C#

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace DataGridView
{
    class ExcelExport
    {

        public void Export(string fileName, string DataSource,string FileName)
        {
            /*fileName is the name of the file will be written.
            DataSource is the DataGridView name used*/
            Microsoft.Office.Interop.Excel._Application app = new Microsoft.Office.Interop.Excel.Application();
            Microsoft.Office.Interop.Excel._Workbook workbook = app.Workbooks.Add(Type.Missing);
            Microsoft.Office.Interop.Excel._Worksheet worksheet = null;
            worksheet = workbook.Sheets["Sheet1"];
            worksheet = workbook.ActiveSheet;
            worksheet.Name = fileName;
            for (int i = 1; i < DataSource.Columns.Count + 1; i++)
            {
                worksheet.Cells[1, i] = DataSource.Columns[i - 1].HeaderText;
            }
            for (int i = 0; i < DataSource.Rows.Count; i++)
            {
                for (int j = 0; j < DataSource.Columns.Count; j++)
                {
                    worksheet.Cells[i + 2, j + 1] = DataSource.Rows[i].Cells[j].Value.ToString();
                }
            }
            var saveFileDialoge = new SaveFileDialog();
            saveFileDialoge.FileName = FileName;
            saveFileDialoge.DefaultExt = ".xlsx";
            if (saveFileDialoge.ShowDialog() == DialogResult.OK)
            {
                workbook.SaveAs(saveFileDialoge.FileName, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Microsoft.Office.Interop.Excel.XlSaveAsAccessMode.xlExclusive, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing);

            }
        }
    }
}

public void Export(string fileName, string DataSource,string FileName)
        {
            /*fileName is the name of the file will be written.
            DataSource is the DataGridView name used*/
...
            for (int i = 1; i < DataSource.Columns.Count + 1; i++)

private DataGridView DataSource = new DataGridView();

myMethod(datatGridView1);

private DataTable theSource = new DataTable();
private void FrmMain_Shown(object sender, EventArgs e)
    {
    theSource.Columns.Add("Column 1");
    theSource.Columns.Add("Column 2");
    theSource.Rows.Add("cccc1111", 666);
    myDataGridView.DataSource = theSource;
    }
private void AddButton_Click(object sender, EventArgs e)
    {
    theSource.Rows.Add(DateTime.Now, "hello");
    }

private void OutputButton_Click(object sender, EventArgs e)
    {
    MyDLLAssemblyClass.OutputToXLSX(theSource, @"d:\Test Data\myFile.xslx");
    }

private static void OutputToXLSX(DataTable dt, string outputPath)
    {
    ...
    }