using System;

using System.Diagnostics;

using System.Runtime.CompilerServices;

using System.Threading.Tasks;

​

public class Program

{

    public static async Task Main()

    {

        var test = new Test();

​

        var task1 = Task.Run(async () => await test.AsyncString());

        var task2 = Task.Run(async () => await test.AsyncString());

        var task3 = Task.Run(async () => await test.AsyncString());

​

        var results = await Task.WhenAll(task1, task2, task3);

​

        Console.WriteLine(string.Join(", ", results));

    }

}

​

public sealed class Test

{

    public Test()

    {

        _lazyString = new AsyncLazy<string>(longRunningOperation);

    }

​

    public async Task<string> AsyncString()

    {

        Console.WriteLine($"{_sw.Elapsed}: Started awaiting lazy string.");

        var result = await _lazyString;

        Console.WriteLine($"{_sw.Elapsed}: Finished awaiting lazy string.");

​

        return result;

    }

​

    async Task<string> longRunningOperation()

    {

        Console.WriteLine($"{_sw.Elapsed}: longRunningOperation() started.");

        await Task.Delay(4000);

        Console.WriteLine($"{_sw.Elapsed}: longRunningOperation() finished.");

        return "finished";

    }

​

    readonly Stopwatch _sw = Stopwatch.StartNew();

​

    readonly AsyncLazy<string> _lazyString;

}

​

public class AsyncLazy<T> : Lazy<Task<T>>

{

    public AsyncLazy(Func<T> valueFactory) :

        base(() => Task.Factory.StartNew(valueFactory))

    { }

​

    public AsyncLazy(Func<Task<T>> taskFactory) :

        base(() => Task.Factory.StartNew(taskFactory).Unwrap())

    { }

​

    public TaskAwaiter<T> GetAwaiter() { return Value.GetAwaiter(); }

}

​