class ClassA
    {
        public event Action eventTest;
        public void fun1(int a,int b)
        {
            Console.WriteLine("fun1 start");
            Console.WriteLine("a+b=" + (a + b).ToString());
            Console.WriteLine("fun1 sleep start");

            eventTest?.Invoke();
          //  Thread.Sleep(1000);
            Console.WriteLine("fun1 sleep stop");

           // return true;
        }
    }


class ClassB
    {
        public delegate void dlg(int num1, int num2);
        public void fun(int a, int b,dlg fa)
        {
            Console.WriteLine("classB start");
            fa(a, b);
            Console.WriteLine("classB stop");
        }
    }



//**main form 按钮
private void button18_Click(object sender, EventArgs e)
        {
            ClassA A = new ClassA();
            ClassB B = new ClassB();
            A.eventTest += () =>
            {
                Console.WriteLine("eventTest start");
                Console.WriteLine("eventTest sleep start");
                Thread.Sleep(1000);
                Console.WriteLine("eventTest sleep end");
            };

            Console.WriteLine("mainform start");
            B.fun(2, 3, A.fun1);
            Console.WriteLine("mainform stop");
        }

输出结果:

mainform start
classB start
fun1 start
a+b=5
fun1 sleep start
eventTest start
eventTest sleep start
eventTest sleep end
fun1 sleep stop
classB stop
mainform stop