Make checkbox bound to nullable bool transition from null to true - wpf

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.

Answer 3

You can handle the Click event and implement a logic like this:
private void CheckBox_Click(object sender, RoutedEventArgs e)
{
CheckBox cb = sender as CheckBox;
switch (cb.IsChecked)
{
case null:
cb.IsChecked = false;
break;
case true:
cb.IsChecked = true;
break;
case false:
if (cb.IsThreeState) {
cb.IsChecked = null;
} else {
cb.IsChecked = true;
}
break;
}
e.Handled = true;
}

Answer 4

I have met this problem recently. I look all answers of this question, but none seem fit me. The accepted answer is fail when I set IsChecked equals false in ViewModel. I decompile the checkbox in wpf.
As you can see, when you click a null IsChecked checkbox, IsChecked property will switch to false.
protected internal virtual void OnToggle()
{
bool? flag;
if (this.IsChecked == true)
{
flag = (this.IsThreeState ? null : new bool?(false));
}
else
{
flag = new bool?(this.IsChecked.HasValue);
}
base.SetCurrentValueInternal(ToggleButton.IsCheckedProperty, flag);
}
So you can Create a new class inherit CheckBox and override OnToggle method like this:
protected override void OnToggle()
{
bool? flag;
if (this.IsChecked == true)
{
flag = this.IsThreeState ? null : new bool?(false);
}
else
flag = true;
// what a pity this method is internal
// actually you can call this method by reflection
base.SetCurrentValueInternal(ToggleButton.IsCheckedProperty, flag);
}
The other way is:
protected override void OnToggle()
{
if (this.IsChecked == null)
this.IsChecked = false;
base.OnToggle();
}

Answer 5

The easiest way would be to simply handle the click event and set the control to true if its current state is null, optionally setting a flag for your internal code tracking after first click.