public async Task Test() {
            await AAA();
            await BBB();
        }
        async Task<string> AAA() {
            await Task.CompletedTask;
            return Guid.NewGuid().ToString("N");
        }
        /// <summary>
        /// 这样写更好
        /// </summary> 
        Task<string> BBB() {
            return Task.FromResult(Guid.NewGuid().ToString("N"));
            //return Task.CompletedTask;  //没有返回值时这样写
        }

 public async Task TestAsync() {
     await AAA();
     await BBB();
 }
 /// <summary>
 /// 完全没有必要用async修饰，直接返回任务就行了
 /// </summary>
 Task<string> AAA() {
     return Task.Run(async () => {
         await Task.Delay(3000);
         return Guid.NewGuid().ToString("N");
     });
 }
 /// <summary>
 /// 用async修饰后，里面必须要await，这样就多一个状态机
 /// </summary> 
 async Task<string> BBB() {
     return await Task.Run(async () => {
         await Task.Delay(3000);
         return Guid.NewGuid().ToString("N");
     });
 }