VB Guru please explain me how this can work ?

[aName]

How can this work ?
http://msdn.microsoft.com/library/d...lrfsystemwindowsformsdragactionclasstopic.asp


In Vbasic( see code below) e is pass ByVal so a copy is put on the stack when the handler is called and it is not suppose to have side effect.

But they clearly do side effect e.Action = DragAction.Cancel


To do that shouldn't we have at least ByRef Argument ????

WTF ?




PS
The c++ version is ok 'cause they pass a pointer to e







Private Sub ListDragSource_QueryContinueDrag(ByVal sender As Object, ByVal e As QueryContinueDragEventArgs) Handles ListDragSource.QueryContinueDrag
' Cancel the drag if the mouse moves off the form.
Dim lb as ListBox = CType(sender, System.Windows.Forms.ListBox)

If Not (lb is nothing) Then

Dim f as Form = lb.FindForm()

' Cancel the drag if the mouse moves off the form. The screenOffset
' takes into account any desktop bands that may be at the top or left
' side of the screen.
If (((Control.MousePosition.X - screenOffset.X) < f.DesktopBounds.Left) Or _
((Control.MousePosition.X - screenOffset.X) > f.DesktopBounds.Right) Or _
((Control.MousePosition.Y - screenOffset.Y) < f.DesktopBounds.Top) Or _
((Control.MousePosition.Y - screenOffset.Y) > f.DesktopBounds.Bottom)) Then

e.Action = DragAction.Cancel
End If
End if
End Sub





[C++]
private:
void ListDragSource_QueryContinueDrag(Object* sender,
System::Windows::Forms::QueryContinueDragEventArgs* e) {
// Cancel the drag if the mouse moves off the form.
ListBox* lb = dynamic_cast<ListBox*>(sender);

if (lb != 0) {

Form* f = lb->FindForm();

// Cancel the drag if the mouse moves off the form. The screenOffset
// takes into account any desktop bands that may be at the top or left
// side of the screen.
if (((Control::MousePosition.X - screenOffset.X) < f->DesktopBounds.Left) ||
((Control::MousePosition.X - screenOffset.X) > f->DesktopBounds.Right) ||
((Control::MousePosition.Y - screenOffset.Y) < f->DesktopBounds.Top) ||
((Control::MousePosition.Y - screenOffset.Y) > f->DesktopBounds.Bottom))
{

e->Action = DragAction::Cancel;
}
}
}

 

[Chris Dunaway]

How can this work ?

Because the QueryContinueDragEventArgs argument (e) is a reference
type. When a reference type is passed ByVal, then what is passed is a
*copy* of the reference. But changing the properties etc of the copy
of the reference changes the original object.

Please see the following article for a more in depth explanation:

http://www.yoda.arachsys.com/csharp/parameters.html


<snip>

 

[Chris Dunaway]

How can this work ?

Because the QueryContinueDragEventArgs argument (e) is a reference
type. When a reference type is passed ByVal, then what is passed is a
*copy* of the reference. But changing the properties etc of the copy
of the reference changes the original object.

Please see the following article for a more in depth explanation:

http://www.yoda.arachsys.com/csharp/parameters.html


<snip>