When I write exporting into xls using wpf framework in C#. I also encounter with this issue and I solve it with following code.Try to see from this code 
 Microsoft.Office.Interop.Excel._Application app = new Microsoft.Office.Interop.Excel.Application();
            Microsoft.Office.Interop.Excel._Workbook workbook = app.Workbooks.Add(Type.Missing);
            Microsoft.Office.Interop.Excel._Workbook workb = app.Workbooks.Open("c:\\Users\\Jack\\Documents\\Document1.xlsx", Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing);
            Microsoft.Office.Interop.Excel._Worksheet worksheet = null;
            worksheet = workb.Sheets[1];
           // worksheet = workbook.Sheets[1];
            //worksheet = workbook.ActiveSheet;
            //worksheet.Name = "birinchi";
            for (int i = 0; i < Table.Columns.Count; i++) 
            {
             //   worksheet.Cells[1, i+1] = Table.Columns[i].Header;
            }
                for (int i = 0; i < Table.Columns.Count; i++)
                {
                    for (int j = 0; j < Table.Items.Count; j++)
                    {
                        TextBlock b = Table.Columns[i].GetCellContent(Table.Items[j]) as TextBlock;
                        worksheet.Cells[j + 2, i + 1] = b.Text;
                        //MessageBox.Show(b.Text);
                    }
                }
                Microsoft.Win32.SaveFileDialog dlg = new Microsoft.Win32.SaveFileDialog();
                dlg.FileName = "Document";
                dlg.DefaultExt = ".xlsx";
                Nullable<bool> result = dlg.ShowDialog();
                if (result == true)
                {
                   workbook.SaveAs(dlg.FileName, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Microsoft.Office.Interop.Excel.XlSaveAsAccessMode.xlExclusive, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing);
                }
                app.Quit();