直接贴出代码：

using System;
using System.Collections.Generic;
using System.Threading.Tasks;

namespace AsyncBreakfast
{
    
    internal class Bacon {
     }
    internal class Coffee {
     }
    internal class Egg {
     }
    internal class Juice {
     }
    internal class Toast {
     }

    class Program
    {
    
        public static async Task Run()
        {
    
            Coffee cup = PourCoffee();
            Console.WriteLine("咖啡做好了");

            var eggsTask = FryEggsAsync(2);
            var baconTask = FryBaconAsync(3);
            var toastTask = MakeToastWithButterAndJamAsync(2);

            Juice oj = PourOJ();
            Console.WriteLine("橙汁倒好了");

            var breakfastTasks = new List<Task> {
     eggsTask, baconTask, toastTask };
            while (breakfastTasks.Count > 0)
            {
    
                Task finishedTask = await Task.WhenAny(breakfastTasks);
                if (finishedTask == eggsTask)
                {
    
                    Console.WriteLine("鸡蛋做好了");
                }
                else if (finishedTask == baconTask)
                {
    
                    Console.WriteLine("培根做好了");
                }
                else if (finishedTask == toastTask)
                {
    
                    Console.WriteLine("面包做好了");
                }
                breakfastTasks.Remove(finishedTask);
            }

            Console.WriteLine("完美的一顿早餐!");
        }

        static async Task<Toast> MakeToastWithButterAndJamAsync(int number)
        {
    
            var toast = await ToastBreadAsync(number);
            ApplyButter(toast);
            ApplyJam(toast);

            return toast;
        }

        private static Juice PourOJ()
        {
    
            Console.WriteLine("倒一杯橙汁");
            return new Juice();
        }

        private static void ApplyJam(Toast toast) =>
            Console.WriteLine("往面包上加点果酱");

        private static void ApplyButter(Toast toast) =>
            Console.WriteLine("往面包上加点奶油");

        private static async Task<Toast> ToastBreadAsync(int slices)
        {
    
            for (int slice = 0; slice < slices; slice++)
            {
    
                Console.WriteLine("把一片面包放进烤面包机");
            }
            Console.WriteLine("开始烤面包...");
            await Task.Delay(15000);
            Console.WriteLine("从面包机里拿出面包");

            return new Toast();
        }

        private static async Task<Bacon> FryBaconAsync(int slices)
        {
    
            Console.WriteLine($"将 {
      slices} 片培根放到平底锅中");
            Console.WriteLine("先将一面煎至金黄...");
            await Task.Delay(10000);
            for (int slice = 0; slice < slices; slice++)
            {
    
                Console.WriteLine("将培根翻一边");
            }
            Console.WriteLine("将另一面煎至金黄...");
            await Task.Delay(10000);
            Console.WriteLine("把煎好的培根装盘");

            return new Bacon();
        }

        private static async Task<Egg> FryEggsAsync(int howMany)
        {
    
            Console.WriteLine("热锅准备煎蛋");
            await Task.Delay(3000);
            Console.WriteLine($"打 {
      howMany} 个鸡蛋到碗里...");
            Console.WriteLine("煎蛋中 ...");
            await Task.Delay(5000);
            Console.WriteLine("将鸡蛋装盘");

            return new Egg();
        }

        private static Coffee PourCoffee()
        {
    
            Console.WriteLine("正在制作咖啡...");
            return new Coffee();
        }
    }
}

在Main函数中异步调用：

using System;
using System.Collections;
using System.Threading.Tasks;

public sealed class Program
{
    
	public static async Task Main()
    {
    
    	await AsyncBreakfast.Program.Run();
    }
}

可以清楚的看到控制台的执行结果：

总结：

通过async关键字将方法声明为异步方法，在遇到await关键字前会先执行方法内的同步代码，遇到await后跳出去执行方法外的同步代码，异步执行await的代码。
等到await代码执行完毕，继续执行await后面的同步代码。
异步方法会返回一个Task实例，可以通过Task的WhenAny方法监听Task是否执行完毕。