Why is my code complaining about DependencyProperty.UnsetValue if the property has value?

Question

I'm trying to add a new trigger to a custom control in WPF programmatically by adding a new style. Control inherits from ToggleButton, and trigger should rely on IsCheckedProperty of the button. Even though I specifically assign value to IsChecked, InvalidOperationException is still thrown. Snippet:

protected BalanceSheetButton(OperationSummaryModel summaryModel, OperationFlow operationFlow, 
StackPanel mainPanel)
{
    this.IsChecked = true;
    RegisterStyle();
}

private void RegisterStyle()
{
    var a = ((DependencyObject)this).ReadLocalValue(IsCheckedProperty);
    var style = new Style();

    var trigger = new Trigger();
    trigger.Property = IsCheckedProperty;
    trigger.Setters.Add(new Setter { Property = ForegroundProperty, Value = this.Foreground });
    style.Triggers.Add(trigger);
    Style = style;
}

The exception: '{DependencyProperty.UnsetValue}' is not a valid value for property 'IsChecked'.

this.ReadLocalValue(IsCheckedProperty) returns assigned value, so why would it complain about the UnsetValue?

I tried assigning a value to IsChecked specifically to no avail. Both this.GetValue(IsCheckedProperty) and this.ReadLocalValue(IsCheckedProperty) return the assigned value.

Answer 1

You forgot to specify the value for the trigger. Just add

trigger.Value = true;

the UnsetValue in the message was the default value for trigger.Value.