Is it possible to define a list of any type that implements multiple interfaces?

Question

Consider the following class hierarchy:

public interface X { void Foo(); }

public interface Y { void Bar(); }

public class A : X, Y
{
    public void Foo() {}
    public void Bar() {}
}

public class B : X, Y
{
    public void Foo() {}
    public void Bar() {}
}

Is there any way I can define a list (or any generic type for that matter) which can contain both A's and B's, while allowing me to treat the contents of said list as both X and Y? I.e. something that will allow me to write something along the lines of this:

var list = ???
list.Add(new A());
list.Add(new B());
list.First().Foo();
list.Last().Bar();

Edit

Just to clarify, the types I'm currently dealing with are ObservableCollection<T> (A) and ReadOnlyObservableCollection<T> (B), where the interfaces I'm interested in are IList<T> (X) and INotifyCollectionChanged (Y). Clearly I'm unable to change their class hierarchy to accomodate my needs, so I need a different workaround.

Answer 1

No, unless you declare another interface:

IAB : IA, IB {}

and make both classes implement it.

You could also implement your own collection class, something like List<IFirst, ISecond>, which would allow that.

Answer 2

It is likely you don't want to modify the class A and B So a wrapper will do the job

public class XYWrapper : X, Y
{

  private dynamic InternalObject { get; set; }

  public void Foo() { InternalObject.Foo(); }
  public void Bar() { InternalObject.Bar(); }

  public static implicit operator XYWrapper(A value)
  {
    var instance = new XYWrapper();
    instance.InternalObject = value;
    return instance;
  }

  public static implicit operator XYWrapper(B value)
  {
    var instance = new XYWrapper();
    instance.InternalObject = value;
    return instance;
  }
}

So you use it in this way:

var list = new List<XYWrapper>();
list.Add(new A());
list.Add(new B());
list.First().Foo();
list.Last().Bar();