How to reference a generic type in the DataType attribute of a DataTemplate?

Question

I have a ViewModel defined like this:

public class LocationTreeViewModel : 
    ObservableCollection, INotifyPropertyChanged
               


        

Answer 1

No, you cannot express a generics type in XAML. You will have to create a concrete type that extends your generic one ...

public class FooLocationTreeViewModel : LocationTreeViewModel<Foo>
{
}

Answer 2

I just implemented a workaround that is certainly less than perfect, and does require a bit of code in your ViewModel (which. because the VM shouldn't know about the view, breaks strict MVVM).

Define your generic type, and then define a class of that type with the lowest-common-ancestor as the type argument:

class GenericClass<T> { }

class Class1 : GenericClass<Apples> { }

class Class2 : GenericClass<Oranges> { }

class WorkaroundClass : GenericClass<Fruit> { }

In your viewmodel you'll need to declare your bound member as the ancestor type, and cast it.

// instead of:
// Apple DisplayFruit => GetGrannySmith();

Fruit DisplayFruit => (Fruit)GetGrannySmith();

In your xaml, you can then bind your data template to the ancestor class:

<DataTemplate DataType="{x:Type WorkaroundClass}"/>

I'm pretty sure that because the Generic parent is, well, generic, you shouldn't run into any actual cases where the differences between your type arguments cause any problems.