MVVM UserControl and binding when using that control

Question

I made a UserControl with the MVVM pattern, where the UserControl's "intelligence" is in its viewModel.

I want to user that UserControl in different views (xaml) so the developer of that view doesn't have to mind about how it is done.

I added some dependencyProperties in my UserControl so the end-programmer could give the control some context informations.

But I have some binding issues.

In the client.xaml:

<Grid>
<MyUserControl MyDependencyProperty0={Binding ClientViewModelProperty0}/>
</Grid>

and in myusercontrol.xaml

<Grid>
<TextBlock Text={Binding TextToDisplay}/>
</Grid>

where TextToDisplay is a property of MyUserControlViewModel. I only need the ClientViewModelProperty0 to be set once, I do not need the clientViewModel to be set as the DataContext of MyUserControl since it has its own dataContext(its view-model)

I assume the solution would be a different Binding Expression syntax (relative source? self?) but I cannot see which one...

Is it yet again time for me to copypaste my "creating a view model for a user control is a code smell" answer? The reason why you're having the issue is because of this. Think for a second: Does a TextBox control have a TextBoxViewModel, or does it have properties and events that your view model binds to? Then why does your UserControl not behave the same? [One of my "don't do it!" answers](http://stackoverflow.com/questions/25795871/wpf-databinding-with-mvvm-and-user-controls/25796096#25796096) — , Oct 02 '14 at 14:54

score 2 · Answer 1 · answered Oct 02 '14 at 15:04

Reusable controls tend to follow a somewhat different design than full-blown application views. Specifically, they don't follow MVVM in quite the same way.

Remember that in WPF, controls are "lookless": their visual appearance is governed by templates. The underlying class is the "model" for the control. Like @Will mentioned in his comment, a TextBox does not have a TextBoxViewModel; the TextBox instance is the "view model". The "view" is the template that gets applied. While a UserControl is a bit different from a templated control (its content is self-contained, so it's effectively both the "view" and the "view model"), the same basic rules apply:

When you create your own reusable controls, put your properties and behavior in the control class itself. That way, when you plug it in to a view, you can set the parameters however you like, e.g., by binding them against the parent view model. A reusable control should never rely on some external/ambient view model being present.

MVVM UserControl and binding when using that control

1 Answers1