With extension methods you can easily add new methods to existing types without creating new derived type or modifing the original type. Anytime you write a new method that has an input parameter with the keyword: this, is going to be an extension method of the type after the word this.

Here I show you a simple extension method that extends DataTable with a new method called: ExportToExcel

This methods creates a new Excel file from a DataTable. Of course you have to add Microsoft.Office.Interop.Excel as reference and set the following using:

using Excel = Microsoft.Office.Interop.Excel;

and the only static method in your new static class:

public static bool ExportToExcel(this DataTable tbl, string excelFilePath = null)
        {
            try
            {
                bool result = false;
                if (tbl == null || tbl.Columns.Count == 0) throw new Exception("No input table!\n");

                var excelApp = new Excel.Application();
                excelApp.Workbooks.Add();
                Excel._Worksheet workSheet = excelApp.ActiveSheet;

                //heading
                for (var i = 0; i < tbl.Columns.Count; i++)
                {
                    workSheet.Cells[1, i + 1] = tbl.Columns[i].ColumnName;
                }

                // rows
                for (var i = 0; i < tbl.Rows.Count; i++)
                {
                    for (var j = 0; j < tbl.Columns.Count; j++)
                    {
                        workSheet.Cells[i + 2, j + 1] = tbl.Rows[i][j];
                    }
                }

                workSheet.Columns.AutoFit();

                if (!string.IsNullOrEmpty(excelFilePath))
                {
                    try
                    {
                        workSheet.SaveAs(excelFilePath);
                        excelApp.Quit();
                        result = true;
                    }
                    catch (Exception ex)
                    {
                        throw new Exception("Excel file could not be saved! Check filepath.\n", ex);
                    }
                }
                else
                {
                    excelApp.Visible = true;
                }

                return result;
            }
            catch
            {
                throw;
            }
        }

After all you can use this method this way:

DataTable dt = new DataTable();
...
// your code here that fills up the DataTable
...
if (dt.ExportToExcel(saveFileDialog1.FileName)) MessageBox.Show("Completed");