Hello Kiet,

Maybe one possible approach to solve this task using Infragisitcs Excel engine, could be :

private void ultraButton2_Click(object sender, EventArgs e)
{
    Infragistics.Documents.Excel.Workbook wb = new Infragistics.Documents.Excel.Workbook();
    wb = Infragistics.Documents.Excel.Workbook.Load("..\\..\\Test.xlsx");
    Infragistics.Documents.Excel.Worksheet worksheet = wb.Worksheets["Sheet1"];
    Infragistics.Documents.Excel.Worksheet worksheet2 = wb.Worksheets["Sheet2"];

    int ii = worksheet.Rows.Count();
    foreach (WorksheetRow item in worksheet2.Rows)
    {
        ii++;
        foreach (WorksheetCell cell in item.Cells)
        {
            worksheet.Rows[ii].SetCellValue(cell.ColumnIndex, cell.Value);
        }
    }

    wb.Save("..\\..\\Test.xlsx");
    Process.Start("..\\..\\Test.xlsx");
}

Another possible solution could be without using Infragistics Excel engine. If you choose this scenario, you should add Microsoft.Office.Interop.Excel library. In this case the sample code will be:

private void ultraButton2_Click(object sender, EventArgs e)
{
  
    Excel.Application app = new Excel.Application();
    app.Workbooks.Add("..\\..\\Test1.xlsx");
    int worksheetCount = app.Workbooks[0].Worksheets.Count;

    app.Workbooks[0].Activate();
    for (int j = 1; j <= worksheetCount; j++)
    {
        Excel._Worksheet ws = (Excel._Worksheet)app.Workbooks[0].Worksheets[j];
        ws.Select(Type.Missing);
        ws.Cells.Select();

        Excel.Range sel = (Excel.Range)app.Selection;
        sel.Copy(Type.Missing);

        Excel._Worksheet sheet = (Excel._Worksheet)app.Workbooks[1].Worksheets.Add(Type.Missing, Type.Missing, Type.Missing, Type.Missing);
        sheet.Paste(Type.Missing, Type.Missing);
    }
}

Let me know if you have any further questions.

Regards