title: When would ShowDialog() return null?
body: <p>WPF's <a href="http://msdn.microsoft.com/en-us/library/system.windows.window.showdialog.aspx" rel="noreferrer"><code>Window.ShowDialog</code></a> method returns a nullable boolean. So does <a href="http://msdn.microsoft.com/en-us/library/ms614338.aspx" rel="noreferrer"><code>CommonDialog.ShowDialog</code></a>.</p>
  
  <p>Now, I understand cases where these would return false (user clicked Cancel or pressed Esc), and when they would return true (code sets <code>Window.DialogResult</code> to true, probably in response to OK being clicked). But null?</p>
  
  <p>My first thought is that clicking the title bar's Close button might return null. But the docs state (and I confirmed by testing) that <a href="http://msdn.microsoft.com/en-us/library/system.windows.window.dialogresult.aspx" rel="noreferrer">the title-bar Close button is treated as a Cancel</a>.</p>
  
  <p>So when would <code>Window.ShowDialog</code> or <code>CommonDialog.ShowDialog</code> ever return null?</p>