FrameworkElement`s DataContext Property does NOT inherit down the element tree

Question

Hello WPF Pros at least I hope some of you read this!

DataContext is a property on FrameworkElement (base class for all WPF Controls) and is implemented as a DependencyProperty. That means all the descendant elements in the logical tree share the same DataContext.

So the ContentControl should do it with its descendant elements right?

I have a scenario where that is NOT the case and I would like to know WHAT is the cause of that misbehaviour ?!

That you understand a bit more about it please read this thread ( dont NOT want to copy everything here) where the trouble starts...:

WPF: Can not find the Trigger target 'cc'. The target must appear before any Setters, Triggers

and to say it in short words: My DataTemplates within the ContentControl do have a dead DataContext that means there is NOTHING to bind to it, what is actually not possible...

Every Element down the ContentControl has NOTHING set in the DataContext Property ???

Solution 1

DataContext is a property on FrameworkElement (base class for all WPF Controls) and is implemented as a DependencyProperty. That means all the descendant elements in the logical tree share the same DataContext.

The fact that it's a dependency property doesn't imply inheritance... It's true for DataContext, but only because the dependency property has the FrameworkPropertyMetadataOptions.Inherits flag in its metadata.

So the ContentControl should do it with its descendant elements right?

ContentControl is a bit special: the DataContext of its descendants (the visual tree built from the DataTemplate) is actually be the Content of the ContentControl. So if your ContentControl has no content, the DataContext inside it is null.

Solution 2

This worked for me:

<ContentControl ContentTemplate="{StaticResource NotesTemplate}"
                Content="{Binding}"
                DataContext="{Binding HeightField}"/>

Without the Content="{Binding}", the DataContext was NULL

Comments

[Elisabeth]

My ContentControl HAS a DataContext like PersonListViewModel, but the problem seems to be one Node down the ContentPresenter and its Content Property which is not set and so the DataContext is NOT either. Ok I understand your answer although it still sounds a bit odd to me. So the solution that Meleak offered me was actually no workaround rather the only thing he could/must do. Any other solution/opinion on this matter ?

[Thomas Levesque]

Your ContentControl has a DataContext, but does it have a Content ?

[Elisabeth]

There is no "Content" Property in the ContentControl using Snoop. In XAML there is... ?? One Node down there is the ContentPresenter which has a Content Property and that is empty.

[Thomas Levesque]

By default Snoop doesn't show the properties that are set to their default value, you can display them with the "D" button. Anyway, that probably means the Content is null, which explains why you have no DataContext in the descendants

[Elisabeth]

Yes the "D" up right in the corner of snoop... Your were right the Content is empty so no DataContext. The question just is why is the Content Property empty ?

[Thomas Levesque]

Well, perhaps because you didn't set it... I can only make assumptions since you didn't post the code

[Elisabeth]

ah sorry I thought you know that link from the other thread with Meleak: sendspace.com/file/a427u1 is it ok for you to download this VS2010 project? posting all code here will be a lot prolly?

[Admin]

Regarding this accepted answer, why is this the case? It seems like a nasty gotcha for which I can see no reason, but I'm sure there is a reason.

[Thomas Levesque]

@ardave, I don't understand your question. Why is what the case?

[Admin]

Why is it the case that ContentControls need their Content to be set for their DataContexts to have a value, whereas so many other WPF entities support direct DataContext inheritance?

[Thomas Levesque]

@ardave, my guess is that if it were the case, it would cause nesting problems with implicit data templates. For instance, assume that you have a DataTemplate for type X (declared in resources with type X as the key, so it's picked up automatically in ContentControls), and that DataTemplate contains a ContentControl. If you didn't set the Content on this ContentControl, and the DataContext is inherited as the content, then this ContentControl would use the same DataTemplate to display its content, causing infinite nesting...

[Thomas Levesque]

Anyway, that's probably a conscious design decision, that might have nothing to do with the explanation above... you'd have to ask Microsoft for the real reasons.

[Peter Taylor]

This can be simplified slightly to <ContentControl ContentTemplate="{StaticResource NotesTemplate}" Content="{Binding HeightField}"/>