Why is CheckBox.IsChecked property Nullable<bool>?

Question

Why is the IsChecked property of a checkbox control in WPF of type bool? (or Nullable<bool>). I mean how can a checkbox control have the value of null?

Answer 1

Yes the null value exists and appears as a filled box. It indicates "Not Applicable" to the system.

Answer 2

According to the documentation, the IsChecked property has three different possible states:

 

So, when IsChecked is set to null, the check box will show an "indeterminate" state. This is commonly represented as a shaded, or greyed-out, control.

Answer 3

Checkboxes can have a 3rd, grayed, indeterminate state.

Answer 4

Don't forget to set the checkbox's property IsThreeState to true to enable this functionality.

Answer 5

Because WPF supports binding. If we bind a DB boolean column value to a checkbox. That column may have True/False/Null values. That means it has three values for a boolean field. So the WPF UI also should handle the three state.