using System;

using System.Threading.Tasks;

​

public class Program

{

    public static void Main()

    {

        var taskCreator = new TaskCreator();

        Console.WriteLine("Before Expression");

        var expr =

            from a in taskCreator.CreateTask1()

            from b in taskCreator.CreateTask2()

            from c in taskCreator.CreateTask3()

            select c + (b + a);

        Console.WriteLine("Before Result");

        var result = expr.Result;

        Console.WriteLine(result);

    }

}

​

public class TaskCreator

{

    public async Task<int> CreateTask1()

    {

        Console.WriteLine("CreateTask1");

        return await Task.FromResult(4);

    }

​

    public async Task<int> CreateTask2()

    {

        Console.WriteLine("CreateTask2");

        return await Task.FromResult(6);

    }

​

    public async Task<string> CreateTask3()

    {

        Console.WriteLine("CreateTask3");

        return await Task.FromResult("The result is: ");

    }

}

​

public static class TaskExtensions

{

    public static async Task<B> Bind<A, B>(this Task<A> @this, Func<A, Task<B>> fn)

    {

        return await fn(await @this);

    }

​

    public static async Task<B> Map<A, B>(this Task<A> @this, Func<A, B> fn)

    {

        return await Task.FromResult(fn(await @this));

    }

​

    public static Task<C> SelectMany<A, B, C>(this Task<A> @this, Func<A, Task<B>> fn, Func<A, B, C> select)

    {

        Console.WriteLine("SelectMany");

        return @this.Bind(a => fn(a).Map(b => select(a, b)));

    }

}