泛型错误 The type parameter 'T' cannot be used with the 'as' operator

泛型错误
The type parameter 'T' cannot be used with the 'as' operator because it does not have a class type constraint nor a 'class' constraint

如果T是接口
这是错误的 ：return System.Activator.CreateInstance(reType) as T;
正确的写法 T t1=(T)System.Activator.CreateInstance(reType) ;


错误的
   public class EventIDDictionary<T> where T : MKProtocol
    {
        private MKHandler defaultHandler;
        private Dictionary<String, Type> productModuleDictionary = new Dictionary<string, Type>();

        private Dictionary<String, Type> ActionDictionary = new Dictionary<string, Type>();
        public void AddDictionary(String eventId, Type type)
        {
            if (type == null)
            {
                throw new ServerInitException("增加AddDictionary,eventId=" + eventId + ",type不能为null");
            }
            if (String.IsNullOrEmpty(eventId))
            {
                throw new ServerInitException("增加AddDictionary,eventId不能为空,type=" + type.FullName);
            }
            if(eventId.Length==12){
                if (ActionDictionary.ContainsKey(eventId))
                {
                    throw new ServerInitException("增加AddDictionary,eventId=" + eventId + "type=" + type.FullName + "已经存在,现有type=" + ActionDictionary[eventId].FullName);
                }
                ActionDictionary.Add(eventId, type);
            }
            else if (eventId.Length == 6)
            {
                if (productModuleDictionary.ContainsKey(eventId))
                {
                    throw new ServerInitException("增加AddDictionary,eventId=" + eventId + "type=" + type.FullName + "已经存在,现有type=" + productModuleDictionary[eventId].FullName);
                }
                productModuleDictionary.Add(eventId, type);
            }
            else
            {
                throw new ServerInitException("增加AddDictionary,eventId位数不对=" + eventId + "type=" + type.FullName);
            }
           
        }
        public void DeleteDictionary(String eventId)
        {
        
            if (String.IsNullOrEmpty(eventId))
            {
                throw new ServerInitException("删除DeleteDictionary,eventId不能为空");
            }
            if (eventId.Length == 12)
            {
                if (ActionDictionary.ContainsKey(eventId))
                {
                    ActionDictionary.Remove(eventId);
                }
              
            }
            if (eventId.Length == 6)
            {
                if (productModuleDictionary.ContainsKey(eventId))
                {
                    productModuleDictionary.Remove(eventId);
                }
             
            }
        }
        public virtual T GetHandler(MKEntity entity)
        {
            Type reType=null;

            if (entity == null || String.IsNullOrEmpty(entity.EventID))
                return default(T);
            if (entity.EventID.Length != 12)
            {
                return default(T);
            }
            if (ActionDictionary.ContainsKey(entity.EventID))
            {
                reType= ActionDictionary[entity.EventID];
            }
            if (productModuleDictionary.ContainsKey(entity.EventID.Substring(0, 6)))
            {
                reType= productModuleDictionary[entity.EventID.Substring(0, 6)];
            }
           
            return System.Activator.CreateInstance(reType) as T;


        }
    }

这是正确的
  public class EventIDDictionary<T> where T : MKProtocol
    {
        private MKHandler defaultHandler;
        private Dictionary<String, Type> productModuleDictionary = new Dictionary<string, Type>();

        private Dictionary<String, Type> ActionDictionary = new Dictionary<string, Type>();
        public void AddDictionary(String eventId, Type type)
        {
            if (type == null)
            {
                throw new ServerInitException("增加AddDictionary,eventId=" + eventId + ",type不能为null");
            }
            if (String.IsNullOrEmpty(eventId))
            {
                throw new ServerInitException("增加AddDictionary,eventId不能为空,type=" + type.FullName);
            }
            if(eventId.Length==12){
                if (ActionDictionary.ContainsKey(eventId))
                {
                    throw new ServerInitException("增加AddDictionary,eventId=" + eventId + "type=" + type.FullName + "已经存在,现有type=" + ActionDictionary[eventId].FullName);
                }
                ActionDictionary.Add(eventId, type);
            }
            else if (eventId.Length == 6)
            {
                if (productModuleDictionary.ContainsKey(eventId))
                {
                    throw new ServerInitException("增加AddDictionary,eventId=" + eventId + "type=" + type.FullName + "已经存在,现有type=" + productModuleDictionary[eventId].FullName);
                }
                productModuleDictionary.Add(eventId, type);
            }
            else
            {
                throw new ServerInitException("增加AddDictionary,eventId位数不对=" + eventId + "type=" + type.FullName);
            }
           
        }
        public void DeleteDictionary(String eventId)
        {
        
            if (String.IsNullOrEmpty(eventId))
            {
                throw new ServerInitException("删除DeleteDictionary,eventId不能为空");
            }
            if (eventId.Length == 12)
            {
                if (ActionDictionary.ContainsKey(eventId))
                {
                    ActionDictionary.Remove(eventId);
                }
              
            }
            if (eventId.Length == 6)
            {
                if (productModuleDictionary.ContainsKey(eventId))
                {
                    productModuleDictionary.Remove(eventId);
                }
             
            }
        }
        public virtual T GetHandler(MKEntity entity)
        {
            Type reType=null;

            if (entity == null || String.IsNullOrEmpty(entity.EventID))
                return default(T);
            if (entity.EventID.Length != 12)
            {
                return default(T);
            }
            if (ActionDictionary.ContainsKey(entity.EventID))
            {
                reType= ActionDictionary[entity.EventID];
            }
            if (productModuleDictionary.ContainsKey(entity.EventID.Substring(0, 6)))
            {
                reType= productModuleDictionary[entity.EventID.Substring(0, 6)];
            }
            T t1=(T)System.Activator.CreateInstance(reType) ;
            return t1;


        }
    }