C# 的 Func<T> Action<T> Predicate<T>

         void Demo_Func_Action( )
        {
            Func<double, double, double> func = UseByFunc;
            Action<double, double> action = UseByAction;
            Predicate<double> predicate = UseByPredicate;

            Console.WriteLine("Func 返回：{0}", UseByFunc(10, 20));  
            Console.WriteLine("Predicate 返回：{0}", predicate(1));
        }

        // 将会给Func用，因为Func是可以返回结果的
        public double UseByFunc(double x, double y)
        {
            return x + y;
        } 

        //给Action用，因为Action是不返回的
        public void UseByAction(double x, double y)
        {
            Console.WriteLine("Action 内部结果:{0}",x-y);
        }

        public bool UseByPredicate(double a) {
            return true;
        }