why don't people wrap DependencyProperties in a generic class?

Question

I didn't like how verbose dp's are, since most of the code is just repeated, I just wrapped it in a generic class.

Having seen quite allot of sample code, I was wondering why more people aren't doing the same.

I haven't come across any problems to speak of in my demo application, and it makes the ViewModels easier to manage.

Sample:

class GenericDependancyProperty<T> : DependencyObject
{
    // Value dependency property
    public static readonly DependencyProperty ValueProperty =
        DependencyProperty.Register( "Value", typeof( T ), typeof( GenericDependancyProperty ),
            new FrameworkPropertyMetadata( (T)default(T),
                new PropertyChangedCallback( OnValueChanged ) ) );

    // getter/setter for the Value dependancy property
    public T Value
    {
        get { return (T)GetValue( ValueProperty ); }
        set { SetValue( ValueProperty, value ); }
    }

    // Handles changes to the Value property.       
    private static void OnValueChanged( DependencyObject d, DependencyPropertyChangedEventArgs e )
    {
        GenericDependancyProperty<T> target = (GenericDependancyProperty<T>)d;
        T oldValue = (T)e.OldValue;
        T newValue = target.Value;
        target.OnValueChanged( oldValue, newValue );
    }

    // Provides derived classes an opportunity to handle changes to the Value property. 
    protected virtual void OnValueChanged( T oldValue, T newValue )
    {
        if ( ValueChanged != null )
        {
            ValueChanged( newValue );
        }
    }

    // Value changed event
    public event Action<T> ValueChanged;
}

Is this a bad idea?

Answer 1

It is not a bad idea, and well worth a try, but it will not work!

You have essentially defined a single dependency property named "Value". This will be OK if you only ever access it via your CLR wrapper (i.e. the get / set code for your Value property). However, much of the framework affects the dependency property directly. For example, style setters, animations will not be able to use your dependency property.

I too share your pain with the DP boilerplate code, which is why I came up with a declarative solution:

[DependencyPropertyDecl("Maximum", typeof(double), 0.0)]
[DependencyPropertyDecl("Minimum", typeof(double), 0.0)]
public partial class RangeControl : UserControl
{
    ...
}

The actual dependency properties are generated by a T4 template within Visual Studio.

https://blog.scottlogic.com/2009/08/18/declarative-dependency-property-definition-with-t4-dte.html