data binding - Change Background Color for WPF textbox in changed-state

Question

I have a class EmployeeViewModel with 2 properties "FirstName" and "LastName". The class also has a dictionary with the changes of the properties. (The class implements INotifyPropertyChanged and IDataErrorInfo, everything is fine.

In my view there is a textbox:

<TextBox x:Name="firstNameTextBox" Text="{Binding Path=FirstName}" />

How can I change the background color of the textbox, if the original value changed? I thought about creating a trigger which sets the background color but to what should I bind? I don't want to created an additional property for every control which holds the state wheter the one was changed or not.

Thx

Answer 1

Just use a MultiBinding with the same property twice but have Mode=OneTime on one of the bindings. Like this:

Public Class MVCBackground
    Implements IMultiValueConverter

    Public Function Convert(ByVal values() As Object, ByVal targetType As System.Type, ByVal parameter As Object, ByVal culture As System.Globalization.CultureInfo) As Object Implements System.Windows.Data.IMultiValueConverter.Convert
        Static unchanged As Brush = Brushes.Blue
        Static changed As Brush = Brushes.Red

        If values.Count = 2 Then
            If values(0).Equals(values(1)) Then
                Return unchanged
            Else
                Return changed
            End If
        Else
            Return unchanged
        End If
    End Function

    Public Function ConvertBack(ByVal value As Object, ByVal targetTypes() As System.Type, ByVal parameter As Object, ByVal culture As System.Globalization.CultureInfo) As Object() Implements System.Windows.Data.IMultiValueConverter.ConvertBack
        Throw New NotImplementedException()
    End Function
End Class

And in the xaml:

<TextBox Text="{Binding TestText}">
    <TextBox.Background>
        <MultiBinding Converter="{StaticResource BackgroundConverter}">
            <Binding Path="TestText"    />
            <Binding Path="TestText" Mode="OneTime" />
        </MultiBinding>
    </TextBox.Background>
</TextBox>

No extra properties or logic required and you could probably wrap it all into your own markup extension. Hope that helps.