how to change the excel format text to number

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C#

ASP.NET

C#4.0

hi all,
i am exporting gridview after export the data it is in the form of text, while calculations i need to change the format.


so my problem is while exporting the data i need to change to number format. is it possible. below my code

public void xlsexport(System.Data.DataTable dtGridsource, string Filename)
{
    int rowIndex = -1;
    int columnIndex = -1;

    string filename = Filename;
    HttpContext.Current.Response.ContentType = "application/vnd.ms-excel";
    HttpContext.Current.Response.AddHeader("Content-Disposition", string.Format("attachment;filename={0}", filename));

    HSSFWorkbook hssfworkbook;
    hssfworkbook = new HSSFWorkbook();

    ////create a entry of DocumentSummaryInformation
    DocumentSummaryInformation dsi = PropertySetFactory.CreateDocumentSummaryInformation();
    dsi.Company = "Adya Wifi";
    hssfworkbook.DocumentSummaryInformation = dsi;

    ////create a entry of SummaryInformation
    SummaryInformation si = PropertySetFactory.CreateSummaryInformation();
    si.Subject = dtGridsource.TableName.ToString();
    hssfworkbook.SummaryInformation = si;

    ISheet sheet1 = hssfworkbook.CreateSheet("Sheet1");

    IRow row = sheet1.CreateRow(0);
    for (int i = 0; i < dtGridsource.Columns.Count; i++)
    {
        row.CreateCell(i).SetCellValue(dtGridsource.Columns[i].ColumnName.ToString());
    }

    for (rowIndex = 0; rowIndex < dtGridsource.Rows.Count; rowIndex++)
    {
        IRow row1 = sheet1.CreateRow(rowIndex + 1);
        for (columnIndex = 0; columnIndex < dtGridsource.Columns.Count; columnIndex++)
        {
            row1.CreateCell(columnIndex).SetCellValue(dtGridsource.Rows[rowIndex][columnIndex].ToString());
        }
    }

    MemoryStream file = new MemoryStream();
    hssfworkbook.Write(file);

    HttpContext.Current.Response.BinaryWrite(file.GetBuffer());
    HttpContext.Current.ApplicationInstance.CompleteRequest();
    HttpContext.Current.Response.Flush();
    HttpContext.Current.Response.Clear();
}