泛型约束

        /// <summary>
        /// 默认构造函数约束
        /// </summary>
        /// <typeparam name="U"></typeparam>
        public class GetEntity<U> where U : new()
        {
            U u = new U();
        }
        /*---------派生约束----------*/
        /*
         * 如果希望在泛型内部使用类型参数实例的某些方法，就必须使用派生约束
         */
        public interface ICommon
        {
            void Add(object obj);
            void Update(object obj);
           

        }
        public class Common: ICommon
        {
            public virtual void Add(object obj)
            {

            }
            public virtual void Update(object obj)
            {

            }

        }
        public class UserDao:Common 
        {
            public override void Add(object obj)
            {
                Console.WriteLine("插入一条User记录");
            }
            public override void Update(object obj)
            {
                Console.WriteLine("更新一条User记录");
            }
        }
        public class Base
        {
            public virtual void Open()
            {

            }
            public virtual void Close()
            {

            }
        }
        /// <summary>
        /// 只派生接口约束
        /// </summary>
        /// <typeparam name="T"></typeparam>
        public class Operate1<T> where T : ICommon,new ()
        {
           
            public void Add(object obj)
            {   
                T t = new T();
                Console.WriteLine("Add:");
                t.Add(obj);
            }
            public void Update(object obj)
            {
                T t = new T();
                Console.WriteLine("Update:");
                t.Update(obj);
                
            }
        }
        /// <summary>
        /// 派生类和接口：
        /// 可以在一个类型参数上添加多个接口实现约束和一个基类继承约束。
        /// 这种约束可以和默认构造函数约束同时使用，这时默认构造函数约束必须出现在列表尾部。
        /// 不能使用类型参数T的静态成员
        /// </summary>
        /// <typeparam name="T"></typeparam>
        public class Operate2<T> :Base where T :ICommon, new()
        {

            public void Add(object obj)
            {
                Base b = new Base();
                T t = new T();
                b.Open();
                Console.WriteLine("Add:");
                t.Add(obj);
                b.Close();

            }
            public void Update(object obj)
            {
                T t = new T();
                Console.WriteLine("Update:");
                t.Update(obj);

            }
            
            

        }
调用实现
       public static void FanxingYueShu()
        {
            Operate1<Common> common = new Operate1<Common>();
            User user=new User ();
            user.Id=1;
            user.UserName="carl";
            user.UserPwd = "carl";
            common.Add(user);
            common.Update(user);
            Operate2<Common> common2 = new Operate2<Common>();
            common2.Add(new object());
            common2.Update(new object());
        }