How to create WPF UserControl having a Canvas as the "main panel"? - xaml

Question

I want to create a UserControl having a Canvas inside it. Then I can use it like this in XAML:
<Window ...>
<Grid>
<local:MyCanvasLikeControl>
<Path .../>
<Path .../>
<Polygon.../>
</local:MyCanvasLikeControl>
</Grid>
</Window>
I have tried some things, but always get this error: "Property Content can only be set once". I know of ControlTemplates and such, but could not find my way by reading the docs by myself.
My goal is to have an equivalent to this:
<Window ...>
<Grid ...>
<Border ....>
<Canvas ...>
<Path ...>
<Path ...>
<Polygon ...>
</Canvas>
</Border>
</Grid>
</Window>
But moving the "Border / Canvas" to a UserControl named "MyCanvasLikeControl"

Answer 1

It's the same type of scenario as if you to stuff a bunch of elements of any other type into a control that would produce the same error like for example;
<UserControl>
<Grid/>
<StackPanel/>
<Canvas/>
<!-- you get the idea -->
</UserControl>
It's just telling you that you need to specify a parent capable of holding your content to act as the single content of its parent, which in this case would be your control. So to fix this, it's luckily simple;
<local:MyCanvasLikeControl>
<Grid>
<Path .../>
<Path .../>
<Polygon.../>
</Grid>
</local:MyCanvasLikeControl>
Or switch Grid with Canvas or whatever you want so long as it can host children elements. Hope this helps.
EDIT:
Ok, so I think the definition of requirement may have been a bit over-complicated in its explanation. If I'm understanding you correctly then here's why you're breaking.
You have your first layer of elements that contain your UserControl, but the way you're trying to insert content into it isn't the way that works, you'll need to provide that ability by specifying how you wish to allow it to be added to your control via a ContentPresenter or ContentControl etc. So instead it would be more like this inside your external UserControl;
<Border ....>
<Canvas ...>
<ContentPresenter/> or <ContentControl/> etc.
</Canvas>
</Border>
So then you can inject that stuff in there the way you want.
<Window ...>
<Grid>
<local:MyCanvasLikeControl>
<local:MyCanvasLikeControl.Content>
<Path .../>
<Path .../>
<Polygon.../>
</local:MyCanvasLikeControl.Content>
</local:MyCanvasLikeControl>
</Grid>
</Window>
Make sense?