Cast type as specific MyCollection

I have a custom collection type ObservableStateCollection that, for simplistic purposes, looks like:

public class ObservableStateCollection<T> : IList<T>, INotifyCollectionChanged, INotifyPropertyChanged where T : StateObservable
{
    private List<T> _items;
    private List<T> _deleted;

    public IEnumerator<T> GetEnumerator()
    {
        return _items.GetEnumerator();
    }

    public IEnumerable<StateObservable> GetAll()
    {
        return _items.Concat(_deleted);
    }

    //...
} 

Note that type T must be derived from StateObservable.

Now, I'm neck deep in reflection. I'll spare the details of 'why', and just show you where I'm currently at. I need to check if a particular property on my model is an ObservableStateCollection<T> and use a foreach to loop through the GetAll() method.

Currently, I'm at:

 if(prop.PropertyType.GetGenericTypeDefinition() == typeof(ObservableStateCollection<>))
 {
       var collection = (ObservableStateCollection<StateObservable>)prop.GetValue(model, null);
       foreach (var e in collection.GetAll())
       {
            //act on ObservableStateCollection<StateObservable>                      
       }
 }

which throws an exception on the line var collection = ..., as I can't cast ObservableStateCollection<DerivedType> to ObservableStateCollection<BaseType>

What are my options here? How to I get a strongly typed object back that I can call GetAll on?