WPF Border DesiredHeight

Question

The following Microsoft example code contains the following:

<Grid>
...  
  <Border Name="Content" ... >
...  
  </Border>
</Grid>
<ControlTemplate.Triggers>
  <Trigger Property="IsExpanded" Value="True">
     <Setter TargetName="ContentRow" Property="Height"
             Value="{Binding ElementName=Content,Path=DesiredHeight}" />
  </Trigger>
...
</ControlTemplate.Triggers>

When run, however, this code generates the following databinding error:

System.Windows.Data Error: 39 : BindingExpression path error: 'DesiredHeight' property not found on 'object' ''Border' (Name='Content')'. BindingExpression:Path=DesiredHeight; DataItem='Border' (Name='Content'); target element is 'RowDefinition' (HashCode=2034711); target property is 'Height' (type 'GridLength')

Despite this error, the code works correctly. I have looked through the documentation and DesiredHeight does not appear to be a member of Border. Can anyone explain where DesiredHeight is coming from? Also, is there any way to resolve/suppress this error so my program output is clean?

Answer 1

You can see that property in the code part of your application

Edit:

Border content = new Border();
int desiredHeight = content.DesiredSize.Height;
int desiredWidth = content.DesiredSize.Width;

To solve the problem try binding it to the Height attribute, since DesiredHeight doesn't seem to be available in the XAML markup of the Border control.

Answer 2

I came up upon the same issue in my application. In the end I changed the code so that I toggled the visibility of the content between Collapsed and Visible, and replaced the Grid with a StackPanel.

I've generally found the quality of the MS control template samples to be pretty good, but the error with this one was a bit frustrating.

Answer 3

DesiredHeight comes from content element and it is a valid binding. I think the reason your binding does not resolve is because DesiredHeight relies on Height property and you haven't set a fixed Height in your template so it evaluates to Double.Nan

Answer 4

I've ran into this. Along the lines of what user275587 said, their example works because the trigger removes the Heigth="0" on the RowDefination.

So I switch the height setting/trigger logic , so the RowDefination has no Height set

<Grid.RowDefinitions>
     <RowDefinition Height="Auto"/>
     <RowDefinition Name="ContentRow" />
</Grid.RowDefinitions>
...
<ControlTemplate.Triggers>
     <Trigger Property="IsExpanded" Value="False">
             <Setter TargetName="ContentRow" Property="Height" Value="0" />
     </Trigger>
</ControlTemplate.Triggers>