WPF User Control hell with MVVM and Dependency Properties

Question

This is what I'm trying to do:

• I'm writing a UserControl that I want to be consumed by other developers.

• I want end users to be able to use my control using Dependency Properties.

  <lib:ControlView ControlsText={Binding Path=UsersOwnViewModelText} />

• I'm using the MVVM pattern.

• I'm binding my ViewModels to their View's using <DataTemplates>.

  <DataTemplate DataType="{x:Type local:ControlViewModel}">
<local:ControlView />
</DataTemplate>

So I have two questions:

1. Am I right in thinking that if a UserControl is being consumed in XAML then the UserControl must set the ViewModel as its DataContext when the control's Loaded event fires instead of using the <DataTemplate> method?

2. How do I allow users to data bind to my control's dependency properties while still being data bound to my ViewModel?

Answer 1

You should separate the two use cases:

1. The (user) control that will be consumed by other developers.
2. The user control that will be consumed by your application.

Importantly, the latter depends on the former - not vice versa.

Use case 1 would use dependency properties, template bindings, all the things that go into making a regular WPF control:

MyControl.cs:

public class MyControl : Control
{
    // dependency properties and other logic
}

Generic.xaml:

<ControlTemplate Type="local:MyControl">
    <!-- define the default look in here, using template bindings to bind to your d-props -->
</ControlTemplate>

You would then define use case 2 as:

MyViewModel.cs:

public class MyViewModel : ViewModel
{
    // properties and business logic
}

MyView.xaml:

<UserControl ...>
    <local:MyControl SomeProperty="{Binding SomePropertyOnViewModel}" .../>
</UserControl>

Best of both worlds with a clean separation. Other developers depend only on the control, which could (and probably should) be in a completely different assembly than your view model and view.

HTH,
Kent

Answer 2

First off, I don't think MVVM is a good choice if you are developing a UserControl that will be consumed by others. A lookless control is what you really should be developing. Jeremiah Morrill has a blog post about this subject.

With that said, you can set the datacontext with XAML if you have a default public constructor.

Inside ControlView.xaml put:

<UserControl.DataContext>
    <local:ControlViewModel />
</UserControl.DataContext>