Why use a owner window in MessageBox.Show?

Question

MessageBox.Show has forms like MessageBox.Show( ownerWindow, .... ).

What do I gain by assigning a owner window?

Answer 1

if i'm not wrong this prevents to owner window to Focus() until the messagebox is closed.

Answer 2

Setting the owner causes the owner to be disabled while the message box is open.

If you don't set the owner then the user can click something else or even close the owner while the message box is open, then when the message box closes and the code after the call to MessageBox.Show runs your program can be in an unknown state - or if the owner was closed you are now running code inside a window that no longer exists and any calls to methods of WinForms or WPF (or for that matter also WinAPI and any other framework) are likely to cause a crash.

Answer 3

A message box is a modal form, which means that its parent window is disabled until the message box is dismissed.

If a Show() overload is called that does not take an owner identifier, then a parent form is usually chosen automatically. I can't find anything in the documentation that describes how that form is chosen, but my experience is that if the message box is displayed inside the GUI thread (i.e., the main thread, or message pump thread), then the active window for that thread is chosen as the parent.

Other threads may create message boxes with no parent form. This could be a bad situation, because it could sit behind the active window, and the user will not even know it is there until they close the program. To avoid this, you could pass the handle of the application's main window to the Show method, which will disable that window for the duration of the message box.

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ADDED: I've been thinking about this and now I'm not so sure. The snippet from reflector that you gave below makes me think that maybe the thread doesn't matter. (I did say that I couldn't find anything in the documentation!)

I'd have to go back and look at the code to make sure, but I think the message boxes that I used to lose behind the main form may have actually been custom message box forms, in which case my experience is faulty, and you never ever have to supply a parent form parameter.

Sorry for the confusion.

My recommendation now is to never supply this parameter unless you need something other than the active window to be the main form.

Answer 4

Using Net Reflector, I just found this code in Messagebox.Show:

else if (owner == IntPtr.Zero)
    owner = UnsafeNativeMethods.GetActiveWindow();

So if you do not nested ownership ( window--(owns)-->window--(owns)->messageBox), leaving out the ownerWindow sets the owner you would normally choose.

Answer 5

It turns out that window ownership is not transitive. In the case where you have form, spawning a form, spawning a MessageBox, The MessageBox.Show needs to use the ownerWindow parameter. The original form should be set as the owner window of the MessageBox.

Answer 6

The documentation seems to imply that the only purpose of the owner parameter is if you specify MB_HELP, the message box knows which window it should send the WM_HELP message to.

http://msdn.microsoft.com/en-us/library/ms645505%28VS.85%29.aspx

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Oh, just realised the OP was about .net - I gave an answer about winapi - sorry!