in your drag drop event:	 
if(!e.Data.GetDataPresent(DataFormats.FileDrop)) return;
            
string[] files = (string[])e.Data.GetData(DataFormats.FileDrop);



in your drag enter event:
 if(e.Data.GetDataPresent(DataFormats.FileDrop))
 	e.Effect = DragDropEffects.Copy;
 else
 	e.Effect = DragDropEffects.None;