private void textBox1_DragEnter(object sender, DragEventArgs e)
{
if (e.Data.GetDataPresent(DataFormats.FileDrop, false))
{
e.Effect = DragDropEffects.All;
}
}

private void textBox1_DragDrop(object sender, DragEventArgs e)
{
this.textBox1.Text = GetDragData((string[])e.Data.GetData(DataFormats.FileDrop));
}