できるかどうか不明ですが
private void _DragEnter(object sender, System.Windows.Forms.DragEventArgs e)
{
if (e.Data.GetDataPresent(DataFormats.FileDrop))
e.Effect = DragDropEffects.Copy;
else
e.Effect = DragDropEffects.None;
}

private void _DragDrop(object sender, System.Windows.Forms.DragEventArgs e)
{
string[] strs = e.Data.GetData(DataFormats.FileDrop, false) as string[];

if (strs == null || strs.Length == 0)
{
return;
}

for (int c = 0; c < strs.Length; c++)
{
if (Directory.Exists(strs[c]))
{
string[] fileNames = Directory.GetFiles(strs[c]);
}
else
{

}
}
}