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

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>

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}}" />