Make checkbox bound to nullable bool transition from null to true

Question

I have a checkbox with its IsChecked property bound to a nullable bool. When my control first loads the value is null and the checkbox appears greyed out. This is what I want.

When the user clicks the checkbox, it moves to the false/Unchecked state.

However, 99% of the time the user is going to want to tick the checkbox - which currently means double clicking the checkbox.

How can I make the value move from null to true when the user first clicks the checkbox?

Answer 1

I had the same problem and ran into this question. This is a late response but I think this is the best solution :)

With the help of IsThreeState and TargetNullValue you can accomplish this

 <CheckBox  IsThreeState="False" IsChecked="{Binding YOUR_NULLABLE_PROPERTY, TargetNullValue=false}" />

The only caveat is that it will toggle between null and true. There will never be a false value.

Answer 2

You can just modify the setter of the bound property to check whether the previous value is null and if it is, set the value to true. Something like this:

public bool? MyBoolProperty 
{
   get { return _myBoolProperty; }
   set 
   {
       _myBoolProperty = (_myBoolProperty != null || value == null) ? value : true;
       RaisePropertyChanged("MyBoolProperty");       
   }
}

The binding system will re-read the property after it sets it, so the new value will be reflected by the CheckBox.