为什么在WPF CheckBox中IsChecked属性为可空布尔值？

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.

Answer 2

如果真是这样，那么可能是三个状态的复选框，请参阅http://msdn.microsoft.com/zh-cn/library/system.windows.forms.checkbox.threestate.aspx进行解释。

Answer 3

Null表示CheckBox是不确定的（既不为空白，也不为选中状态，而是通过一行，或者该框内部有一个灰色框）

Answer 4

If we want to bind a value from DB and if that value is null then we have to handle that also. A boolean DB value can be True, False or NULL. So the UI checkbox also must be handle the there states. That's why WPF CheckBox has three state.