public static void AutoSizeColumns( DataGrid grid ) {
  System.Drawing.Graphics g = System.Drawing.Graphics.FromHwnd( grid.Handle );
  GridColumnStylesCollection cs = grid.TableStyles[0].GridColumnStyles;
  object ds = grid.DataSource;
 
  for ( int style = 0; style < cs.Count; style++ ) {
    int rowCount = ( ds is DataView ) ? ( ds as DataView ).Count : ( ds as DataTable ).Rows.Count;
    string format = "{0}";
 
    if ( cs[style] is DataGridTextBoxColumn ) {
      format = "{0:" + ( cs[style] as DataGridTextBoxColumn ).Format + "}";
    }
 
    // The default minimum width is the header text
    System.Drawing.SizeF maxWidth = g.MeasureString( cs[style].HeaderText, grid.Font );
 
    // Find the widest row in the grid for this column
    for ( int i = 0; i < rowCount; i++ ) {
      System.Drawing.SizeF cellWidth = g.MeasureString( string.Format( format, grid[i, style] ), grid.Font );
 
      if ( cellWidth.Width > maxWidth.Width ) {
        maxWidth = cellWidth;
      }
    }
 
    cs[style].Width = (int)maxWidth.Width + 8;
  }
 
  g.Dispose();
}