private void gridSource_MouseDown(object sender, MouseEventArgs e)
{
   if (e.Button == MouseButtons.Left)
   {
      DataGridView.HitTestInfo info = this.gridSource.HitTest(e.X, e.Y);
      if (info.RowIndex >= 0)
      {
         if (info.RowIndex >= 0 && info.ColumnIndex >= 0)
         {
            string text = (String)this.gridSource.Rows[info.RowIndex].Cells[info.ColumnIndex].Value;
            if (text != null)
            {
               this.gridSource.DoDragDrop(text, DragDropEffects.Copy);
            }
         }
      }
   }
}

private void gridTarget_DragDrop(object sender, DragEventArgs e)
{
   //得到要拖拽到的位置
   Point p = this.gridTarget.PointToClient(new Point(e.X, e.Y));
   DataGridView.HitTestInfo hit = this.gridTarget.HitTest(p.X, p.Y);
   if (hit.Type == DataGridViewHitTestType.Cell)
   {
      DataGridViewCell clickedCell = this.gridTarget.Rows[hit.RowIndex].Cells[hit.ColumnIndex];
      clickedCell.Value = (System.String)e.Data.GetData(typeof(System.String));
      //如果只想允许拖拽到某一个特定列，比如Target Field Expression，则先要判断列是否为Target Field Expression，如下：
      //if (0 == string.Compare(clickedCell.OwningColumn.Name, "Target Field Expression"))
      //{
      // clickedCell.Value = (System.String)e.Data.GetData(typeof(System.String));
      //}
   }
}

private void gridTarget_DragEnter(object sender, DragEventArgs e)
{
   e.Effect = DragDropEffects.Copy;
}