using Excel = Microsoft.Office.Interop.Excel;
    private MemoryStream ExportDataSetToExcel()
    {
        System.Data.DataTable employeeTable = new System.Data.DataTable("Employee");
        employeeTable.Columns.Add("Employee ID");
        employeeTable.Columns.Add("Employee Name");
        employeeTable.Rows.Add("1", "ABC");
        employeeTable.Rows.Add("2", "DEF");
        employeeTable.Rows.Add("3", "PQR");
        employeeTable.Rows.Add("4", "XYZ");

        ////Create a Department Table
        System.Data.DataTable departmentTable = new System.Data.DataTable("Department");
        departmentTable.Columns.Add("Department ID");
        departmentTable.Columns.Add("Department Name");
        departmentTable.Rows.Add("1", "IT");
        departmentTable.Rows.Add("2", "HR");
        departmentTable.Rows.Add("3", "Finance");

        ////Create a DataSet with the existing DataTables
        DataSet ds = new DataSet("Organization");
        ds.Tables.Add(employeeTable);
        ds.Tables.Add(departmentTable);

        ////Creae an Excel application instance
        Excel.Application excelApp = new Excel.Application();
        excelApp.Visible = true;
        Excel.Workbook excelWorkBook = excelApp.Workbooks.Add();

        foreach (System.Data.DataTable table in ds.Tables)
        {
            ////Add a new worksheet to workbook with the Datatable name
            Microsoft.Office.Interop.Excel.Worksheet excelWorkSheet = excelWorkBook.Sheets.Add();
            excelWorkSheet.Name = table.TableName;

            for (int i = 1; i < table.Columns.Count + 1; i++)
            {
                excelWorkSheet.Cells[1, i] = table.Columns[i - 1].ColumnName;
                excelWorkSheet.Cells[1, i].Font.Bold = true;
            }

            for (int j = 0; j < table.Rows.Count; j++)
            {
                for (int k = 0; k < table.Columns.Count; k++)
                {
                    excelWorkSheet.Cells[j + 2, k + 1] = table.Rows[j].ItemArray[k].ToString();
                    excelWorkSheet.Cells[j + 2, k + 1].Interior.Color = Excel.XlRgbColor.rgbRed;
                }
            }
        }

        MemoryStream ms = new MemoryStream();

        ////excelWorkBook.SaveAs(ms, Type.Missing, Type.Missing, Type.Missing, true, false, Excel.XlSaveAsAccessMode.xlNoChange, Type.Missing, Type.Missing, Type.Missing);
        excelWorkBook.Close();
        excelApp.Quit();
        return ms;
    }