from1.cs

private void grid_liste_piecejointe_DragEnter(object sender, DragEventArgs e)
{
    if (e.Data.GetDataPresent(DataFormats.FileDrop))
        e.Effect = DragDropEffects.Move;
    else
        e.Effect = DragDropEffects.None;    
}

private void grid_liste_piecejointe_DragDrop(object sender, DragEventArgs e) {
    var dropped = ((string[])e.Data.GetData(DataFormats.FileDrop));
    var files = dropped.ToList();

    if (!files.Any())
        return;

    foreach (string drop in dropped)
        if (Directory.Exists(drop))
            files.AddRange(Directory.GetFiles(drop, filterFile, SearchOption.AllDirectories));

    foreach (string file in files)
    {
        if (!checkAttachmentExists(file) && checkExtensionFile(file))
        {
            PieceJointe pj = new PieceJointe();
            pj.setPiecejointe(file);
            list_pj.Add(pj);
        }
    }
}