diff --git a/ClosedXML/ClosedXML/ClosedXML/Excel/Cells/XLCell.cs b/ClosedXML/ClosedXML/ClosedXML/Excel/Cells/XLCell.cs
index 07a9abf..6e2dfbd 100644
--- a/ClosedXML/ClosedXML/ClosedXML/Excel/Cells/XLCell.cs
+++ b/ClosedXML/ClosedXML/ClosedXML/Excel/Cells/XLCell.cs
@@ -136,7 +136,7 @@
                 if (HasRichText)
                     return _richText.ToString();
 
-                return StringExtensions.IsNullOrWhiteSpace(_cellValue) ? FormulaA1 : _cellValue;
+                return XLHelper.IsNullOrWhiteSpace(_cellValue) ? FormulaA1 : _cellValue;
             }
         }
 
@@ -268,7 +268,7 @@
         public T GetValue<T>()
         {
             var val = Value;
-            if (!StringExtensions.IsNullOrWhiteSpace(FormulaA1))
+            if (!XLHelper.IsNullOrWhiteSpace(FormulaA1))
                 return (T)Convert.ChangeType(String.Empty, typeof(T));
             if (val is TimeSpan)
             {
@@ -378,7 +378,7 @@
             get
             {
                 string fA1 = FormulaA1;
-                if (!StringExtensions.IsNullOrWhiteSpace(fA1))
+                if (!XLHelper.IsNullOrWhiteSpace(fA1))
                 {
                     if (fA1[0] == '{')
                         fA1 = fA1.Substring(1, fA1.Length - 2);
@@ -551,7 +551,7 @@
                             if (!hasTitles)
                             {
                                 string fieldName = GetFieldName(m.GetType().GetCustomAttributes(true));
-                                if (StringExtensions.IsNullOrWhiteSpace(fieldName))
+                                if (XLHelper.IsNullOrWhiteSpace(fieldName))
                                     fieldName = m.GetType().Name;
 
                                 SetValue(fieldName, fRo, co);
@@ -578,7 +578,7 @@
                             if (!hasTitles)
                             {
                                 foreach (string fieldName in from DataColumn column in ((DataRow) m).Table.Columns
-                                                             select StringExtensions.IsNullOrWhiteSpace(column.Caption)
+                                                             select XLHelper.IsNullOrWhiteSpace(column.Caption)
                                                                         ? column.ColumnName
                                                                         : column.Caption)
                                 {
@@ -633,7 +633,7 @@
                                     if ((info as IEnumerable) == null)
                                     {
                                         string fieldName = GetFieldName(info.GetCustomAttributes(true));
-                                        if (StringExtensions.IsNullOrWhiteSpace(fieldName))
+                                        if (XLHelper.IsNullOrWhiteSpace(fieldName))
                                             fieldName = info.Name;
 
                                         SetValue(fieldName, fRo, co);
@@ -647,7 +647,7 @@
                                     if ((info as IEnumerable) == null)
                                     {
                                         string fieldName = GetFieldName(info.GetCustomAttributes(true));
-                                        if (StringExtensions.IsNullOrWhiteSpace(fieldName))
+                                        if (XLHelper.IsNullOrWhiteSpace(fieldName))
                                             fieldName = info.Name;
 
                                         SetValue(fieldName, fRo, co);
@@ -966,9 +966,9 @@
         {
             get
             {
-                if (StringExtensions.IsNullOrWhiteSpace(_formulaA1))
+                if (XLHelper.IsNullOrWhiteSpace(_formulaA1))
                 {
-                    if (!StringExtensions.IsNullOrWhiteSpace(_formulaR1C1))
+                    if (!XLHelper.IsNullOrWhiteSpace(_formulaR1C1))
                     {
                         _formulaA1 = GetFormulaA1(_formulaR1C1);
                         return FormulaA1;
@@ -988,7 +988,7 @@
 
             set
             {
-                _formulaA1 = StringExtensions.IsNullOrWhiteSpace(value) ? null : value;
+                _formulaA1 = XLHelper.IsNullOrWhiteSpace(value) ? null : value;
 
                 _formulaR1C1 = null;
             }
@@ -998,7 +998,7 @@
         {
             get
             {
-                if (StringExtensions.IsNullOrWhiteSpace(_formulaR1C1))
+                if (XLHelper.IsNullOrWhiteSpace(_formulaR1C1))
                     _formulaR1C1 = GetFormulaR1C1(FormulaA1);
 
                 return _formulaR1C1;
@@ -1006,7 +1006,7 @@
 
             set
             {
-                _formulaR1C1 = StringExtensions.IsNullOrWhiteSpace(value) ? null : value;
+                _formulaR1C1 = XLHelper.IsNullOrWhiteSpace(value) ? null : value;
 
 // FormulaA1 = GetFormulaA1(value);
             }
@@ -1223,7 +1223,7 @@
         {
             var style = GetStyleForRead();
             return _dataType == XLCellValues.Number
-                   && StringExtensions.IsNullOrWhiteSpace(style.NumberFormat.Format)
+                   && XLHelper.IsNullOrWhiteSpace(style.NumberFormat.Format)
                    && ((style.NumberFormat.NumberFormatId >= 14
                         && style.NumberFormat.NumberFormatId <= 22)
                        || (style.NumberFormat.NumberFormatId >= 45
@@ -1234,7 +1234,7 @@
         {
             string format = String.Empty;
             var style = GetStyleForRead();
-            if (StringExtensions.IsNullOrWhiteSpace(style.NumberFormat.Format))
+            if (XLHelper.IsNullOrWhiteSpace(style.NumberFormat.Format))
             {
                 var formatCodes = GetFormatCodes();
                 if (formatCodes.ContainsKey(style.NumberFormat.NumberFormatId))
@@ -1451,7 +1451,7 @@
         private string GetFormula(string strValue, FormulaConversionType conversionType, int rowsToShift,
                                   int columnsToShift)
         {
-            if (StringExtensions.IsNullOrWhiteSpace(strValue))
+            if (XLHelper.IsNullOrWhiteSpace(strValue))
                 return String.Empty;
 
             string value = ">" + strValue + "<";
@@ -1726,7 +1726,7 @@
         internal static String ShiftFormulaRows(String formulaA1, XLWorksheet worksheetInAction, XLRange shiftedRange,
                                                 int rowsShifted)
         {
-            if (StringExtensions.IsNullOrWhiteSpace(formulaA1)) return String.Empty;
+            if (XLHelper.IsNullOrWhiteSpace(formulaA1)) return String.Empty;
 
             string value = formulaA1; // ">" + formulaA1 + "<";
 
@@ -1952,7 +1952,7 @@
         internal static String ShiftFormulaColumns(String formulaA1, XLWorksheet worksheetInAction, XLRange shiftedRange,
                                                    int columnsShifted)
         {
-            if (StringExtensions.IsNullOrWhiteSpace(formulaA1)) return String.Empty;
+            if (XLHelper.IsNullOrWhiteSpace(formulaA1)) return String.Empty;
 
             string value = formulaA1; // ">" + formulaA1 + "<";
 
diff --git a/ClosedXML/ClosedXML/ClosedXML/Excel/DataValidation/XLDataValidation.cs b/ClosedXML/ClosedXML/ClosedXML/Excel/DataValidation/XLDataValidation.cs
index 7923cb6..658ae1b 100644
--- a/ClosedXML/ClosedXML/ClosedXML/Excel/DataValidation/XLDataValidation.cs
+++ b/ClosedXML/ClosedXML/ClosedXML/Excel/DataValidation/XLDataValidation.cs
@@ -41,9 +41,9 @@
             return
                 AllowedValues != XLAllowedValues.AnyValue
                 || (ShowInputMessage &&
-                  (!StringExtensions.IsNullOrWhiteSpace(InputTitle) || !StringExtensions.IsNullOrWhiteSpace(InputMessage)))
+                  (!XLHelper.IsNullOrWhiteSpace(InputTitle) || !XLHelper.IsNullOrWhiteSpace(InputMessage)))
                 ||(ShowErrorMessage &&
-                  (!StringExtensions.IsNullOrWhiteSpace(ErrorTitle) || !StringExtensions.IsNullOrWhiteSpace(ErrorMessage)));
+                  (!XLHelper.IsNullOrWhiteSpace(ErrorTitle) || !XLHelper.IsNullOrWhiteSpace(ErrorMessage)));
 
         }
 
diff --git a/ClosedXML/ClosedXML/ClosedXML/Excel/Ranges/XLRangeBase.cs b/ClosedXML/ClosedXML/ClosedXML/Excel/Ranges/XLRangeBase.cs
index 4081f65..22225c5 100644
--- a/ClosedXML/ClosedXML/ClosedXML/Excel/Ranges/XLRangeBase.cs
+++ b/ClosedXML/ClosedXML/ClosedXML/Excel/Ranges/XLRangeBase.cs
@@ -974,7 +974,7 @@
         {
             foreach (XLWorksheet ws in Worksheet.Workbook.WorksheetsInternal)
             {
-                foreach (XLCell cell in ws.Internals.CellsCollection.GetCells(c => !StringExtensions.IsNullOrWhiteSpace(c.FormulaA1)))
+                foreach (XLCell cell in ws.Internals.CellsCollection.GetCells(c => !XLHelper.IsNullOrWhiteSpace(c.FormulaA1)))
                     using (var asRange = AsRange())
                         cell.ShiftFormulaColumns(asRange, numberOfColumns);
             }
@@ -1180,7 +1180,7 @@
             using (var asRange = AsRange())
                 foreach (XLWorksheet ws in Worksheet.Workbook.WorksheetsInternal)
                 {
-                    foreach (XLCell cell in ws.Internals.CellsCollection.GetCells(c => !StringExtensions.IsNullOrWhiteSpace(c.FormulaA1)))
+                    foreach (XLCell cell in ws.Internals.CellsCollection.GetCells(c => !XLHelper.IsNullOrWhiteSpace(c.FormulaA1)))
                             cell.ShiftFormulaRows(asRange, numberOfRows);
                 }
 
@@ -1337,7 +1337,7 @@
                 XLCell cell in
                     Worksheet.Workbook.Worksheets.Cast<XLWorksheet>().SelectMany(
                         xlWorksheet => (xlWorksheet).Internals.CellsCollection.GetCells(
-                            c => !StringExtensions.IsNullOrWhiteSpace(c.FormulaA1))))
+                            c => !XLHelper.IsNullOrWhiteSpace(c.FormulaA1))))
             {
                 if (shiftDeleteCells == XLShiftDeletedCells.ShiftCellsUp)
                     cell.ShiftFormulaRows((XLRange)shiftedRangeFormula, numberOfRows * -1);
@@ -1621,7 +1621,7 @@
         public IXLRangeBase Sort(String columnsToSortBy, XLSortOrder sortOrder = XLSortOrder.Ascending, Boolean matchCase = false, Boolean ignoreBlanks = true)
         {
             SortColumns.Clear();
-            if (StringExtensions.IsNullOrWhiteSpace(columnsToSortBy))
+            if (XLHelper.IsNullOrWhiteSpace(columnsToSortBy))
             {
                 columnsToSortBy = String.Empty;
                 Int32 maxColumn = ColumnCount();
diff --git a/ClosedXML/ClosedXML/ClosedXML/Excel/Tables/XLTable.cs b/ClosedXML/ClosedXML/ClosedXML/Excel/Tables/XLTable.cs
index 3521773..afed638 100644
--- a/ClosedXML/ClosedXML/ClosedXML/Excel/Tables/XLTable.cs
+++ b/ClosedXML/ClosedXML/ClosedXML/Excel/Tables/XLTable.cs
@@ -65,7 +65,7 @@
                     foreach (var cell in headersRow.Cells())
                     {
                         var name = cell.GetString();
-                        if (StringExtensions.IsNullOrWhiteSpace(name)) 
+                        if (XLHelper.IsNullOrWhiteSpace(name)) 
                         {
                             name = "Column" + (cellPos + 1);
                             cell.SetValue(name);
@@ -395,7 +395,7 @@
             Int32 co = 1;
             foreach (IXLCell c in range.Row(1).Cells())
             {
-                if (StringExtensions.IsNullOrWhiteSpace(((XLCell)c).InnerText))
+                if (XLHelper.IsNullOrWhiteSpace(((XLCell)c).InnerText))
                     c.Value = GetUniqueName("Column" + co.ToStringLookup());
                 _uniqueNames.Add(c.GetString());
                 co++;
@@ -445,7 +445,7 @@
                     Int32 co = 1;
                     foreach (IXLCell c in headersRow.Cells())
                     {
-                        if (StringExtensions.IsNullOrWhiteSpace(((XLCell)c).InnerText))
+                        if (XLHelper.IsNullOrWhiteSpace(((XLCell)c).InnerText))
                             c.Value = GetUniqueName("Column" + co.ToStringLookup());
                         _uniqueNames.Add(c.GetString());
                         co++;
diff --git a/ClosedXML/ClosedXML/ClosedXML/Excel/XLWorkbook_Load.cs b/ClosedXML/ClosedXML/ClosedXML/Excel/XLWorkbook_Load.cs
index f1d2a9e..c801704 100644
--- a/ClosedXML/ClosedXML/ClosedXML/Excel/XLWorkbook_Load.cs
+++ b/ClosedXML/ClosedXML/ClosedXML/Excel/XLWorkbook_Load.cs
@@ -776,7 +776,7 @@
                 {
                     if (cell.CellValue != null)
                     {
-                        if (!StringExtensions.IsNullOrWhiteSpace(cell.CellValue.Text))
+                        if (!XLHelper.IsNullOrWhiteSpace(cell.CellValue.Text))
                         {
                             var sharedString = sharedStrings[Int32.Parse(cell.CellValue.Text)];
 
@@ -871,7 +871,7 @@
                     xlCell.Style.NumberFormat.NumberFormatId = Int32.Parse(numberFormatId);
 
 
-                if (!StringExtensions.IsNullOrWhiteSpace(xlCell.Style.NumberFormat.Format))
+                if (!XLHelper.IsNullOrWhiteSpace(xlCell.Style.NumberFormat.Format))
                     xlCell._dataType = GetDataTypeFromFormat(xlCell.Style.NumberFormat.Format);
                 else if ((numberFormatId >= 14 && numberFormatId <= 22) || (numberFormatId >= 45 && numberFormatId <= 47))
                     xlCell._dataType = XLCellValues.DateTime;
@@ -1283,7 +1283,7 @@
             foreach (DataValidation dvs in dataValidations.Elements<DataValidation>())
             {
                 String txt = dvs.SequenceOfReferences.InnerText;
-                if (StringExtensions.IsNullOrWhiteSpace(txt)) continue;
+                if (XLHelper.IsNullOrWhiteSpace(txt)) continue;
                 foreach (var dvt in txt.Split(' ').Select(rangeAddress => ws.Range(rangeAddress).DataValidation))
                 {
                     if (dvs.AllowBlank != null) dvt.IgnoreBlanks = dvs.AllowBlank;
diff --git a/ClosedXML/ClosedXML/ClosedXML/Excel/XLWorkbook_Save.cs b/ClosedXML/ClosedXML/ClosedXML/Excel/XLWorkbook_Save.cs
index 892801a..e1fd261 100644
--- a/ClosedXML/ClosedXML/ClosedXML/Excel/XLWorkbook_Save.cs
+++ b/ClosedXML/ClosedXML/ClosedXML/Excel/XLWorkbook_Save.cs
@@ -175,7 +175,7 @@
                     var vmlDrawingPart = worksheetPart.VmlDrawingParts.FirstOrDefault();
                     if (vmlDrawingPart == null)
                     {
-                        if (StringExtensions.IsNullOrWhiteSpace(worksheet.LegacyDrawingId))
+                        if (XLHelper.IsNullOrWhiteSpace(worksheet.LegacyDrawingId))
                         {
                             worksheet.LegacyDrawingId = context.RelIdGenerator.GetNext(RelType.Workbook);
                             worksheet.LegacyDrawingIsNew = true;
@@ -347,7 +347,7 @@
 
             if (Properties.Manager != null)
             {
-                if (!StringExtensions.IsNullOrWhiteSpace(Properties.Manager))
+                if (!XLHelper.IsNullOrWhiteSpace(Properties.Manager))
                 {
                     if (properties.Manager == null)
                         properties.Manager = new Manager();
@@ -360,7 +360,7 @@
 
             if (Properties.Company == null) return;
 
-            if (!StringExtensions.IsNullOrWhiteSpace(Properties.Company))
+            if (!XLHelper.IsNullOrWhiteSpace(Properties.Company))
             {
                 if (properties.Company == null)
                     properties.Company = new Company();
@@ -602,7 +602,7 @@
                             LocalSheetId = sheetId,
                             Text = nr.ToString()
                         };
-                    if (!StringExtensions.IsNullOrWhiteSpace(nr.Comment))
+                    if (!XLHelper.IsNullOrWhiteSpace(nr.Comment))
                         definedName.Comment = nr.Comment;
                     definedNames.AppendChild(definedName);
                 }
@@ -653,7 +653,7 @@
                         Name = nr.Name,
                         Text = nr.ToString()
                     };
-                if (!StringExtensions.IsNullOrWhiteSpace(nr.Comment))
+                if (!XLHelper.IsNullOrWhiteSpace(nr.Comment))
                     definedName.Comment = nr.Comment;
                 definedNames.AppendChild(definedName);
             }
@@ -694,7 +694,7 @@
                         w.Internals.CellsCollection.GetCells(
                             c => ((c.DataType == XLCellValues.Text && c.ShareString) || c.HasRichText)
                                  && (c as XLCell).InnerText.Length > 0
-                                 && StringExtensions.IsNullOrWhiteSpace(c.FormulaA1)
+                                 && XLHelper.IsNullOrWhiteSpace(c.FormulaA1)
                             )))
             {
                 c.DataType = XLCellValues.Text;
@@ -849,7 +849,7 @@
                 var cellsWithoutFormulas = new HashSet<String>();
                 foreach (var c in worksheet.Internals.CellsCollection.GetCells())
                 {
-                    if (StringExtensions.IsNullOrWhiteSpace(c.FormulaA1))
+                    if (XLHelper.IsNullOrWhiteSpace(c.FormulaA1))
                         cellsWithoutFormulas.Add(c.Address.ToStringRelative());
                     else
                     {
@@ -1635,7 +1635,7 @@
                             tableColumn1.TotalsRowFormula = new TotalsRowFormula(xlField.TotalsRowFormulaA1);
                     }
 
-                    if (!StringExtensions.IsNullOrWhiteSpace(xlField.TotalsRowLabel))
+                    if (!XLHelper.IsNullOrWhiteSpace(xlField.TotalsRowLabel))
                         tableColumn1.TotalsRowLabel = xlField.TotalsRowLabel;
                 }
                 tableColumns1.AppendChild(tableColumn1);
@@ -2577,7 +2577,7 @@
                     StrokeWeight = String.Format("{0}pt", c.Comment.Style.ColorsAndLines.LineWeight),
                     InsetMode = c.Comment.Style.Margins.Automatic ? InsetMarginValues.Auto : InsetMarginValues.Custom
                 };
-            if (!StringExtensions.IsNullOrWhiteSpace(c.Comment.Style.Web.AlternateText))
+            if (!XLHelper.IsNullOrWhiteSpace(c.Comment.Style.Web.AlternateText))
                 shape.Alternate = c.Comment.Style.Web.AlternateText;
 
 
@@ -2918,7 +2918,7 @@
         {
             var differentialFormat = new DifferentialFormat();
             differentialFormat.Append(GetNewFont(new FontInfo {Font = cf.Style.Font as XLFont}, false));
-            if (!StringExtensions.IsNullOrWhiteSpace(cf.Style.NumberFormat.Format))
+            if (!XLHelper.IsNullOrWhiteSpace(cf.Style.NumberFormat.Format))
             {
                 var numberFormat = new NumberingFormat
                     {
@@ -3545,7 +3545,7 @@
         {
             var newXLNumberFormat = new XLNumberFormat();
 
-            if (nf.FormatCode != null && !StringExtensions.IsNullOrWhiteSpace(nf.FormatCode.Value))
+            if (nf.FormatCode != null && !XLHelper.IsNullOrWhiteSpace(nf.FormatCode.Value))
                 newXLNumberFormat.Format = nf.FormatCode.Value;
             else if (nf.NumberFormatId != null)
                 newXLNumberFormat.NumberFormatId = (Int32) nf.NumberFormatId.Value;
@@ -4057,7 +4057,7 @@
                     }
 
                     cell.StyleIndex = styleId;
-                    if (!StringExtensions.IsNullOrWhiteSpace(opCell.FormulaA1))
+                    if (!XLHelper.IsNullOrWhiteSpace(opCell.FormulaA1))
                     {
                         var formula = opCell.FormulaA1;
                         if (formula.StartsWith("{"))
@@ -4149,7 +4149,7 @@
 
                 var protection = xlWorksheet.Protection;
                 sheetProtection.Sheet = protection.Protected;
-                if (!StringExtensions.IsNullOrWhiteSpace(protection.PasswordHash))
+                if (!XLHelper.IsNullOrWhiteSpace(protection.PasswordHash))
                     sheetProtection.Password = protection.PasswordHash;
                 sheetProtection.FormatCells = GetBooleanValue(!protection.FormatCells, true);
                 sheetProtection.FormatColumns = GetBooleanValue(!protection.FormatColumns, true);
@@ -4353,7 +4353,7 @@
                                 Display = hl.Cell.GetFormattedString()
                             };
                     }
-                    if (!StringExtensions.IsNullOrWhiteSpace(hl.Tooltip))
+                    if (!XLHelper.IsNullOrWhiteSpace(hl.Tooltip))
                         hyperlink.Tooltip = hl.Tooltip;
                     hyperlinks.AppendChild(hyperlink);
                 }
@@ -4608,7 +4608,7 @@
             {
                 worksheetPart.Worksheet.RemoveAllChildren<LegacyDrawing>();
                 {
-                    if (!StringExtensions.IsNullOrWhiteSpace(xlWorksheet.LegacyDrawingId))
+                    if (!XLHelper.IsNullOrWhiteSpace(xlWorksheet.LegacyDrawingId))
                     {
                         var previousElement = cm.GetPreviousElementFor(XLWSContentManager.XLWSContents.LegacyDrawing);
                         worksheetPart.Worksheet.InsertAfter(new LegacyDrawing {Id = xlWorksheet.LegacyDrawingId},
diff --git a/ClosedXML/ClosedXML/ClosedXML/Excel/XLWorksheet.cs b/ClosedXML/ClosedXML/ClosedXML/Excel/XLWorksheet.cs
index c862b2c..1d97047 100644
--- a/ClosedXML/ClosedXML/ClosedXML/Excel/XLWorksheet.cs
+++ b/ClosedXML/ClosedXML/ClosedXML/Excel/XLWorksheet.cs
@@ -167,7 +167,7 @@
                     throw new ArgumentException("Worksheet names cannot contain any of the following characters: " +
                                                 InvalidNameChars);
 
-                if (StringExtensions.IsNullOrWhiteSpace(value))
+                if (XLHelper.IsNullOrWhiteSpace(value))
                     throw new ArgumentException("Worksheet names cannot be empty");
 
                 if (value.Length > 31)
@@ -617,7 +617,7 @@
 
         private String ReplaceRelativeSheet(string newSheetName, String value)
         {
-            if (StringExtensions.IsNullOrWhiteSpace(value)) return value;
+            if (XLHelper.IsNullOrWhiteSpace(value)) return value;
 
             var newValue = new StringBuilder();
             var addresses = value.Split(',');
diff --git a/ClosedXML/ClosedXML/ClosedXML/Excel/XLWorksheets.cs b/ClosedXML/ClosedXML/ClosedXML/Excel/XLWorksheets.cs
index 7a08d77..35b7f05 100644
--- a/ClosedXML/ClosedXML/ClosedXML/Excel/XLWorksheets.cs
+++ b/ClosedXML/ClosedXML/ClosedXML/Excel/XLWorksheets.cs
@@ -119,7 +119,7 @@
                     "Can't delete the worksheet because there are multiple worksheets associated with that index.");
 
             var ws = _worksheets.Values.Single(w => w.Position == position);
-            if (!StringExtensions.IsNullOrWhiteSpace(ws.RelId) && !Deleted.Contains(ws.RelId))
+            if (!XLHelper.IsNullOrWhiteSpace(ws.RelId) && !Deleted.Contains(ws.RelId))
                 Deleted.Add(ws.RelId);
 
             _worksheets.RemoveAll(w => w.Position == position);
@@ -155,7 +155,7 @@
 
         public void Rename(String oldSheetName, String newSheetName)
         {
-            if (StringExtensions.IsNullOrWhiteSpace(oldSheetName) || !_worksheets.ContainsKey(oldSheetName)) return;
+            if (XLHelper.IsNullOrWhiteSpace(oldSheetName) || !_worksheets.ContainsKey(oldSheetName)) return;
 
             var ws = _worksheets[oldSheetName];
             _worksheets.Remove(oldSheetName);
diff --git a/ClosedXML/ClosedXML/ClosedXML/ExcelHelper.cs b/ClosedXML/ClosedXML/ClosedXML/ExcelHelper.cs
index f1dcca5..9b85972 100644
--- a/ClosedXML/ClosedXML/ClosedXML/ExcelHelper.cs
+++ b/ClosedXML/ClosedXML/ClosedXML/ExcelHelper.cs
@@ -113,7 +113,7 @@
         public static bool IsValidColumn(string column)
         {
             var length = column.Length;
-            if (StringExtensions.IsNullOrWhiteSpace(column) || length > 3)
+            if (IsNullOrWhiteSpace(column) || length > 3)
                 return false;
 
             var theColumn = column.ToUpper();
@@ -150,7 +150,7 @@
 
         public static bool IsValidA1Address(string address)
         {
-            if (StringExtensions.IsNullOrWhiteSpace(address))
+            if (IsNullOrWhiteSpace(address))
                 return false;
 
             address = address.Replace("$", "");
@@ -194,7 +194,7 @@
             return Convert.ToDouble(pt)*DpiX/72.0;
         }
 
-        internal static IXLTableRows InsertRowsWithoutEvents(Func<Int32, Boolean, IXLRangeRows> insertFunc,
+        internal static IXLTableRows InsertRowsWithoutEvents(Func<int, bool, IXLRangeRows> insertFunc,
                                                              XLTableRange tableRange, Int32 numberOfRows,
                                                              Boolean expandTable)
         {
@@ -229,5 +229,26 @@
                 worksheet.SelectedRanges.Add(range);
             }
         }
+
+        public static bool IsNullOrWhiteSpace(string value)
+        {
+#if NET4
+            return String.IsNullOrWhiteSpace(value);
+#else
+            if (value != null)
+            {
+                var length = value.Length;
+                for (int i = 0; i < length; i++)
+                {
+                    if (!char.IsWhiteSpace(value[i]))
+                    {
+                        return false;
+                    }
+                }
+            }
+            return true;
+#endif
+
+        }
     }
 }
\ No newline at end of file
diff --git a/ClosedXML/ClosedXML/ClosedXML/Extensions.cs b/ClosedXML/ClosedXML/ClosedXML/Extensions.cs
index 324f28b..84a9139 100644
--- a/ClosedXML/ClosedXML/ClosedXML/Extensions.cs
+++ b/ClosedXML/ClosedXML/ClosedXML/Extensions.cs
@@ -104,22 +104,6 @@
 
     public static class StringExtensions
     {
-        public static bool IsNullOrWhiteSpace(string value)
-        {
-            if (value != null)
-            {
-                var length = value.Length;
-                for (int i = 0; i < length; i++)
-                {
-                    if (!char.IsWhiteSpace(value[i]))
-                    {
-                        return false;
-                    }
-                }
-            }
-            return true;
-        }
-
         private static readonly Regex RegexNewLine = new Regex(@"((?<!\r)\n|\r\n)");
         public static String FixNewLines(this String value)
         {
@@ -194,7 +178,7 @@
         }
         public static Double GetWidth(this IXLFontBase fontBase, String text, Dictionary<IXLFontBase, Font> fontCache)
         {
-            if (StringExtensions.IsNullOrWhiteSpace(text))
+            if (XLHelper.IsNullOrWhiteSpace(text))
                 return 0;
 
             var font = GetCachedFont(fontBase, fontCache);