Display a default DataTemplate in a ContentControl when its content is null or empty?

Question

I would think this is possible, but the obvious way isn't working.

Currently, I'm doing this:

<ContentControl
    Content="{Binding HurfView.EditedPart}">
    <ContentControl.Resources>
        <Style
            TargetType="ContentControl"
            x:Key="emptytemplate">
            <Style.Triggers>
                <DataTrigger
                    Binding="{Binding RelativeSource={x:Static RelativeSource.Self}, Path=Content}"
                    Value="{x:Null}">
                    <Setter
                        Property="ContentControl.Template">
                        <Setter.Value>
                            <ControlTemplate>
                                <Grid
                                    HorizontalAlignment="Stretch"
                                    VerticalAlignment="Stretch">
                                    <TextBlock>EMPTY!</TextBlock>
                                </Grid>
                            </ControlTemplate>
                        </Setter.Value>
                    </Setter>
                </DataTrigger>
            </Style.Triggers>
        </Style>
    </ContentControl.Resources>
</ContentControl>

I'm not getting any binding errors and this compiles. However, it doesn't produce the expected result. I've also tried the obvious:

<DataTemplate DataType="{x:Null}"><TextBlock>Hurf</TextBlock></DataTemplate>

This won't compile. And attempting to set the content twice fails as well:

<ContentControl
    Content="{Binding HurfView.EditedPart}">
        <TextBlock>DEFAULT DISPLAY</TextBlock>
</ContentControl>

Can I do this without writing a custom template selector?

Thessalonians

Answer 1

Simple, you have to bind the content property in the style. Styles won't overwrite a value on a control if there's a binding present, even if the value evaluates to Null. Try this.

<ContentControl>
    <ContentControl.Style>
        <Style TargetType="ContentControl">
            <Setter Property="Content" Value="{Binding HurfView.EditedPart}" />
            <Style.Triggers>
                <DataTrigger Binding="{Binding RelativeSource={x:Static RelativeSource.Self}, Path=Content}" Value="{x:Null}">
                    <Setter Property="ContentControl.Template">
                        <Setter.Value>
                            <ControlTemplate>
                                <Grid HorizontalAlignment="Stretch" VerticalAlignment="Stretch">
                                    <TextBlock>EMPTY!</TextBlock>
                                </Grid>
                            </ControlTemplate>
                        </Setter.Value>
                    </Setter>
                </DataTrigger>
            </Style.Triggers>
        </Style>
    </ContentControl.Style>
</ContentControl>

Decosta

No problem it's a very common mistake people make when trying to do more complex things with styles and triggers. – Decosta

Every time I come back for this I'm going to leave a comment. Thanks. – Thessalonians

Y Halo thar. Best solution to a failed Converter as well. Just return null from the conversion process and trap for it this way. Can't do it using Validation, unfortunately... – Thessalonians

It's the gift that keeps on giving, and the beauty of Stackoverflow. – Decosta

Well, at least 100 rep this time. – Thessalonians

Its so much easier to come back here than to look through my codebase to find another example. Maybe I should just create a custom ContentControl... – Thessalonians

Hmm this won't work for me, I'm using the TransitionControl from here, and it crashes because Template.FindName("PART_ContentHost", this); returns null... – Sepulveda

@Sepulveda Name the Grid (or other top-level panel) PART_ContentHost – Decosta

Answer 2

Since I stumbled upon this question and had the same problem today, I wanted to contribute another way how I solved the problem. Since I did not like to add another style trigger I used the property TargetNullValue which seems to be a bit more readable than the accepted solution (which works nevertheless):

    <ContentControl>
      <ContentControl.Content>
        <Binding Path="ContentViewModel">
          <Binding.TargetNullValue>
            <Grid HorizontalAlignment="Stretch" VerticalAlignment="Stretch">
              <TextBlock>EMPTY!</TextBlock>
            </Grid>
          </Binding.TargetNullValue>
        </Binding>
      </ContentControl.Content>
    </ContentControl>

Hierology

this is an elegant solution with the TargetNullValue. Nice find – Pedo

Works, but in some cases like mine it produces Binding-Errors. Although I also think this one is more readable, check for new Binding-Errors when using it. – Particular

Also have a look here: [#22316199 to learn about the FallbackValue attribute in case TargetNullValue alone does not do the trick – Nildanile

Answer 3

You could return DBNull.Value as the FallbackValue of the Binding for the Content of the ContentControl, and create a DataTemplate for DBNull :

<DataTemplate DataType="{x:Type system:DBNull}">
    <!-- The default template -->
</DataTemplate>

...

<ContentControl Content="{Binding HurfView.EditedPart, FallbackValue={x:Static system:DBNull.Value}}" />

Escapee

That's kinda weird... It was my understanding the preferred method was to return DependencyProperty.UnsetValue. I've done this in some type converters and it worked well... – Thessalonians

@Will, yes, that would probably be better. I wrote that answer more than one year ago, and my understanding of WPF at the time was still very incomplete... – Escapee

@Will can you add an answer using DependencyProperty.UnsetValue – Zhao

@Zhao um, no? Because I have no idea why you would need one? Is the selected answer not correct? – Thessalonians

I used TargetNullValue={x:Static system:DBNull.Value} and <DataTemplate DataType="{x:Type system:DBNull}">, and works great – Imprecision