c# - Does MessageBox.Show() automatically marshall to the UI Thread?

Question

I launch a thread via ThreadPool.QueueUserWorkItem which has a messagebox dialogue in it:

System.Windows.Forms.DialogResult dr = System.Windows.Forms.MessageBox.Show("would you like to download upgrade in background? ..", "Upgrade Available", MessageBoxButtons.YesNo);

It seems to work fine however I am a little suspicious after some customers suggested they were not getting the message popping up. I had the feeling in .NET framework 2.0+ you do not need to marshal this particular call, it does it for you. Correct?

This is a semi-related topic for interest: Why use a owner window in MessageBox.Show?

Answer 1

No, it doesn't Marshal to the UI thread. If you think about it, it wouldn't be possible for it to do so.

It's possible to have more than one UI thread in an application. Some programs, such as internet explorer, have many UI threads. Which UI thread would the .Show call pick?

It's also possible to use MessageBox.Show in an application that has no UI threads. You can very well call this in a Console application and it will work.

MessageBox.Show will show UI on the thread it is called from. If there isn't already a message pump running on the thread it will setup a temporary one in order to function. It will tear it down after the Show call completes.