Description: Implementation of the System.Windows.Forms.DataGridView that shows automatically formatted and sized row numbers in the row header.

Link: http://www.codekeep.net/snippets/34d04a2b-7876-441a-b0ea-c5960dde2b7d.aspx

using System;
using System.Collections.Generic;
using System.Text;
using System.Windows.Forms;
using System.Drawing;

namespace DataGridView2
{
    /// <summary>
    /// display a datagrid with row numbers
    /// </summary>
    class DataGridView2 : DataGridView
    {
        /// <summary>
        /// draw the row number on the header cell, auto adjust the column width
        /// to fit the row number
        /// </summary>
        /// <param name="e"></param>
        protected override void OnRowPostPaint(DataGridViewRowPostPaintEventArgs e)
        {
            base.OnRowPostPaint(e);

            // get the row number in leading zero format,
            //  where the width of the number = the width of the maximum number
            int RowNumWidth = this.RowCount.ToString().Length;
            StringBuilder RowNumber = new StringBuilder(RowNumWidth);
            RowNumber.Append(e.RowIndex + 1);
            while (RowNumber.Length < RowNumWidth)
                RowNumber.Insert(0, "0");

            // get the size of the row number string
            SizeF Sz = e.Graphics.MeasureString(RowNumber.ToString(), this.Font);

            // adjust the width of the column that contains the row header cells
            if (this.RowHeadersWidth < (int)(Sz.Width + 20))
                this.RowHeadersWidth = (int)(Sz.Width + 20);

            // draw the row number
            e.Graphics.DrawString(
                RowNumber.ToString(),
                this.Font,
                SystemBrushes.ControlText,
                e.RowBounds.Location.X + 15,
                e.RowBounds.Location.Y + ((e.RowBounds.Height - Sz.Height) / 2));
        }
    }
}