泛型方法中的泛型类型转换

我有以下类（其中一些在PRISM框架中，无法更改）：

public abstract class NetworkEventBase<T> : CompositePresentationEvent<T> where T : NetworkEventPayload { }
public class NetworkEventPayload { }
public class TestEvent : NetworkEventBase<TestPayload> { }
public class TestPayload : NetworkEventPayload { }
// the following classes are PRISM classes:
public class CompositePresentationEvent<TPayload> : EventBase { }
public abstract class EventBase { }

现在，我需要在IEventAggregator的decorator中将TestEvent的一个实例转换为它的基类NetworkEventBase。IEventAggregator看起来像：

public interface IEventAggregator
{
    TEventType GetEvent<TEventType>() where TEventType : EventBase, new();
}

现在，在我的装饰器中，我尝试这样转换：

public class MessageBusAdapterInjectorDecorator : IEventAggregator {
    ...
    public TEventType GetEvent<TEventType>() where TEventType : EventBase, new()
    {
        var aggregatedEvent = this.eventAggregator.GetEvent<TEventType>();
        var networkEvent = aggregatedEvent as NetworkEventBase<NetworkEventPayload>;
        if (networkEvent != null)
        {
            networkEvent.MessageBusAdapter = this.messageBusAdapter;
        }
        return aggregatedEvent;
    }
}

但是，即使aggregatedEvent的运行时类型为TestEvent，networkEvent也始终为null。