Tim Van Wassenhove

Earlier today i was refactoring some graphical components. I wanted to use an interface instead of concrete implementations for my drag and drop code… Although i was using an instance of Foo, and thus IFoo, the following code never allowed me to drop:

interface IFoo { }
class Foo : IFoo { }

private void label1_MouseDown(object sender, MouseEventArgs e)
{
 this.DoDragDrop(new Foo(), DragDropEffects.All);
}

private void Form1_DragEnter(object sender, DragEventArgs e)
{
 // for an instance of Foo it returns false,
 // if you use typeof(Foo) it returns true though...
 if (e.Data.GetDataPresent(typeof(IFoo)))
 {
  e.Effect = DragDropEffects.All;
 }
}

Simply wrapping the concrete instance in a DataObject results in the desired behaviour:

private void label1_MouseDown(object sender, MouseEventArgs e)
{
 this.DoDragDrop(new DataObject(typeof(IFoo).FullName,new Foo()), DragDropEffects.All);
}