async・await／非同期処理のサンプル

検証用C#コード.cs

//Microsoft (R) Visual C# Compiler version 3.4.0-beta4-19562-05 (ff930dec)
//Copyright (C) Microsoft Corporation. All rights reserved.


using System;
using System.Threading;
using System.Threading.Tasks;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;

namespace Rextester
{
    public class Program
    {
        public static void Main(string[] args)
        {
            Task<string> task = F1Async();
            Console.WriteLine(task.Result);
        }
        
        static async Task<string> F1Async(){
            Console.WriteLine( "F1 start.");
            var ret = "10";
            
            ret =  await F3Async();
            Task.Run(()=>F2(ret));

            Console.WriteLine ("F1 end. ");
            return "ALL End ret:"+ ret;
        }
        
        static void F2(string ret){
            Console.WriteLine( "F2 [ret from F3orF1?:"+ret+"] start.");            
            Console.WriteLine( "F2 [ret from F3orF1?:"+ret+"] end.");
        }
        
        static async Task<string> F3Async(){
            Console.WriteLine( "F3 start.");
            Thread.Sleep(2000);
            Console.WriteLine( "F3 end.");
            return "-1";
        }
    }
}

オンラインツール：https://rextester.com/