Question

I have a UserControl that exists of only two TextBlocks:

<UserControl [...] x:Name="root">
    <StackPanel Orientation="Vertical">
        <TextBlock Text="{Binding Text1, ElementName=root}" /> 
        <TextBlock Text="{Binding Text2, ElementName=root}" />
    </StackPanel> 
</UserControl>

The corresponding code-behind looks like this:

public static readonly DependencyProperty Text1Property = DependencyProperty.Register("Text1", typeof(String), typeof(CmpText)); 
public static readonly DependencyProperty Text2Property = DependencyProperty.Register("Text2", typeof(String), typeof(CmpText)); 

public string Text1 
{ 
    get { return (string)GetValue(Text1Property); } 
    set { SetValue(Text1Property, value); } 
} 

public string Text2 
{ 
    get { return (string)GetValue(Text2Property); } 
    set { SetValue(Text2Property, value); } 
}

And this is how I use this UserControl in MainWindow.xml:

<local:CmpText Text1="{Binding Password1}" Text2="{Binding Password2}" />

What I basically want is the second TextBlock’s background to change its color to red if both Text1 and Text2 are unequal.

I tried to use a helper property in the code-behind:

public bool IsEqual { get { return Text1 == Text2; } }

And set the second TextBlock’s Style to

<Style TargetType="{x:Type TextBlock}"> 
     <Style.Triggers> 
         <DataTrigger Binding="{Binding IsEqual, ElementName=root}" Value="True"> 
             <Setter Property="Background" Value="Red"/> 
         </DataTrigger> 
     </Style.Triggers> 
 </Style>

However, IsEqual always turns out to be ‘true’ (and the TextBlock’s background is always red) even if the Text1 & Text2 properties don’t match. I think my helper property ‘IsEqual’ compares the default values of Text1 & Text2, which happen to be NULL (I don’t have any way to confirm that, since I cannot debug the GUI). So the evaluation of IsEqual seems to happen before my text properties get assigned any values. I want the evaluation to happen after the text properties get assigned.

I don’t know how to proceed. Can you help?

Answer 1

Currently there is no way that WPF could find out that IsEqual has changed, so the binding won’t be re-evaluated.

You could do three things:

1. Make IsEqual another dependency property and add PropertyChangedCallbacks to the Text1 and Text2 which, whenever they change, update IsEqual.

2. Or, implement INotifyPropertyChanged and raise the PropertyChanged event for IsEqual in the Text1 and Text2 PropertyChangedCallbacks.

3. Or, use a MultiBinding in conjunction with a IMultiValueConverter to bind the Background property directly to Text1 and Text2. The converter would get two strings as input and return a Brush.