模板型问题

Question

下面是一段愚蠢的代码片段，我想保留一份请求列表，以便将它们与响应相匹配。我从中删除了很多代码以简化它，它可能没有多大意义。

以下代码中的注释表示错误。模板类型我做错了什么？

    static void Main(string[] args)
    {
        Request request = new Request();
        Response response = SendReceive<Response>(request);
        Console.ReadLine();
    }

    static T SendReceive<T>(IMessage request) where T: IMessage
    {
        T results = default(T);

        OutstandingRequest<T> outstandingRequest = new OutstandingRequest<T>();
        _waitingForResponses.Add(outstandingRequest as OutstandingRequest<IMessage>);
        // Above line of code adds a null to the list because outstandingRequest (type OutstandingRequest<T>) 
        //is not of type OutstandingRequest(IMessage) even though T is restricted to be of type IMessage

        // Do stuff like send, and wait for a response, return results.

        return results;
    }
    static List<OutstandingRequest<IMessage>> _waitingForResponses = new List<OutstandingRequest<IMessage>>();

    public interface IMessage
    {
        void Qaax();
    }
    public class Request : IMessage
    {
        public void Qaax() { }
    }
    public class Response : IMessage
    {
        public void Qaax() { }
    }

    public class OutstandingRequest<T> where T : IMessage
    {
        public T Response { get; set; }
    }

}