Display a default DataTemplate in a ContentControl when its content is null or empty?
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07-07-2019 - |
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?
Solution
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>
OTHER TIPS
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>
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}}" />