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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...

MelT
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    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

1 Answers1

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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.

Mike Strobel
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  • There should be a good technique to effectively test UI controls. Mixing the View and the ViewModel in one is saying "no" to testability. I know from my experience that part of the logic of UI controls can be extracted to VMs so they can be tested without creating any instance of the actual control. Unfortunately, I haven't found a good strategy for this. – SuperJMN Jul 27 '22 at 22:29