How to merge Excel cell in c# when cell range in not static or fix?

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C#

hi,
i want to merge a cell where i want to merge a excel cell in c# where i have not static of fix cell range.

Below in my code for excel generat
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workbookGrid = new Workbook();
workbookGrid.DocumentProperties.Author = "DiaSales";
workbookGrid.DocumentProperties.Title = "DiaSales Sales Mail";

//Create a sheet
workbookGrid.Worksheets.Add("Sales Mail");
Spire.Xls.Worksheet worksheet;
worksheet = workbookGrid.Worksheets[0];
worksheet.Name = "Sales Mail2";

int j = 0;

DataTable DTsalesMail = new TRN_Sales_Order().GetSalesMailData();

foreach (DataColumn dc in DTsalesMail.Columns)
{
j = j + 1;
worksheet[1, j].Style.Font.IsBold = true;
worksheet[1, j].Style.Font.FontName = "Calibri";
worksheet[1, j].Style.Color = Color.LightGray;

}


worksheet.InsertDataTable(DTsalesMail, true, 1, 1, -1, -1);

SaveFileDialog saveFileDialog1 = new SaveFileDialog();
saveFileDialog1.Filter = "XLS Files (*.xls)|*.xls";
saveFileDialog1.Title = "Sales Mail";
saveFileDialog1.FileName = "Sales Mail";

if ((saveFileDialog1.ShowDialog() == System.Windows.Forms.DialogResult.OK))
{
string svFileName = saveFileDialog1.FileName;
if (!string.IsNullOrEmpty(saveFileDialog1.FileName))
{
workbookGrid.SaveToFile(saveFileDialog1.FileName);

//Remove Unnecessory Sheet
Microsoft.Office.Interop.Excel.Application ExcelApp = default(Microsoft.Office.Interop.Excel.Application);
Microsoft.Office.Interop.Excel.Workbook ExcelworkbookGrid = default(Microsoft.Office.Interop.Excel.Workbook);
Microsoft.Office.Interop.Excel.Worksheet ExcelSheet = default(Microsoft.Office.Interop.Excel.Worksheet);
string sMySheet = "SalesMail1";

ExcelApp = (Microsoft.Office.Interop.Excel.Application)Interaction.CreateObject("Excel.Application");
ExcelApp.AlertBeforeOverwriting = false;
ExcelApp.DisplayAlerts = false;
ExcelworkbookGrid = ExcelApp.Workbooks.Open(System.IO.Path.GetFullPath(saveFileDialog1.FileName));
recall1:

if (ExcelworkbookGrid.Sheets.Count > 1)
{
ExcelSheet = ExcelworkbookGrid.Worksheets[2];
ExcelSheet.Activate();

if (ExcelSheet.Name != sMySheet)
{
ExcelSheet.Delete();
}
goto recall1;
}

ExcelSheet = ExcelworkbookGrid.Worksheets[1];
ExcelSheet.Activate();

ExcelworkbookGrid.DoNotPromptForConvert = true;
ExcelworkbookGrid.SaveLinkValues = false;
ExcelworkbookGrid.UpdateRemoteReferences = false;

ExcelworkbookGrid.Save();

ExcelworkbookGrid.Close();
ExcelworkbookGrid = null;

ExcelApp.Quit();
ExcelApp = null;
}
}