Why is IsChecked property nullable boolean in WPF CheckBox?

Question

Is there a case when IsChecked property on WPF CheckBox null ?

I guess that will answer my question from title of question.

Answer 1

It is null when you don't know if its checked or not: http://msdn.microsoft.com/en-us/library/system.windows.controls.primitives.togglebutton.ischecked.aspx

In many systems a checkbox like this is shaded instead of being checked.