private void Form5_Load(object sender, EventArgs e)
{
 this.AllowDrop=true;
}
private void Form5_DragEnter(object sender, DragEventArgs e)
{
 if (e.Data.GetDataPresent(DataFormats.FileDrop))
 {
  string[] files = (string[])e.Data.GetData(DataFormats.FileDrop);
  foreach (string f in files)
  {
   listBox1.Items.Add(f);
  }
 }
}