Toolstip controls can't get focus

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In my Winform I have one ToolStripButton on the toolstrip and one textbox in
the content panel. The textbox has focus in the winfrom. When I click on one
toolstrip button, i want to do some validation on the Leave event of the
textbox. The problem is that event I click the toolstrip button, the focus
won't change and the Leave event of the textbox won't get fired.

How do i make the Leave event happen?

Thanks.
Zongwen Feng