Drag and drop broken in Designer Host?

[DrBonzo]

I've inherited a body of C# code that has a 'Designer Host' (DH) based UI.
Custom controls are selected from a 'toolbox' window and placed/sized
in the DH interface. I'm trying to add a feature that allows the user to
drag data, specifically a string, from another window to one of these
controls,
without success. Part of the control definition is:

[Designer(typeof (TactControlDes igner))]
public class TactControl : UserControl
{
...
protected void tactDragOver(ob ject sender,
System.Windows. Forms.DragEvent Args e) {
Console.WriteLi ne("drag over tact");
e.Effect = DragDropEffects .Link;
}

protected void tactDragDrop(ob ject sender,
System.Windows. Forms.DragEvent Args e) {
string dropArg = (string) e.Data.GetData( typeof (System.String) );
if (dropArg != null) {
MessageBox.Show (dropArg);
}
}

public TactControl()
{
// This call is required by the Windows.Forms Form Designer.
InitializeCompo nent();

// TODO: Add any initialization after the InitializeCompo nent call
this.AllowDrop = true;
this.DragOver += new System.Windows. Forms.DragEvent Handler(tactDra gOver);
this.DragDrop += new System.Windows. Forms.DragEvent Handler(tactDra gDrop);

}
. . . neither of the drag event handlers ever get called.

I had thought that this DH technology (about which I'm extremely ignorant)
might not be creating windows for these controls, but Spy++ shows otherwise.

The definition of the DH class being used starts off:

[ProvideProperty ("Name", typeof(ICompone nt))]
internal class DesignerHost : IDesignerHost, IContainer,
IComponentChang eService, IExtenderProvid er, ITypeDescriptor FilterService,
IExtenderListSe rvice, IExtenderProvid erService
{

I'm guessing that the DH stuff might mess with the drag-drop procedure
in order to implement its own toolbox interactions, but I've no idea how,
and no idea where to look.

Any thoughts or pointers would be appreciated . . .