Ouch.  Looks like a bug to me.

 

I recently used a custom control as the content of the header without
issues, so if you need to do more than basic UI in the header that could
be an option.

 

Carl.

 

Carl Scarlett

Senior .NET/WPF Developer, UX Designer | Genesis

IT & Change Management | Bankwest

A: Level 5, 199 Hay Street | Perth | Western Australia | 6004

P: (08) 9449 8451

M: 0408 913 870

E: [email protected]

 

Description: test

 

 

 

From: [email protected] [mailto:[email protected]]
On Behalf Of Steven Nagy <[email protected]>
Sent: Monday, 12 December 2011 9:21 AM
To: "[email protected]" <[email protected]>
Subject: Expander Header via style throws error

 

Hiyas,

 

Really simple to recreate this issue, just paste in this code to a new
WPF project:

 

    <Grid>        

        <Grid.RowDefinitions>

            <RowDefinition Height="*" />

            <RowDefinition Height="*" />

        </Grid.RowDefinitions>

        <Grid.Resources>

            <Style x:Key="ExpanderStyle" TargetType="Expander">

                <Setter Property="Header">

                    <Setter.Value>

                        <Grid>

                            <TextBlock Text="Foo" />

                        </Grid>

                    </Setter.Value>

                </Setter>

            </Style>

        </Grid.Resources>

 

        <Expander Style="{StaticResource ExpanderStyle}" />

        <Expander Style="{StaticResource ExpanderStyle}" Grid.Row="1" />

    </Grid>

 

Essentially, 2 expanders using the same style to set the header.

 

Throws this exception: "'Set property
'System.Windows.FrameworkElement.Style' threw an exception.' Line number
'23' and line position '10'"
Inner exception: "Specified element is already the logical child of
another element. Disconnect it first."

 

However if you sacrifice the complex header content for something simple
like this, it works fine:

<Setter Property="Header" Value="Foo" />

 

Any ideas why this is the case?

Using style inheritance from the same base style causes same issue.

However if I blanket copy paste the style with a new name and point the
expander to that, then it also works fine. So it's not the header
content that's the problem specifically.

I'm guessing some sort of virtualisation similar to what the tab control
does with tab pages under binding scenarios; it is using the same actual
Grid control and applying it to both headers, thus the inner exception
above. No idea how to fix it though (other than copying pasting styles
which I really don't want to do).

 

Cheers,

Steve

 


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