Question

I have two classes defined like

public class PostleitzahlList : ObservableCollection<Postleitzahl> {
}

public class Postleitzahl : IPostleitzahl {
}

Now I have a Service-Class which contains

PostleitzahlList _postleitzahlList;

This Serviceclass has also to implement a Property of a Service-Interface which returns _postleitzahlList – but this Interface only knows IPostleitzahl – it doesn’t know PostleitzahlList or Postleitzahl. this Property should be used for Binding in WPF.

I am trying now to declare and implement this Property. I have tried

    public ObservableCollection<IPostleitzahl> PostleitzahlList {
        get { return this._postleitzahlList; }
    }

and

    public IList<IPostleitzahl> PostleitzahlList {
        get { return this._postleitzahlList; }
    }

But both does not work.
The fallowing seems to work:

    public IEnumerable<IPostleitzahl> PostleitzahlList {
        get { return this._postleitzahlList; }
    }

I ask me now
1. why does the first and second try not working?
2. what is the best solution to solve this?

Answer 1

The problem is with covariance. An ObservableCollection<Postleitzahl> isn’t an ObservableCollection<IPostleitzahl>, and ditto with lists. Here’s an example of why not:

ObservableCollection<string> strings = new ObservableCollection<string>();

// This is invalid, but it's what you're trying to do, effectively.
ObservableCollection<object> objects = strings;

// This would have to work... it's fine...
objects.Add(new object());

// And this should be fine too...
string x = strings[0];

… but as you can see, you’re now trying to fetch a non-string reference and store it in a string variable. The only viable outcome would be an execution-time failure… and half the point of generics is to push error detection to compile-time.

Now IEnumerable<T> is covariant in T because you can’t add any items via it – that makes it safe to apply that sort of conversion:

// There's nothing you can do to violate type safety here...
Observable<string> strings = new ObservableCollection<string>();
IEnumerable<object> objects = strings;

For more information, read up on covariance and contravariance in generics in MSDN.

Do you need the IPostleitzahl interface? If you just exposed the properties via ObservableCollection<Postleitzahl> or IList<Postleitzahl> it would be fine. Alternatively, you could change your variable to be an ObservableCollection<IPostleitzahl> and just happen to populate it by creating Postleitzahl instances.