Are generic UserControls and generic Dependency properties possible?

Question

I have an abstract generic class that defines a generic dependency property the type of which is to be defined by subclassing classes. This property is somehow not recognized as a dependency property so that I get an error message during runtime when binding to this property. Additionally during compilation time the constructor cannot call InitializeComponent. Why is that?

The generic abstract class MyClass:

abstract public class MyClass<T,U> : UserControl {
    protected MyClass() {
        InitializeComponent(); // Here is one error: Cannot be found
    }

    abstract protected U ListSource;

    private static void DPChanged
    (DependencyObject d, DependencyPropertyChangedEventArgs e) {
        var myClassObj = (MyClass) d;
        myClassObj.DataContext = myClassObj.ListSource;
    }

    // Causes a binding error at runtime => DP (of the concrete subclass)
    // is not recognized as a dependency property 
    public static readonly DependencyProperty DPProperty = 
        DependencyProperty.Register(
        "DP", 
        typeof(T), 
        typeof(MyClass), 
        new PropertyMetadata(null, DPChanged));

    public T DP {
        get { return (T) GetValue(DPProperty); }
        set { SetValue(DPProperty, value); }
    }
}

The corresponding XAML:

<UserControl x:Class="Path.of.Namespace.MyClass"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
    <ListView>
        <!-- Some stuff for the list view - used for all subclasses -->
    </ListView>
</UserControl>

A concrete subclass MySubClass:

public partial class MySubClass : MyClass<ClassWithAList, List<int>> {
    public MySubClass() {
        InitializeComponent(); // Another error: Cannot be found
    }

    protected List<int> ListSource {
        get { return new List<int>(); } // Just a dummy value

    }
}

The corresponding XAML:

<local:MySubClass xmlns:local="Path.of.Namespace.MySubClass" />

P.S. I am also not quite sure if the partial stuff is done correctly - R# suggests to remove these keywords.

Similar question here that might help: http://stackoverflow.com/questions/3629403/wpf-dependency-properties-set-in-xaml-when-base-class-is-generic — Colin Smith, Sep 01 '12 at 12:28
You inherit from ListView in code behind and have XAML that claims you inherit from UserControl, that cannot work... — H.B., Sep 01 '12 at 14:16
@H.B. Ah sorry, the sample code was erroneous. I just updated it. — Bastian, Sep 02 '12 at 13:40

score 1 · Accepted Answer · answered Sep 15 '12 at 00:02

I think there are several things wrong with your code, but first, let me explain partial in this context:

partial is used to declare one class in separate files. As visual studio is creating a MySubClass.g.cs with a partial class MySubClass (btw. this is where the method InitializeComponent is declared) from your MySubClass.xaml you'd have this class declaration twice and thus to get this to compile you will need the partial keyword.

Now to the rest: XAML and generics don't do well together, meaning: not at all. your 'corresponding XAML' is declaring a MyClass. But a MyClass without generic parameters doesn't exist. You could at this point change x:Class="Path.of.Namespace.MyClass" to x:Class="Path.of.Namespace.MySubClass", that should work.

but now the next thing is: how do you want to access that dependecy property in xaml? it is a static member in a generic type, so you have to specify it like this in code: MyClass<ClassWithAList, List<int>>.DPProperty. The problem is again: you can't do that in xaml

to solve your problem it might be a good idea to tell us what you want to accomplish

Thank you for the good explanation. Actually what I tried to achieve was also possible without using generics. I just wanted to avoid defining Dependency Properties anew in each subclass - now I just stick to the more verbose solution that works as well (by not using generics). — Bastian, Sep 19 '12 at 19:27

score 0 · Answer 2 · edited Sep 13 '12 at 18:07

0

I think the problem could be that you are writing typeof(MyClass), and should be typeof(MyClass<T,U>).

edited Sep 13 '12 at 18:07

McGarnagle

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answered Sep 13 '12 at 17:40

Raúl Otaño

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Are generic UserControls and generic Dependency properties possible?

2 Answers2