WPF binding for a converter not working

I have a custom converter that has a DefaultText property. All my converter does is return the DefaultText if the string is null or empty. I can't seem to get it to work though. Here's what I've got. This is the converter class.

public class DisplayValueConverter : DependencyObject, IValueConverter
{
    public static readonly DependencyProperty DefaultTextProperty = DependencyProperty.Register( "DefaultText",
                                                                                                 typeof ( string ),
                                                                                                 typeof ( DisplayValueConverter ) );

    public string DefaultText
    {
        get { return ( string ) GetValue( DefaultTextProperty ); }
        set { SetValue( DefaultTextProperty, value ); }
    }

    public object Convert( object value, Type targetType, object parameter, CultureInfo culture )
    {

        string empty = ( parameter != null ) ? parameter as string : DefaultText;
        return ( value != null && !string.IsNullOrEmpty( value.ToString().Trim() ) ) ? value.ToString() : empty;
    }

    public object ConvertBack( object value, Type targetType, object parameter, CultureInfo culture )
    {
        return null;
    }
}

And this is my xaml putting it into use.

        <TextBox Grid.Column="1"
                 Grid.Row="3"
                 VerticalAlignment="Center"
                 Margin="0,0,10,0" >
        开发者_JAVA百科    <TextBox.Text>
                <Binding Path="DataSource.Payee"
                         Mode="TwoWay"
                         NotifyOnSourceUpdated="True"
                         NotifyOnTargetUpdated="True"
                         NotifyOnValidationError="True"
                         ValidatesOnDataErrors="True"
                         UpdateSourceTrigger="PropertyChanged">
                    <Binding.Converter>
                        <k:DisplayValueConverter DefaultText="{Binding ElementName=This, Path=Test, Mode=TwoWay}" />
                    </Binding.Converter>
                    <Binding.ValidationRules>
                        <vr:RequiredField Label="Payee" />
                    </Binding.ValidationRules>
                </Binding>
            </TextBox.Text>
        </TextBox>

I've verified that the DataContext has an object, and that the Path works. So I'm not sure what I'm doing wrong.

I think the problem is with your use of Binding.ElementName. Because your value converter isn't actually part of the visual or logical tree, the binding engine has no way of knowing what tree it needs to traverse in order to find the element with the matching ElementName.

In this scenario, your best bet is going to be to specifically set the binding's Source property from code behind, or maybe creating a custom markup extension that would grab the right object for you.

just to add comment to the answer#1 - even if the converter would be in the visual or logical tree you should not assume that DefaultText will be set before the execution of Convert(...) in the converter.

In my case, I had forgotten that WPF requires that bindings to code-behind values must be explicitly declared as public properties with getters and setters.

So this was failing:

public partial class MyPage: Page
{
    public bool ShowLabel;  //No get/set !

    // constructor etc...
}

<Page x:Name="pageName">

    <Label Content="Hello" 
           Visibility="{Binding 
                           Path=ShowLabel, 
                           ElementName=pageName, 
                           Converter={StaticResource VisibilityConverter}}"/>

</Page>

This was not converting, at all. Breakpoints were not hitting. Nothing.

By adding a getter and setter to the properties I was using, the page recognised the values and the bindings/converter worked as required.

public partial class MyPage: Page
{
    public bool ShowLabel { get; set; }
}