Why is IsChecked property nullable boolean in WPF CheckBox?

wpf checkbox

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.

wpf checkbox

If that is the case then it's probably a three-state-checkbox, have a look at http://msdn.microsoft.com/en-us/library/system.windows.forms.checkbox.threestate.aspx for an explaination.

wpf checkbox

Null means the CheckBox is indeterminate (is neither blank nor checked, instead it either has a line through it, or the box has a gray box inside it)