet)
+		return
+	}
+	ws = new(xlsxWorksheet)
+	if _, ok := f.xmlAttr[name]; !ok {
+		d := f.xmlNewDecoder(bytes.NewReader(namespaceStrictToTransitional(f.readXML(name))))
+		f.xmlAttr[name] = append(f.xmlAttr[name], getRootElement(d)...)
+	}
+	if err = f.xmlNewDecoder(bytes.NewReader(namespaceStrictToTransitional(f.readXML(name)))).
+		Decode(ws); err != nil && err != io.EOF {
+		err = fmt.Errorf("xml decode error: %s", err)
+		return
+	}
+	err = nil
+	if f.checked == nil {
+		f.checked = make(map[string]bool)
+	}
+	if ok = f.checked[name]; !ok {
+		checkSheet(ws)
+		if err = checkRow(ws); err != nil {
 			return
 		}
-		err = nil
-		if f.checked == nil {
-			f.checked = make(map[string]bool)
-		}
-		if ok = f.checked[name]; !ok {
-			checkSheet(ws)
-			if err = checkRow(ws); err != nil {
-				return
-			}
-			f.checked[name] = true
-		}
-		f.Sheet[name] = ws
+		f.checked[name] = true
 	}
-
+	f.Sheet.Store(name, ws)
 	return
 }
 
@@ -375,7 +378,7 @@ func (f *File) AddVBAProject(bin string) error {
 		})
 	}
 	file, _ := ioutil.ReadFile(bin)
-	f.XLSX["xl/vbaProject.bin"] = file
+	f.Pkg.Store("xl/vbaProject.bin", file)
 	return err
 }
 
diff --git a/excelize_test.go b/excelize_test.go
index e3cfa53..22e39d1 100644
--- a/excelize_test.go
+++ b/excelize_test.go
@@ -343,13 +343,17 @@ func TestSetCellHyperLink(t *testing.T) {
 	f = NewFile()
 	_, err = f.workSheetReader("Sheet1")
 	assert.NoError(t, err)
-	f.Sheet["xl/worksheets/sheet1.xml"].Hyperlinks = &xlsxHyperlinks{Hyperlink: make([]xlsxHyperlink, 65530)}
+	ws, ok := f.Sheet.Load("xl/worksheets/sheet1.xml")
+	assert.True(t, ok)
+	ws.(*xlsxWorksheet).Hyperlinks = &xlsxHyperlinks{Hyperlink: make([]xlsxHyperlink, 65530)}
 	assert.EqualError(t, f.SetCellHyperLink("Sheet1", "A65531", "https://github.com/360EntSecGroup-Skylar/excelize", "External"), ErrTotalSheetHyperlinks.Error())
 
 	f = NewFile()
 	_, err = f.workSheetReader("Sheet1")
 	assert.NoError(t, err)
-	f.Sheet["xl/worksheets/sheet1.xml"].MergeCells = &xlsxMergeCells{Cells: []*xlsxMergeCell{{Ref: "A:A"}}}
+	ws, ok = f.Sheet.Load("xl/worksheets/sheet1.xml")
+	assert.True(t, ok)
+	ws.(*xlsxWorksheet).MergeCells = &xlsxMergeCells{Cells: []*xlsxMergeCell{{Ref: "A:A"}}}
 	err = f.SetCellHyperLink("Sheet1", "A1", "https://github.com/360EntSecGroup-Skylar/excelize", "External")
 	assert.EqualError(t, err, `cannot convert cell "A" to coordinates: invalid cell name "A"`)
 }
@@ -376,7 +380,9 @@ func TestGetCellHyperLink(t *testing.T) {
 	f = NewFile()
 	_, err = f.workSheetReader("Sheet1")
 	assert.NoError(t, err)
-	f.Sheet["xl/worksheets/sheet1.xml"].Hyperlinks = &xlsxHyperlinks{
+	ws, ok := f.Sheet.Load("xl/worksheets/sheet1.xml")
+	assert.True(t, ok)
+	ws.(*xlsxWorksheet).Hyperlinks = &xlsxHyperlinks{
 		Hyperlink: []xlsxHyperlink{{Ref: "A1"}},
 	}
 	link, target, err = f.GetCellHyperLink("Sheet1", "A1")
@@ -384,7 +390,9 @@ func TestGetCellHyperLink(t *testing.T) {
 	assert.Equal(t, link, true)
 	assert.Equal(t, target, "")
 
-	f.Sheet["xl/worksheets/sheet1.xml"].MergeCells = &xlsxMergeCells{Cells: []*xlsxMergeCell{{Ref: "A:A"}}}
+	ws, ok = f.Sheet.Load("xl/worksheets/sheet1.xml")
+	assert.True(t, ok)
+	ws.(*xlsxWorksheet).MergeCells = &xlsxMergeCells{Cells: []*xlsxMergeCell{{Ref: "A:A"}}}
 	link, target, err = f.GetCellHyperLink("Sheet1", "A1")
 	assert.EqualError(t, err, `cannot convert cell "A" to coordinates: invalid cell name "A"`)
 	assert.Equal(t, link, false)
@@ -1112,8 +1120,8 @@ func TestSetSheetRow(t *testing.T) {
 	assert.EqualError(t, f.SetSheetRow("Sheet1", "", &[]interface{}{"cell", nil, 2}),
 		`cannot convert cell "" to coordinates: invalid cell name ""`)
 
-	assert.EqualError(t, f.SetSheetRow("Sheet1", "B27", []interface{}{}), `pointer to slice expected`)
-	assert.EqualError(t, f.SetSheetRow("Sheet1", "B27", &f), `pointer to slice expected`)
+	assert.EqualError(t, f.SetSheetRow("Sheet1", "B27", []interface{}{}), ErrParameterInvalid.Error())
+	assert.EqualError(t, f.SetSheetRow("Sheet1", "B27", &f), ErrParameterInvalid.Error())
 	assert.NoError(t, f.SaveAs(filepath.Join("test", "TestSetSheetRow.xlsx")))
 }
 
@@ -1198,7 +1206,7 @@ func TestContentTypesReader(t *testing.T) {
 	// Test unsupported charset.
 	f := NewFile()
 	f.ContentTypes = nil
-	f.XLSX["[Content_Types].xml"] = MacintoshCyrillicCharset
+	f.Pkg.Store("[Content_Types].xml", MacintoshCyrillicCharset)
 	f.contentTypesReader()
 }
 
@@ -1206,22 +1214,22 @@ func TestWorkbookReader(t *testing.T) {
 	// Test unsupported charset.
 	f := NewFile()
 	f.WorkBook = nil
-	f.XLSX["xl/workbook.xml"] = MacintoshCyrillicCharset
+	f.Pkg.Store("xl/workbook.xml", MacintoshCyrillicCharset)
 	f.workbookReader()
 }
 
 func TestWorkSheetReader(t *testing.T) {
 	// Test unsupported charset.
 	f := NewFile()
-	delete(f.Sheet, "xl/worksheets/sheet1.xml")
-	f.XLSX["xl/worksheets/sheet1.xml"] = MacintoshCyrillicCharset
+	f.Sheet.Delete("xl/worksheets/sheet1.xml")
+	f.Pkg.Store("xl/worksheets/sheet1.xml", MacintoshCyrillicCharset)
 	_, err := f.workSheetReader("Sheet1")
 	assert.EqualError(t, err, "xml decode error: XML syntax error on line 1: invalid UTF-8")
 
 	// Test on no checked worksheet.
 	f = NewFile()
-	delete(f.Sheet, "xl/worksheets/sheet1.xml")
-	f.XLSX["xl/worksheets/sheet1.xml"] = []byte(`<worksheet xmlns="http://schemas.openxmlformats.org/spreadsheetml/2006/main"><sheetData/></worksheet>`)
+	f.Sheet.Delete("xl/worksheets/sheet1.xml")
+	f.Pkg.Store("xl/worksheets/sheet1.xml", []byte(`<worksheet xmlns="http://schemas.openxmlformats.org/spreadsheetml/2006/main"><sheetData/></worksheet>`))
 	f.checked = nil
 	_, err = f.workSheetReader("Sheet1")
 	assert.NoError(t, err)
@@ -1232,7 +1240,7 @@ func TestRelsReader(t *testing.T) {
 	f := NewFile()
 	rels := "xl/_rels/workbook.xml.rels"
 	f.Relationships.Store(rels, nil)
-	f.XLSX[rels] = MacintoshCyrillicCharset
+	f.Pkg.Store(rels, MacintoshCyrillicCharset)
 	f.relsReader(rels)
 }
 
diff --git a/file.go b/file.go
index fa73ec8..1786e27 100644
--- a/file.go
+++ b/file.go
@@ -26,18 +26,17 @@ import (
 //    f := NewFile()
 //
 func NewFile() *File {
-	file := make(map[string][]byte)
-	file["_rels/.rels"] = []byte(XMLHeader + templateRels)
-	file["docProps/app.xml"] = []byte(XMLHeader + templateDocpropsApp)
-	file["docProps/core.xml"] = []byte(XMLHeader + templateDocpropsCore)
-	file["xl/_rels/workbook.xml.rels"] = []byte(XMLHeader + templateWorkbookRels)
-	file["xl/theme/theme1.xml"] = []byte(XMLHeader + templateTheme)
-	file["xl/worksheets/sheet1.xml"] = []byte(XMLHeader + templateSheet)
-	file["xl/styles.xml"] = []byte(XMLHeader + templateStyles)
-	file["xl/workbook.xml"] = []byte(XMLHeader + templateWorkbook)
-	file["[Content_Types].xml"] = []byte(XMLHeader + templateContentTypes)
 	f := newFile()
-	f.SheetCount, f.XLSX = 1, file
+	f.Pkg.Store("_rels/.rels", []byte(XMLHeader+templateRels))
+	f.Pkg.Store("docProps/app.xml", []byte(XMLHeader+templateDocpropsApp))
+	f.Pkg.Store("docProps/core.xml", []byte(XMLHeader+templateDocpropsCore))
+	f.Pkg.Store("xl/_rels/workbook.xml.rels", []byte(XMLHeader+templateWorkbookRels))
+	f.Pkg.Store("xl/theme/theme1.xml", []byte(XMLHeader+templateTheme))
+	f.Pkg.Store("xl/worksheets/sheet1.xml", []byte(XMLHeader+templateSheet))
+	f.Pkg.Store("xl/styles.xml", []byte(XMLHeader+templateStyles))
+	f.Pkg.Store("xl/workbook.xml", []byte(XMLHeader+templateWorkbook))
+	f.Pkg.Store("[Content_Types].xml", []byte(XMLHeader+templateContentTypes))
+	f.SheetCount = 1
 	f.CalcChain = f.calcChainReader()
 	f.Comments = make(map[string]*xlsxComments)
 	f.ContentTypes = f.contentTypesReader()
@@ -48,8 +47,9 @@ func NewFile() *File {
 	f.WorkBook = f.workbookReader()
 	f.Relationships = sync.Map{}
 	f.Relationships.Store("xl/_rels/workbook.xml.rels", f.relsReader("xl/_rels/workbook.xml.rels"))
-	f.Sheet["xl/worksheets/sheet1.xml"], _ = f.workSheetReader("Sheet1")
 	f.sheetMap["Sheet1"] = "xl/worksheets/sheet1.xml"
+	ws, _ := f.workSheetReader("Sheet1")
+	f.Sheet.Store("xl/worksheets/sheet1.xml", ws)
 	f.Theme = f.themeReader()
 	return f
 }
@@ -165,20 +165,22 @@ func (f *File) writeToZip(zw *zip.Writer) error {
 		}
 		stream.rawData.Close()
 	}
-
-	for path, content := range f.XLSX {
-		if _, ok := f.streams[path]; ok {
-			continue
-		}
-		fi, err := zw.Create(path)
+	var err error
+	f.Pkg.Range(func(path, content interface{}) bool {
 		if err != nil {
-			return err
+			return false
 		}
-		_, err = fi.Write(content)
+		if _, ok := f.streams[path.(string)]; ok {
+			return true
+		}
+		var fi io.Writer
+		fi, err = zw.Create(path.(string))
 		if err != nil {
-			return err
+			return false
 		}
-	}
+		_, err = fi.Write(content.([]byte))
+		return true
+	})
 
-	return nil
+	return err
 }
diff --git a/file_test.go b/file_test.go
index dbbf75a..d86ce53 100644
--- a/file_test.go
+++ b/file_test.go
@@ -5,6 +5,7 @@ import (
 	"bytes"
 	"os"
 	"strings"
+	"sync"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
@@ -38,19 +39,19 @@ func TestWriteTo(t *testing.T) {
 	{
 		f := File{}
 		buf := bytes.Buffer{}
-		f.XLSX = make(map[string][]byte)
-		f.XLSX["/d/"] = []byte("s")
+		f.Pkg = sync.Map{}
+		f.Pkg.Store("/d/", []byte("s"))
 		_, err := f.WriteTo(bufio.NewWriter(&buf))
 		assert.EqualError(t, err, "zip: write to directory")
-		delete(f.XLSX, "/d/")
+		f.Pkg.Delete("/d/")
 	}
 	// Test file path overflow
 	{
 		f := File{}
 		buf := bytes.Buffer{}
-		f.XLSX = make(map[string][]byte)
+		f.Pkg = sync.Map{}
 		const maxUint16 = 1<<16 - 1
-		f.XLSX[strings.Repeat("s", maxUint16+1)] = nil
+		f.Pkg.Store(strings.Repeat("s", maxUint16+1), nil)
 		_, err := f.WriteTo(bufio.NewWriter(&buf))
 		assert.EqualError(t, err, "zip: FileHeader.Name too long")
 	}
@@ -58,8 +59,8 @@ func TestWriteTo(t *testing.T) {
 	{
 		f := File{}
 		buf := bytes.Buffer{}
-		f.XLSX = make(map[string][]byte)
-		f.XLSX["s"] = nil
+		f.Pkg = sync.Map{}
+		f.Pkg.Store("s", nil)
 		f.streams = make(map[string]*StreamWriter)
 		file, _ := os.Open("123")
 		f.streams["s"] = &StreamWriter{rawData: bufferedWriter{tmp: file}}
diff --git a/lib.go b/lib.go
index 5c9bbf6..00a67d9 100644
--- a/lib.go
+++ b/lib.go
@@ -51,8 +51,8 @@ func ReadZipReader(r *zip.Reader) (map[string][]byte, int, error) {
 
 // readXML provides a function to read XML content as string.
 func (f *File) readXML(name string) []byte {
-	if content, ok := f.XLSX[name]; ok {
-		return content
+	if content, _ := f.Pkg.Load(name); content != nil {
+		return content.([]byte)
 	}
 	if content, ok := f.streams[name]; ok {
 		return content.rawData.buf.Bytes()
@@ -66,7 +66,7 @@ func (f *File) saveFileList(name string, content []byte) {
 	newContent := make([]byte, 0, len(XMLHeader)+len(content))
 	newContent = append(newContent, []byte(XMLHeader)...)
 	newContent = append(newContent, content...)
-	f.XLSX[name] = newContent
+	f.Pkg.Store(name, newContent)
 }
 
 // Read file content as string in a archive file.
diff --git a/merge_test.go b/merge_test.go
index afe75aa..cf460dd 100644
--- a/merge_test.go
+++ b/merge_test.go
@@ -71,13 +71,19 @@ func TestMergeCell(t *testing.T) {
 
 	f = NewFile()
 	assert.NoError(t, f.MergeCell("Sheet1", "A2", "B3"))
-	f.Sheet["xl/worksheets/sheet1.xml"].MergeCells = &xlsxMergeCells{Cells: []*xlsxMergeCell{nil, nil}}
+	ws, ok := f.Sheet.Load("xl/worksheets/sheet1.xml")
+	assert.True(t, ok)
+	ws.(*xlsxWorksheet).MergeCells = &xlsxMergeCells{Cells: []*xlsxMergeCell{nil, nil}}
 	assert.NoError(t, f.MergeCell("Sheet1", "A2", "B3"))
 
-	f.Sheet["xl/worksheets/sheet1.xml"].MergeCells = &xlsxMergeCells{Cells: []*xlsxMergeCell{{Ref: "A1"}}}
+	ws, ok = f.Sheet.Load("xl/worksheets/sheet1.xml")
+	assert.True(t, ok)
+	ws.(*xlsxWorksheet).MergeCells = &xlsxMergeCells{Cells: []*xlsxMergeCell{{Ref: "A1"}}}
 	assert.EqualError(t, f.MergeCell("Sheet1", "A2", "B3"), `invalid area "A1"`)
 
-	f.Sheet["xl/worksheets/sheet1.xml"].MergeCells = &xlsxMergeCells{Cells: []*xlsxMergeCell{{Ref: "A:A"}}}
+	ws, ok = f.Sheet.Load("xl/worksheets/sheet1.xml")
+	assert.True(t, ok)
+	ws.(*xlsxWorksheet).MergeCells = &xlsxMergeCells{Cells: []*xlsxMergeCell{{Ref: "A:A"}}}
 	assert.EqualError(t, f.MergeCell("Sheet1", "A2", "B3"), `cannot convert cell "A" to coordinates: invalid cell name "A"`)
 }
 
@@ -154,16 +160,24 @@ func TestUnmergeCell(t *testing.T) {
 	// Test unmerged area on not exists worksheet.
 	assert.EqualError(t, f.UnmergeCell("SheetN", "A1", "A1"), "sheet SheetN is not exist")
 
-	f.Sheet["xl/worksheets/sheet1.xml"].MergeCells = nil
+	ws, ok := f.Sheet.Load("xl/worksheets/sheet1.xml")
+	assert.True(t, ok)
+	ws.(*xlsxWorksheet).MergeCells = nil
 	assert.NoError(t, f.UnmergeCell("Sheet1", "H7", "B15"))
 
-	f.Sheet["xl/worksheets/sheet1.xml"].MergeCells = &xlsxMergeCells{Cells: []*xlsxMergeCell{nil, nil}}
+	ws, ok = f.Sheet.Load("xl/worksheets/sheet1.xml")
+	assert.True(t, ok)
+	ws.(*xlsxWorksheet).MergeCells = &xlsxMergeCells{Cells: []*xlsxMergeCell{nil, nil}}
 	assert.NoError(t, f.UnmergeCell("Sheet1", "H15", "B7"))
 
-	f.Sheet["xl/worksheets/sheet1.xml"].MergeCells = &xlsxMergeCells{Cells: []*xlsxMergeCell{{Ref: "A1"}}}
+	ws, ok = f.Sheet.Load("xl/worksheets/sheet1.xml")
+	assert.True(t, ok)
+	ws.(*xlsxWorksheet).MergeCells = &xlsxMergeCells{Cells: []*xlsxMergeCell{{Ref: "A1"}}}
 	assert.EqualError(t, f.UnmergeCell("Sheet1", "A2", "B3"), `invalid area "A1"`)
 
-	f.Sheet["xl/worksheets/sheet1.xml"].MergeCells = &xlsxMergeCells{Cells: []*xlsxMergeCell{{Ref: "A:A"}}}
+	ws, ok = f.Sheet.Load("xl/worksheets/sheet1.xml")
+	assert.True(t, ok)
+	ws.(*xlsxWorksheet).MergeCells = &xlsxMergeCells{Cells: []*xlsxMergeCell{{Ref: "A:A"}}}
 	assert.EqualError(t, f.UnmergeCell("Sheet1", "A2", "B3"), `cannot convert cell "A" to coordinates: invalid cell name "A"`)
 
 }
diff --git a/picture.go b/picture.go
index 09283c8..58fa909 100644
--- a/picture.go
+++ b/picture.go
@@ -223,11 +223,12 @@ func (f *File) addSheetPicture(sheet string, rID int) {
 // folder xl/drawings.
 func (f *File) countDrawings() int {
 	c1, c2 := 0, 0
-	for k := range f.XLSX {
-		if strings.Contains(k, "xl/drawings/drawing") {
+	f.Pkg.Range(func(k, v interface{}) bool {
+		if strings.Contains(k.(string), "xl/drawings/drawing") {
 			c1++
 		}
-	}
+		return true
+	})
 	f.Drawings.Range(func(rel, value interface{}) bool {
 		if strings.Contains(rel.(string), "xl/drawings/drawing") {
 			c2++
@@ -305,11 +306,12 @@ func (f *File) addDrawingPicture(sheet, drawingXML, cell, file string, width, he
 // folder xl/media/image.
 func (f *File) countMedia() int {
 	count := 0
-	for k := range f.XLSX {
-		if strings.Contains(k, "xl/media/image") {
+	f.Pkg.Range(func(k, v interface{}) bool {
+		if strings.Contains(k.(string), "xl/media/image") {
 			count++
 		}
-	}
+		return true
+	})
 	return count
 }
 
@@ -318,16 +320,22 @@ func (f *File) countMedia() int {
 // and drawings that use it will reference the same image.
 func (f *File) addMedia(file []byte, ext string) string {
 	count := f.countMedia()
-	for name, existing := range f.XLSX {
-		if !strings.HasPrefix(name, "xl/media/image") {
-			continue
+	var name string
+	f.Pkg.Range(func(k, existing interface{}) bool {
+		if !strings.HasPrefix(k.(string), "xl/media/image") {
+			return true
 		}
-		if bytes.Equal(file, existing) {
-			return name
+		if bytes.Equal(file, existing.([]byte)) {
+			name = k.(string)
+			return false
 		}
+		return true
+	})
+	if name != "" {
+		return name
 	}
 	media := "xl/media/image" + strconv.Itoa(count+1) + ext
-	f.XLSX[media] = file
+	f.Pkg.Store(media, file)
 	return media
 }
 
@@ -468,8 +476,7 @@ func (f *File) GetPicture(sheet, cell string) (string, []byte, error) {
 	}
 	target := f.getSheetRelationshipsTargetByID(sheet, ws.Drawing.RID)
 	drawingXML := strings.Replace(target, "..", "xl", -1)
-	_, ok := f.XLSX[drawingXML]
-	if !ok {
+	if _, ok := f.Pkg.Load(drawingXML); !ok {
 		return "", nil, err
 	}
 	drawingRelationships := strings.Replace(
@@ -532,7 +539,10 @@ func (f *File) getPicture(row, col int, drawingXML, drawingRelationships string)
 			if deTwoCellAnchor.From.Col == col && deTwoCellAnchor.From.Row == row {
 				drawRel = f.getDrawingRelationships(drawingRelationships, deTwoCellAnchor.Pic.BlipFill.Blip.Embed)
 				if _, ok = supportImageTypes[filepath.Ext(drawRel.Target)]; ok {
-					ret, buf = filepath.Base(drawRel.Target), f.XLSX[strings.Replace(drawRel.Target, "..", "xl", -1)]
+					ret = filepath.Base(drawRel.Target)
+					if buffer, _ := f.Pkg.Load(strings.Replace(drawRel.Target, "..", "xl", -1)); buffer != nil {
+						buf = buffer.([]byte)
+					}
 					return
 				}
 			}
@@ -556,7 +566,10 @@ func (f *File) getPictureFromWsDr(row, col int, drawingRelationships string, wsD
 				if drawRel = f.getDrawingRelationships(drawingRelationships,
 					anchor.Pic.BlipFill.Blip.Embed); drawRel != nil {
 					if _, ok = supportImageTypes[filepath.Ext(drawRel.Target)]; ok {
-						ret, buf = filepath.Base(drawRel.Target), f.XLSX[strings.Replace(drawRel.Target, "..", "xl", -1)]
+						ret = filepath.Base(drawRel.Target)
+						if buffer, _ := f.Pkg.Load(strings.Replace(drawRel.Target, "..", "xl", -1)); buffer != nil {
+							buf = buffer.([]byte)
+						}
 						return
 					}
 				}
diff --git a/picture_test.go b/picture_test.go
index be917b8..69873eb 100644
--- a/picture_test.go
+++ b/picture_test.go
@@ -155,7 +155,7 @@ func TestGetPicture(t *testing.T) {
 	assert.Empty(t, raw)
 	f, err = prepareTestBook1()
 	assert.NoError(t, err)
-	f.XLSX["xl/drawings/drawing1.xml"] = MacintoshCyrillicCharset
+	f.Pkg.Store("xl/drawings/drawing1.xml", MacintoshCyrillicCharset)
 	_, _, err = f.getPicture(20, 5, "xl/drawings/drawing1.xml", "xl/drawings/_rels/drawing2.xml.rels")
 	assert.EqualError(t, err, "xml decode error: XML syntax error on line 1: invalid UTF-8")
 }
@@ -173,11 +173,12 @@ func TestAddPictureFromBytes(t *testing.T) {
 	assert.NoError(t, f.AddPictureFromBytes("Sheet1", fmt.Sprint("A", 1), "", "logo", ".png", imgFile))
 	assert.NoError(t, f.AddPictureFromBytes("Sheet1", fmt.Sprint("A", 50), "", "logo", ".png", imgFile))
 	imageCount := 0
-	for fileName := range f.XLSX {
-		if strings.Contains(fileName, "media/image") {
+	f.Pkg.Range(func(fileName, v interface{}) bool {
+		if strings.Contains(fileName.(string), "media/image") {
 			imageCount++
 		}
-	}
+		return true
+	})
 	assert.Equal(t, 1, imageCount, "Duplicate image should only be stored once.")
 	assert.EqualError(t, f.AddPictureFromBytes("SheetN", fmt.Sprint("A", 1), "", "logo", ".png", imgFile), "sheet SheetN is not exist")
 }
@@ -205,6 +206,8 @@ func TestDrawingResize(t *testing.T) {
 	// Test calculate drawing resize with invalid coordinates.
 	_, _, _, _, err = f.drawingResize("Sheet1", "", 1, 1, nil)
 	assert.EqualError(t, err, `cannot convert cell "" to coordinates: invalid cell name ""`)
-	f.Sheet["xl/worksheets/sheet1.xml"].MergeCells = &xlsxMergeCells{Cells: []*xlsxMergeCell{{Ref: "A:A"}}}
+	ws, ok := f.Sheet.Load("xl/worksheets/sheet1.xml")
+	assert.True(t, ok)
+	ws.(*xlsxWorksheet).MergeCells = &xlsxMergeCells{Cells: []*xlsxMergeCell{{Ref: "A:A"}}}
 	assert.EqualError(t, f.AddPicture("Sheet1", "A1", filepath.Join("test", "images", "excel.jpg"), `{"autofit": true}`), `cannot convert cell "A" to coordinates: invalid cell name "A"`)
 }
diff --git a/pivotTable.go b/pivotTable.go
index 3d93260..05ac783 100644
--- a/pivotTable.go
+++ b/pivotTable.go
@@ -13,7 +13,6 @@ package excelize
 
 import (
 	"encoding/xml"
-	"errors"
 	"fmt"
 	"strconv"
 	"strings"
@@ -163,7 +162,7 @@ func (f *File) AddPivotTable(opt *PivotTableOption) error {
 // properties.
 func (f *File) parseFormatPivotTableSet(opt *PivotTableOption) (*xlsxWorksheet, string, error) {
 	if opt == nil {
-		return nil, "", errors.New("parameter is required")
+		return nil, "", ErrParameterRequired
 	}
 	pivotTableSheetName, _, err := f.adjustRange(opt.PivotTableRange)
 	if err != nil {
@@ -192,11 +191,11 @@ func (f *File) parseFormatPivotTableSet(opt *PivotTableOption) (*xlsxWorksheet,
 // adjustRange adjust range, for example: adjust Sheet1!$E$31:$A$1 to Sheet1!$A$1:$E$31
 func (f *File) adjustRange(rangeStr string) (string, []int, error) {
 	if len(rangeStr) < 1 {
-		return "", []int{}, errors.New("parameter is required")
+		return "", []int{}, ErrParameterRequired
 	}
 	rng := strings.Split(rangeStr, "!")
 	if len(rng) != 2 {
-		return "", []int{}, errors.New("parameter is invalid")
+		return "", []int{}, ErrParameterInvalid
 	}
 	trimRng := strings.Replace(rng[1], "$", "", -1)
 	coordinates, err := f.areaRefToCoordinates(trimRng)
@@ -205,7 +204,7 @@ func (f *File) adjustRange(rangeStr string) (string, []int, error) {
 	}
 	x1, y1, x2, y2 := coordinates[0], coordinates[1], coordinates[2], coordinates[3]
 	if x1 == x2 && y1 == y2 {
-		return rng[0], []int{}, errors.New("parameter is invalid")
+		return rng[0], []int{}, ErrParameterInvalid
 	}
 
 	// Correct the coordinate area, such correct C1:B3 to B1:C3.
@@ -600,11 +599,12 @@ func (f *File) addPivotFields(pt *xlsxPivotTableDefinition, opt *PivotTableOptio
 // the folder xl/pivotTables.
 func (f *File) countPivotTables() int {
 	count := 0
-	for k := range f.XLSX {
-		if strings.Contains(k, "xl/pivotTables/pivotTable") {
+	f.Pkg.Range(func(k, v interface{}) bool {
+		if strings.Contains(k.(string), "xl/pivotTables/pivotTable") {
 			count++
 		}
-	}
+		return true
+	})
 	return count
 }
 
@@ -612,11 +612,12 @@ func (f *File) countPivotTables() int {
 // the folder xl/pivotCache.
 func (f *File) countPivotCache() int {
 	count := 0
-	for k := range f.XLSX {
-		if strings.Contains(k, "xl/pivotCache/pivotCacheDefinition") {
+	f.Pkg.Range(func(k, v interface{}) bool {
+		if strings.Contains(k.(string), "xl/pivotCache/pivotCacheDefinition") {
 			count++
 		}
-	}
+		return true
+	})
 	return count
 }
 
diff --git a/pivotTable_test.go b/pivotTable_test.go
index 7098b3a..e746d8d 100644
--- a/pivotTable_test.go
+++ b/pivotTable_test.go
@@ -137,12 +137,12 @@ func TestAddPivotTable(t *testing.T) {
 		ShowLastColumn:  true,
 	}))
 	// Create pivot table with many data, many rows, many cols and defined name
-	f.SetDefinedName(&DefinedName{
+	assert.NoError(t, f.SetDefinedName(&DefinedName{
 		Name:     "dataRange",
 		RefersTo: "Sheet1!$A$1:$E$31",
 		Comment:  "Pivot Table Data Range",
 		Scope:    "Sheet2",
-	})
+	}))
 	assert.NoError(t, f.AddPivotTable(&PivotTableOption{
 		DataRange:       "dataRange",
 		PivotTableRange: "Sheet2!$A$57:$AJ$91",
diff --git a/rows.go b/rows.go
index 6360f4e..229b12d 100644
--- a/rows.go
+++ b/rows.go
@@ -195,9 +195,12 @@ func (f *File) Rows(sheet string) (*Rows, error) {
 	if !ok {
 		return nil, ErrSheetNotExist{sheet}
 	}
-	if f.Sheet[name] != nil {
+	if ws, ok := f.Sheet.Load(name); ok && ws != nil {
+		worksheet := ws.(*xlsxWorksheet)
+		worksheet.Lock()
+		defer worksheet.Unlock()
 		// flush data
-		output, _ := xml.Marshal(f.Sheet[name])
+		output, _ := xml.Marshal(worksheet)
 		f.saveFileList(name, f.replaceNameSpaceBytes(name, output))
 	}
 	var (
diff --git a/rows_test.go b/rows_test.go
index 585fe59..069b668 100644
--- a/rows_test.go
+++ b/rows_test.go
@@ -43,11 +43,13 @@ func TestRows(t *testing.T) {
 	}
 
 	f = NewFile()
-	f.XLSX["xl/worksheets/sheet1.xml"] = []byte(`<worksheet><sheetData><row r="1"><c r="A1" t="s"><v>1</v></c></row><row r="A"><c r="2" t="str"><v>B</v></c></row></sheetData></worksheet>`)
+	f.Pkg.Store("xl/worksheets/sheet1.xml", []byte(`<worksheet><sheetData><row r="1"><c r="A1" t="s"><v>1</v></c></row><row r="A"><c r="2" t="str"><v>B</v></c></row></sheetData></worksheet>`))
+	f.Sheet.Delete("xl/worksheets/sheet1.xml")
+	delete(f.checked, "xl/worksheets/sheet1.xml")
 	_, err = f.Rows("Sheet1")
 	assert.EqualError(t, err, `strconv.Atoi: parsing "A": invalid syntax`)
 
-	f.XLSX["xl/worksheets/sheet1.xml"] = nil
+	f.Pkg.Store("xl/worksheets/sheet1.xml", nil)
 	_, err = f.Rows("Sheet1")
 	assert.NoError(t, err)
 }
@@ -187,7 +189,7 @@ func TestColumns(t *testing.T) {
 
 func TestSharedStringsReader(t *testing.T) {
 	f := NewFile()
-	f.XLSX["xl/sharedStrings.xml"] = MacintoshCyrillicCharset
+	f.Pkg.Store("xl/sharedStrings.xml", MacintoshCyrillicCharset)
 	f.sharedStringsReader()
 	si := xlsxSI{}
 	assert.EqualValues(t, "", si.String())
@@ -874,12 +876,14 @@ func TestErrSheetNotExistError(t *testing.T) {
 
 func TestCheckRow(t *testing.T) {
 	f := NewFile()
-	f.XLSX["xl/worksheets/sheet1.xml"] = []byte(`<?xml version="1.0" encoding="UTF-8" standalone="yes"?><worksheet xmlns="http://schemas.openxmlformats.org/spreadsheetml/2006/main" ><sheetData><row r="2"><c><v>1</v></c><c r="F2"><v>2</v></c><c><v>3</v></c><c><v>4</v></c><c r="M2"><v>5</v></c></row></sheetData></worksheet>`)
+	f.Pkg.Store("xl/worksheets/sheet1.xml", []byte(`<?xml version="1.0" encoding="UTF-8" standalone="yes"?><worksheet xmlns="http://schemas.openxmlformats.org/spreadsheetml/2006/main" ><sheetData><row r="2"><c><v>1</v></c><c r="F2"><v>2</v></c><c><v>3</v></c><c><v>4</v></c><c r="M2"><v>5</v></c></row></sheetData></worksheet>`))
 	_, err := f.GetRows("Sheet1")
 	assert.NoError(t, err)
 	assert.NoError(t, f.SetCellValue("Sheet1", "A1", false))
 	f = NewFile()
-	f.XLSX["xl/worksheets/sheet1.xml"] = []byte(`<?xml version="1.0" encoding="UTF-8" standalone="yes"?><worksheet xmlns="http://schemas.openxmlformats.org/spreadsheetml/2006/main" ><sheetData><row r="2"><c><v>1</v></c><c r="-"><v>2</v></c><c><v>3</v></c><c><v>4</v></c><c r="M2"><v>5</v></c></row></sheetData></worksheet>`)
+	f.Pkg.Store("xl/worksheets/sheet1.xml", []byte(`<?xml version="1.0" encoding="UTF-8" standalone="yes"?><worksheet xmlns="http://schemas.openxmlformats.org/spreadsheetml/2006/main" ><sheetData><row r="2"><c><v>1</v></c><c r="-"><v>2</v></c><c><v>3</v></c><c><v>4</v></c><c r="M2"><v>5</v></c></row></sheetData></worksheet>`))
+	f.Sheet.Delete("xl/worksheets/sheet1.xml")
+	delete(f.checked, "xl/worksheets/sheet1.xml")
 	assert.EqualError(t, f.SetCellValue("Sheet1", "A1", false), `cannot convert cell "-" to coordinates: invalid cell name "-"`)
 }
 
diff --git a/sheet.go b/sheet.go
index 420235c..2b91495 100644
--- a/sheet.go
+++ b/sheet.go
@@ -15,7 +15,6 @@ import (
 	"bytes"
 	"encoding/json"
 	"encoding/xml"
-	"errors"
 	"fmt"
 	"io"
 	"io/ioutil"
@@ -152,25 +151,27 @@ func (f *File) workSheetWriter() {
 	var arr []byte
 	buffer := bytes.NewBuffer(arr)
 	encoder := xml.NewEncoder(buffer)
-	for p, sheet := range f.Sheet {
-		if sheet != nil {
+	f.Sheet.Range(func(p, ws interface{}) bool {
+		if ws != nil {
+			sheet := ws.(*xlsxWorksheet)
 			for k, v := range sheet.SheetData.Row {
-				f.Sheet[p].SheetData.Row[k].C = trimCell(v.C)
+				sheet.SheetData.Row[k].C = trimCell(v.C)
 			}
 			if sheet.SheetPr != nil || sheet.Drawing != nil || sheet.Hyperlinks != nil || sheet.Picture != nil || sheet.TableParts != nil {
-				f.addNameSpaces(p, SourceRelationship)
+				f.addNameSpaces(p.(string), SourceRelationship)
 			}
 			// reusing buffer
 			_ = encoder.Encode(sheet)
-			f.saveFileList(p, replaceRelationshipsBytes(f.replaceNameSpaceBytes(p, buffer.Bytes())))
-			ok := f.checked[p]
+			f.saveFileList(p.(string), replaceRelationshipsBytes(f.replaceNameSpaceBytes(p.(string), buffer.Bytes())))
+			ok := f.checked[p.(string)]
 			if ok {
-				delete(f.Sheet, p)
-				f.checked[p] = false
+				f.Sheet.Delete(p.(string))
+				f.checked[p.(string)] = false
 			}
 			buffer.Reset()
 		}
-	}
+		return true
+	})
 }
 
 // trimCell provides a function to trim blank cells which created by fillColumns.
@@ -213,7 +214,7 @@ func (f *File) setSheet(index int, name string) {
 	}
 	path := "xl/worksheets/sheet" + strconv.Itoa(index) + ".xml"
 	f.sheetMap[trimSheetName(name)] = path
-	f.Sheet[path] = &ws
+	f.Sheet.Store(path, &ws)
 	f.xmlAttr[path] = []xml.Attr{NameSpaceSpreadSheet}
 }
 
@@ -448,7 +449,7 @@ func (f *File) getSheetMap() map[string]string {
 				if strings.HasPrefix(rel.Target, "/") {
 					path = filepath.ToSlash(strings.TrimPrefix(strings.Replace(filepath.Clean(rel.Target), "\\", "/", -1), "/"))
 				}
-				if _, ok := f.XLSX[path]; ok {
+				if _, ok := f.Pkg.Load(path); ok {
 					maps[v.Name] = path
 				}
 			}
@@ -524,10 +525,10 @@ func (f *File) DeleteSheet(name string) {
 			f.deleteSheetFromContentTypes(target)
 			f.deleteCalcChain(sheet.SheetID, "")
 			delete(f.sheetMap, sheetName)
-			delete(f.XLSX, sheetXML)
-			delete(f.XLSX, rels)
+			f.Pkg.Delete(sheetXML)
+			f.Pkg.Delete(rels)
 			f.Relationships.Delete(rels)
-			delete(f.Sheet, sheetXML)
+			f.Sheet.Delete(sheetXML)
 			delete(f.xmlAttr, sheetXML)
 			f.SheetCount--
 		}
@@ -573,7 +574,7 @@ func (f *File) deleteSheetFromContentTypes(target string) {
 //
 func (f *File) CopySheet(from, to int) error {
 	if from < 0 || to < 0 || from == to || f.GetSheetName(from) == "" || f.GetSheetName(to) == "" {
-		return errors.New("invalid worksheet index")
+		return ErrSheetIdx
 	}
 	return f.copySheet(from, to)
 }
@@ -595,12 +596,11 @@ func (f *File) copySheet(from, to int) error {
 	worksheet.Drawing = nil
 	worksheet.TableParts = nil
 	worksheet.PageSetUp = nil
-	f.Sheet[path] = worksheet
+	f.Sheet.Store(path, worksheet)
 	toRels := "xl/worksheets/_rels/sheet" + toSheetID + ".xml.rels"
 	fromRels := "xl/worksheets/_rels/sheet" + strconv.Itoa(f.getSheetID(fromSheet)) + ".xml.rels"
-	_, ok := f.XLSX[fromRels]
-	if ok {
-		f.XLSX[toRels] = f.XLSX[fromRels]
+	if rels, ok := f.Pkg.Load(fromRels); ok && rels != nil {
+		f.Pkg.Store(toRels, rels.([]byte))
 	}
 	fromSheetXMLPath := f.sheetMap[trimSheetName(fromSheet)]
 	fromSheetAttr := f.xmlAttr[fromSheetXMLPath]
@@ -824,9 +824,9 @@ func (f *File) SearchSheet(sheet, value string, reg ...bool) ([]string, error) {
 	if !ok {
 		return result, ErrSheetNotExist{sheet}
 	}
-	if f.Sheet[name] != nil {
+	if ws, ok := f.Sheet.Load(name); ok && ws != nil {
 		// flush data
-		output, _ := xml.Marshal(f.Sheet[name])
+		output, _ := xml.Marshal(ws.(*xlsxWorksheet))
 		f.saveFileList(name, f.replaceNameSpaceBytes(name, output))
 	}
 	return f.searchSheet(name, value, regSearch)
@@ -1483,7 +1483,7 @@ func (f *File) SetDefinedName(definedName *DefinedName) error {
 				scope = f.GetSheetName(*dn.LocalSheetID)
 			}
 			if scope == definedName.Scope && dn.Name == definedName.Name {
-				return errors.New("the same name already exists on the scope")
+				return ErrDefinedNameduplicate
 			}
 		}
 		wb.DefinedNames.DefinedName = append(wb.DefinedNames.DefinedName, d)
@@ -1518,7 +1518,7 @@ func (f *File) DeleteDefinedName(definedName *DefinedName) error {
 			}
 		}
 	}
-	return errors.New("no defined name on the scope")
+	return ErrDefinedNameScope
 }
 
 // GetDefinedName provides a function to get the defined names of the workbook
@@ -1558,7 +1558,7 @@ func (f *File) GroupSheets(sheets []string) error {
 		}
 	}
 	if !inActiveSheet {
-		return errors.New("group worksheet must contain an active worksheet")
+		return ErrGroupSheets
 	}
 	// check worksheet exists
 	wss := []*xlsxWorksheet{}
@@ -1714,8 +1714,7 @@ func (f *File) relsReader(path string) *xlsxRelationships {
 	var err error
 	rels, _ := f.Relationships.Load(path)
 	if rels == nil {
-		_, ok := f.XLSX[path]
-		if ok {
+		if _, ok := f.Pkg.Load(path); ok {
 			c := xlsxRelationships{}
 			if err = f.xmlNewDecoder(bytes.NewReader(namespaceStrictToTransitional(f.readXML(path)))).
 				Decode(&c); err != nil && err != io.EOF {
@@ -1734,6 +1733,8 @@ func (f *File) relsReader(path string) *xlsxRelationships {
 // row to accept data. Missing rows are backfilled and given their row number
 // Uses the last populated row as a hint for the size of the next row to add
 func prepareSheetXML(ws *xlsxWorksheet, col int, row int) {
+	ws.Lock()
+	defer ws.Unlock()
 	rowCount := len(ws.SheetData.Row)
 	sizeHint := 0
 	var ht float64
diff --git a/sheet_test.go b/sheet_test.go
index a721472..268abdc 100644
--- a/sheet_test.go
+++ b/sheet_test.go
@@ -347,9 +347,13 @@ func TestSetActiveSheet(t *testing.T) {
 	f.WorkBook.BookViews = nil
 	f.SetActiveSheet(1)
 	f.WorkBook.BookViews = &xlsxBookViews{WorkBookView: []xlsxWorkBookView{}}
-	f.Sheet["xl/worksheets/sheet1.xml"].SheetViews = &xlsxSheetViews{SheetView: []xlsxSheetView{}}
+	ws, ok := f.Sheet.Load("xl/worksheets/sheet1.xml")
+	assert.True(t, ok)
+	ws.(*xlsxWorksheet).SheetViews = &xlsxSheetViews{SheetView: []xlsxSheetView{}}
 	f.SetActiveSheet(1)
-	f.Sheet["xl/worksheets/sheet1.xml"].SheetViews = nil
+	ws, ok = f.Sheet.Load("xl/worksheets/sheet1.xml")
+	assert.True(t, ok)
+	ws.(*xlsxWorksheet).SheetViews = nil
 	f.SetActiveSheet(1)
 	f = NewFile()
 	f.SetActiveSheet(-1)
@@ -365,14 +369,14 @@ func TestSetSheetName(t *testing.T) {
 
 func TestGetWorkbookPath(t *testing.T) {
 	f := NewFile()
-	delete(f.XLSX, "_rels/.rels")
+	f.Pkg.Delete("_rels/.rels")
 	assert.Equal(t, "", f.getWorkbookPath())
 }
 
 func TestGetWorkbookRelsPath(t *testing.T) {
 	f := NewFile()
-	delete(f.XLSX, "xl/_rels/.rels")
-	f.XLSX["_rels/.rels"] = []byte(`<?xml version="1.0" encoding="UTF-8" standalone="yes"?><Relationships xmlns="http://schemas.openxmlformats.org/package/2006/relationships"><Relationship Id="rId1" Type="http://purl.oclc.org/ooxml/officeDocument/relationships/officeDocument" Target="/workbook.xml"/></Relationships>`)
+	f.Pkg.Delete("xl/_rels/.rels")
+	f.Pkg.Store("_rels/.rels", []byte(`<?xml version="1.0" encoding="UTF-8" standalone="yes"?><Relationships xmlns="http://schemas.openxmlformats.org/package/2006/relationships"><Relationship Id="rId1" Type="http://purl.oclc.org/ooxml/officeDocument/relationships/officeDocument" Target="/workbook.xml"/></Relationships>`))
 	assert.Equal(t, "_rels/workbook.xml.rels", f.getWorkbookRelsPath())
 }
 
diff --git a/sheetpr_test.go b/sheetpr_test.go
index 42e2e0d..53532e9 100644
--- a/sheetpr_test.go
+++ b/sheetpr_test.go
@@ -443,7 +443,9 @@ func TestSheetFormatPrOptions(t *testing.T) {
 func TestSetSheetFormatPr(t *testing.T) {
 	f := NewFile()
 	assert.NoError(t, f.GetSheetFormatPr("Sheet1"))
-	f.Sheet["xl/worksheets/sheet1.xml"].SheetFormatPr = nil
+	ws, ok := f.Sheet.Load("xl/worksheets/sheet1.xml")
+	assert.True(t, ok)
+	ws.(*xlsxWorksheet).SheetFormatPr = nil
 	assert.NoError(t, f.SetSheetFormatPr("Sheet1", BaseColWidth(1.0)))
 	// Test set formatting properties on not exists worksheet.
 	assert.EqualError(t, f.SetSheetFormatPr("SheetN"), "sheet SheetN is not exist")
@@ -452,7 +454,9 @@ func TestSetSheetFormatPr(t *testing.T) {
 func TestGetSheetFormatPr(t *testing.T) {
 	f := NewFile()
 	assert.NoError(t, f.GetSheetFormatPr("Sheet1"))
-	f.Sheet["xl/worksheets/sheet1.xml"].SheetFormatPr = nil
+	ws, ok := f.Sheet.Load("xl/worksheets/sheet1.xml")
+	assert.True(t, ok)
+	ws.(*xlsxWorksheet).SheetFormatPr = nil
 	var (
 		baseColWidth     BaseColWidth
 		defaultColWidth  DefaultColWidth
diff --git a/sparkline.go b/sparkline.go
index 150c0ea..5326c60 100644
--- a/sparkline.go
+++ b/sparkline.go
@@ -467,7 +467,7 @@ func (f *File) parseFormatAddSparklineSet(sheet string, opt *SparklineOption) (*
 		return ws, err
 	}
 	if opt == nil {
-		return ws, errors.New("parameter is required")
+		return ws, ErrParameterRequired
 	}
 	if len(opt.Location) < 1 {
 		return ws, errors.New("parameter 'Location' is required")
diff --git a/sparkline_test.go b/sparkline_test.go
index eac9824..0777ee1 100644
--- a/sparkline_test.go
+++ b/sparkline_test.go
@@ -253,7 +253,9 @@ func TestAddSparkline(t *testing.T) {
 		Style:    -1,
 	}), `parameter 'Style' must betweent 0-35`)
 
-	f.Sheet["xl/worksheets/sheet1.xml"].ExtLst.Ext = `<extLst>
+	ws, ok := f.Sheet.Load("xl/worksheets/sheet1.xml")
+	assert.True(t, ok)
+	ws.(*xlsxWorksheet).ExtLst.Ext = `<extLst>
 	    <ext x14="http://schemas.microsoft.com/office/spreadsheetml/2009/9/main" uri="{05C60535-1F16-4fd2-B633-F4F36F0B64E0}">
 	        <x14:sparklineGroups
 	            xmlns:xm="http://schemas.microsoft.com/office/excel/2006/main">
diff --git a/stream.go b/stream.go
index 054dd8d..4a77b56 100644
--- a/stream.go
+++ b/stream.go
@@ -301,7 +301,7 @@ func (sw *StreamWriter) SetRow(axis string, values []interface{}) error {
 	}
 	if !sw.sheetWritten {
 		if len(sw.cols) > 0 {
-			sw.rawData.WriteString("<cols>" + sw.cols + "</cols>")
+			_, _ = sw.rawData.WriteString("<cols>" + sw.cols + "</cols>")
 		}
 		_, _ = sw.rawData.WriteString(`<sheetData>`)
 		sw.sheetWritten = true
@@ -481,9 +481,9 @@ func (sw *StreamWriter) Flush() error {
 	}
 
 	sheetPath := sw.File.sheetMap[trimSheetName(sw.Sheet)]
-	delete(sw.File.Sheet, sheetPath)
+	sw.File.Sheet.Delete(sheetPath)
 	delete(sw.File.checked, sheetPath)
-	delete(sw.File.XLSX, sheetPath)
+	sw.File.Pkg.Delete(sheetPath)
 
 	return nil
 }
diff --git a/stream_test.go b/stream_test.go
index cf133f1..f911ccc 100644
--- a/stream_test.go
+++ b/stream_test.go
@@ -99,8 +99,8 @@ func TestStreamWriter(t *testing.T) {
 
 	// Test unsupported charset
 	file = NewFile()
-	delete(file.Sheet, "xl/worksheets/sheet1.xml")
-	file.XLSX["xl/worksheets/sheet1.xml"] = MacintoshCyrillicCharset
+	file.Sheet.Delete("xl/worksheets/sheet1.xml")
+	file.Pkg.Store("xl/worksheets/sheet1.xml", MacintoshCyrillicCharset)
 	_, err = file.NewStreamWriter("Sheet1")
 	assert.EqualError(t, err, "xml decode error: XML syntax error on line 1: invalid UTF-8")
 
@@ -145,7 +145,9 @@ func TestStreamTable(t *testing.T) {
 
 	// Verify the table has names.
 	var table xlsxTable
-	assert.NoError(t, xml.Unmarshal(file.XLSX["xl/tables/table1.xml"], &table))
+	val, ok := file.Pkg.Load("xl/tables/table1.xml")
+	assert.True(t, ok)
+	assert.NoError(t, xml.Unmarshal(val.([]byte), &table))
 	assert.Equal(t, "A", table.TableColumns.TableColumn[0].Name)
 	assert.Equal(t, "B", table.TableColumns.TableColumn[1].Name)
 	assert.Equal(t, "C", table.TableColumns.TableColumn[2].Name)
diff --git a/styles.go b/styles.go
index 1ba9f08..235746c 100644
--- a/styles.go
+++ b/styles.go
@@ -15,7 +15,6 @@ import (
 	"bytes"
 	"encoding/json"
 	"encoding/xml"
-	"errors"
 	"fmt"
 	"io"
 	"log"
@@ -1104,14 +1103,14 @@ func parseFormatStyleSet(style interface{}) (*Style, error) {
 	case *Style:
 		fs = *v
 	default:
-		err = errors.New("invalid parameter type")
+		err = ErrParameterInvalid
 	}
 	if fs.Font != nil {
 		if len(fs.Font.Family) > MaxFontFamilyLength {
-			return &fs, errors.New("the length of the font family name must be smaller than or equal to 31")
+			return &fs, ErrFontLength
 		}
 		if fs.Font.Size > MaxFontSize {
-			return &fs, errors.New("font size must be between 1 and 409 points")
+			return &fs, ErrFontSize
 		}
 	}
 	return &fs, err
diff --git a/styles_test.go b/styles_test.go
index e2eed1d..5d452f6 100644
--- a/styles_test.go
+++ b/styles_test.go
@@ -201,7 +201,7 @@ func TestNewStyle(t *testing.T) {
 	_, err = f.NewStyle(&Style{})
 	assert.NoError(t, err)
 	_, err = f.NewStyle(Style{})
-	assert.EqualError(t, err, "invalid parameter type")
+	assert.EqualError(t, err, ErrParameterInvalid.Error())
 
 	_, err = f.NewStyle(&Style{Font: &Font{Family: strings.Repeat("s", MaxFontFamilyLength+1)}})
 	assert.EqualError(t, err, "the length of the font family name must be smaller than or equal to 31")
@@ -261,14 +261,14 @@ func TestStylesReader(t *testing.T) {
 	f := NewFile()
 	// Test read styles with unsupported charset.
 	f.Styles = nil
-	f.XLSX["xl/styles.xml"] = MacintoshCyrillicCharset
+	f.Pkg.Store("xl/styles.xml", MacintoshCyrillicCharset)
 	assert.EqualValues(t, new(xlsxStyleSheet), f.stylesReader())
 }
 
 func TestThemeReader(t *testing.T) {
 	f := NewFile()
 	// Test read theme with unsupported charset.
-	f.XLSX["xl/theme/theme1.xml"] = MacintoshCyrillicCharset
+	f.Pkg.Store("xl/theme/theme1.xml", MacintoshCyrillicCharset)
 	assert.EqualValues(t, new(xlsxTheme), f.themeReader())
 }
 
diff --git a/table.go b/table.go
index 12ef41a..620cf20 100644
--- a/table.go
+++ b/table.go
@@ -105,11 +105,12 @@ func (f *File) AddTable(sheet, hcell, vcell, format string) error {
 // folder xl/tables.
 func (f *File) countTables() int {
 	count := 0
-	for k := range f.XLSX {
-		if strings.Contains(k, "xl/tables/table") {
+	f.Pkg.Range(func(k, v interface{}) bool {
+		if strings.Contains(k.(string), "xl/tables/table") {
 			count++
 		}
-	}
+		return true
+	})
 	return count
 }