Ian Griffiths on UI Thread

The following message was one of Ian Griffiths's posts on discuss.develop.com.

Alois Reisinger wrote:
> Once again some ui thread question.
> Does all the WinForms have to be on the same thread? (i think no).

No - each top level form is allowed to be on its own thread. While people often use the term "The UI thread" as a convenient shorthand, it's slightly misleading, as a Windows Forms application can in fact have several UI threads. (Although in practice a lot of Windows Forms applications only use one thread as a UI thread. In these cases, it makes perfect sense to refer to "the UI thread.")

Note that it's only with top-level windows that you can do this. Everything inside a particular top-level window has to live on the same thread as that top-level window.

Which leads us onto your next question:
> Could there be a problem, if i have a newly created form that
> should be inserted as a child into a mdi parent if they (parent
> and child) were created on different threads?

Yes, that would be a problem.
Here's why. When you add a form as an MDI child of another form, you are effectively doing two things: you are adding the child to the parent's MdiChildren collection, and you are also setting the child's MdiParent property. Doing one of these implicitly does the other for you. If you write this:

child.MdiParent = parent;

then this has the implicit side effect of adding the child to the parent's MdiChildren. Or if you do this:

parent.MdiChildren.Add(child);

then that has the implicit side effect of setting the child's MdiParent property to refer to the parent.

If these two forms have different UI threads, then whichever thread you choose to set up the parent/child association on, it'll be the wrong thread for one or other of them.

And the same problem applies for normal non-MDI parenting too. (Both for Forms and for any other control. And yes, you can nest a form inside another form without resorting to MDI.)