msgbox that disappears automatically after certain time

Not really unless you want to do a dirty trick like simulating keyboard input (no, this is not my advice; sorry if I gave you this idea — it would be pretty bad :-)).

You could create a custom form or window (you did not tag you UI; please see my comment to the question) instead MessageBox. In this case, you could run it as a modal form or window (how inconvenient! please, always tag your UI library!) use a timer to call Hide or Close it.

I could warn you: this would violate expected UI design style and possible irritate the user too much. Is something has OK and/or Cancel button, the user expect to be the one who decides when to close it. Consider using something like ToolTip instead, or some control which behaves like a ToolTip.

—SA

You can create your own messagebox form for that. Overload a constructor which takes message text and timeout value. Write some logic on Form Load which will display a message up to timeout value and then after close automatically.

By itself, no - because the messagebox runs on the primary UI thread. It's a dialog so it is, by the very nature of it, a blocking operation. You are going to have to write a custom dialog that can close itself automatically after a period of time. When the dialog gets displayed, kick off a timer that's set to the duration you want to close the dialog after and, in the timer event, close the dialog.