wpf – Microsoft.Win32.OpenFileDialog.ShowDialog()何时返回null？

OpenFileDialog的 ShowDialog方法返回一个可以为空的布尔值,如果用户单击确定则设置为true,如果单击取消则设置为false.什么时候返回null？文档没有说.

这在下面链接的问题中说明,但我在这里提到 Programming WPF (Chris Sells,Ian Griffiths)说：

ShowDialog will always return true or
false. … Only after a dialog has
been shown but before it’s been closed
is DialogResult null.

类似的问题：When would ShowDialog() return null?

并且：Why is DialogResult a nullable bool in WPF?