public Dictionary<Type, Buisness.MessageObjects.AbstractMessageBroker<AbstractMessage>> BrockerList

 BrockerList.Add(typeof(Buisness.MessageObjects.TextMessage.TextMessage),
                new Buisness.MessageObjects.TextMessage.TextMessageBroker());

  public class TextMessageBroker  : AbstractMessageBroker<TextMessage>

           BrockerList = new Dictionary<Type, Team.Business.MessageObjects.AbstractMessageBroker<AbstractMessage>>();
            Team.Business.MessageObjects.AbstractMessageBroker<AbstractMessage> newTextMessageBroker = 
                (Team.Business.MessageObjects.AbstractMessageBroker<AbstractMessage>) 
                new Business.MessageObjects.TextMessage.TextMessageBroker();

  public abstract class AbstractType {}
  public class ImplementingType : AbstractType{}
  public class MyClass<T>{}

  public class Test
  {
    Test()
    {
      MyClass<ImplementingType> obj = new MyClass<ImplementingType>();
// der cast in der folgenden Zeile produziert bereits einen Fehler
      MyClass<AbstractType> abstractObj = (MyClass<AbstractType>)obj;
    }
  }

Dictionary<string,FileStream> rawDictionary = new Dictionary<string,FileStream>();
Dictionary<string,Stream> pseudoCastedDict = rawDictionary.ToDictionary(kvp => kvp.Key,kvp => (Stream)kvp.Value);

 
   public class MessageHandling
    {
        public Dictionary<Type, object> BrockerList
        {
            get;
            set;
        }

        /// <summary>
        /// Initializes a new instance of the <see cref="T:MessageHandling"/> class.
        /// </summary>
        public MessageHandling()
        {
            BrockerList = new Dictionary<Type, object>();

            Team.Business.MessageObjects.IAbstractMessageBroker<TextMessage> newTextMessageBroker =
                    new Business.MessageObjects.TextMessage.TextMessageBroker();

            BrockerList.Add(typeof(Business.MessageObjects.TextMessage.TextMessage),
                newTextMessageBroker);
        }

        public Team.Business.MessageObjects.AbstractMessageBroker<IAbstractMessage> GetBrockerForType(Type messageType)
        {
            Team.Business.MessageObjects.AbstractMessageBroker<IAbstractMessage> ob =
                (Team.Business.MessageObjects.AbstractMessageBroker<IAbstractMessage>)
                BrockerList[messageType];
            return ob;
        }
    }