Question

Let’s say in some abstract ViewModel base-class I have a plain-old property as follows:

public Size Size
{
    get { return _size; }
    set
    {
        _size = value;
        OnPropertyChanged("Size");
    }
}

I then create a more specific ViewModel, inheriting from the previous one, which contains the following property:

public Rect Rectangle
{
    get { return new Rect(0, 0, _size.Width, _size.Height); }
}

Now, in some View class I bind to the aforementioned ViewModel’s Rectangle property. Everything works fine, until I change the size. When Size changes, Rectangle doesn’t know about it and the change doesn’t propagate to the View. And since Rectangle is in the child class, I can’t simply add an OnPropertyChanged("Rectangle") to the Size setter.

Now imagine that I have many different properties like Rectangle, that all depend on base-class properties, and that none of these changes are being propagated. I need some lightweight and elegant way of chaining change notifications, preferably one that doesn’t require a lot of code and doesn’t force me into using dependency properties.

Obviously there are a lot of ugly solutions here- what I am looking for is something clean and clever. It seems to me this would be a very common scenario, and it seems to me there might be an MVVM-friendly way of doing this.

Answer 1

I recently blogged about this exact problem. I include a [DependsUpon("Size")] attribute with the Rectangle. I REALLY like this approach, because it keeps the dependency knowledge with the code that creates the dependency, not the other way around.

Take a look: http://houseofbilz.com/archive/2009/11/14/adventures-in-mvvm—-dependant-properties-with-inotifypropertychanged.aspx