Data Export from DataGrid to Excel is not working in WPF

private void btnExportToExcel_Click(object sender, RoutedEventArgs e)
       {
           ExportDataToExcel ex = new ExportDataToExcel();
           ex.ExportToExcel();
       }
       public Microsoft.Office.Interop.Excel.Application ExportToExcel()
       {
           int i = 0;
           int j = 0;
           int k = 1;
           j=dgDisplay.Items.Count;//it returns j=0..what went wrong?
           var rows = GetDataGridRows(dgDisplay);
           Microsoft.Office.Interop.Excel.Application ExcelApp = new Microsoft.Office.Interop.Excel.Application();
           Microsoft.Office.Interop.Excel._Workbook ExcelBook;
           Microsoft.Office.Interop.Excel._Worksheet ExcelSheet;
           ExcelBook = (Microsoft.Office.Interop.Excel._Workbook)ExcelApp.Workbooks.Add(1);
           ExcelSheet = (Microsoft.Office.Interop.Excel._Worksheet)ExcelBook.ActiveSheet;
           for (i = 1; i <= dgDisplay.Columns.Count; i++)
           {
               ExcelSheet.Cells[1, i] = dgDisplay.Columns[i - 1].Header.ToString();
           }
           foreach (DataGridRow r in rows)
           {
               DataRowView rv = (DataRowView)r.Item;
               foreach (DataGridColumn column in dgDisplay.Columns)
               {
                   if (column.GetCellContent(r) is TextBlock)
                   {
                       TextBlock cellContent = column.GetCellContent(r) as TextBlock;
                       ExcelSheet.Cells[i + 1, k] = cellContent.Text.Trim();
                       k++;
                       //MessageBox.Show(cellContent.Text);
                   }
               }

           }
           ExcelApp.Visible = true;
           ExcelSheet = null;
           ExcelBook = null;
           ExcelApp = null;

           return ExcelApp;

       }
public IEnumerable<DataGridRow> GetDataGridRows(DataGrid grid)
       {
           var itemsSource = grid.ItemsSource as IEnumerable;
           if (null == itemsSource) yield return null;
           foreach (var item in itemsSource)
           {
               var row = grid.ItemContainerGenerator.ContainerFromItem(item) as DataGridRow;
               if (null != row) yield return row;
           }
       }