 <p>As it turns out, there <em>is</em> a way to do this, but it's not pretty. It involves using the WinForms version of <code>MessageBox</code> and passing an undocumented option as the last property. </p> <pre><code>var result = System.Windows.Forms.MessageBox.Show("Are you sure you want to exit this app?", "Exit", System.Windows.Forms.MessageBoxButtons.YesNo, System.Windows.Forms.MessageBoxIcon.Question, System.Windows.Forms.MessageBoxDefaultButton.Button2, (System.Windows.Forms.MessageBoxOptions)8192 /*MB_TASKMODAL*/); </code></pre> <p>Source: <a href="http://social.msdn.microsoft.com/Forums/vstudio/en-US/8d1bd4a2-455e-4e3f-8c88-7ed49aeabc09/messagebox-is-not-applicationmodal?forum=wpf" rel="nofollow">http://social.msdn.microsoft.com/Forums/vstudio/en-US/8d1bd4a2-455e-4e3f-8c88-7ed49aeabc09/messagebox-is-not-applicationmodal?forum=wpf</a></p> <p>Hopefully this is helpful to somebody else in the future!</p>