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C# 同步 异步 回调 状态机 async await Demo

简介: C# 同步 异步 回调 状态机 async await Demo 我们项目的客户端和服务端通信用的是WCF,我就想,能不能用异步的方式调用WCF服务呢?或者说能不能用async await的方式调用WCF服务呢?
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C# 同步 异步 回调 状态机 async await Demo

cyouhuiquan.jpg

为什么会研究这个?

我们项目的客户端和服务端通信用的是WCF,我就想,能不能用异步的方式调用WCF服务呢?或者说能不能用async await的方式调用WCF服务呢?

然后我发现WCF是通过BeginXXX和EndXXX这种回调的方式实现异步的,似乎不支持async await语法糖,那只能通过状态机的方式实现了,不然多个请求就会写成回调地狱。

如果用Task.Run包起来,里面再写个Wait,这就是假异步,还不如不用异步,按原来非异步的方式写。

研究了回调和状态机之后,又看了相关的博客,就像一篇博客里说的,调用WCF不难,异步调用WCF不难,异步调用WCF并保证调用顺序也不难,难的是实现了这些之后,代码的可读性和可维护性。

所以我的结论是,调用WCF,如果没有特殊需求,还是不要异步的好,写起来复杂。

C# 同步 异步 回调 状态机 async await Demo

主要演示了不使用async、await语法糖,通过回调函数和状态机实现异步

为什么要写这个Demo?

为了帮助理解异步,async、await经过编译生成的状态机代码,有点复杂,这个Demo里写了一个相对简单的状态机代码,便于理解

代码说明

代码中主要写了三种下载文件的示例

  1. 同步方式下载文件,为了防止下载较大文件时卡住界面,代码在线程中执行,文件下载完成之前,始终占用一个线程
  2. 异步方式下载文件,使用了async、await语法糖,下载文件时,可以看到,workerThreads(可用线程数)和completionPortThreads(可用异步线程数)会发生变化,但是不会长时间占用一个线程
  3. 异步方式下载文件,不使用async、await语法糖,通过回调函数和状态机实现,workerThreads(可用线程数)和completionPortThreads(可用异步线程数)会发生变化,但是不会长时间占用一个线程

结论:异步的本质是回调,异步是回调的语法糖

  1. 相比同步方式,使用异步方式下载文件时,忽略掉误差,下载速度并没有更快,异步的主要优点是不会长时间占用一个线程
  2. 在没有async、await语法糖时,使用回调函数和状态机也可以实现异步,但代码写的不够优雅,心智负担重,所以async、await的另一个优点是使代码简单
  3. 异步的本质就是回调,C#异步底层是通过系统级回调和状态机实现的,async、await会被编译成状态机代码,相关于用代码生成器生成了代码,但这个代码自己写的话,心智负担重
  4. 不使用async、await语法糖的前提下,使用状态机可以避免回调地狱
  5. 即使不使用async、await语法糖,依然感受到了异步的侵入性,没办法在底层完全封装起来,代码一直侵入到控件的事件里,使用async、await也是,你从代码的底层,写async一直写到控件的事件层

思考:

经测试,button4_Click、button5_Click、button6_Click在下载大文件时,耗时较长,但是界面并不会卡住,依然可以自由拖动窗体或点击其它按钮

button4_Click、button5_Click、button6_Click方法分别调用了AsyncFunctionWithCallback方法

AsyncFunctionWithCallback方法调用了HttpUtil类的HttpDownloadFileAsyncWithCallback方法

请仔细观察上面的方法,它们没有使用new Thread、Task.Run、Task.Factory.StartNew等线程相关的方法,

也就是说执行长时间操作,没有阻塞界面,但是又没有使用线程,

实际上下载文件过程中,没有使用C#的CLR线程池,测试时,workerThreads数量不会变化,但是你会看到completionPortThreads数量变化,

下载文件的过程使用了异步线程池,但是下载大文件时,并不会长时间占用异步线程池

Demo截图

image

测试代码Form1.cs

using Models;
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.IO;
using System.Linq;
using System.Text;
using System.Threading;
using System.Threading.Tasks;
using System.Windows.Forms;
using Utils;
namespace AsyncAwaitDemo
{
    public partial class Form1 : Form
    {
        private DateTime _dt1;
        private System.Timers.Timer _timer;
        private string _testFileDownloadUrl = "http://desk-fd.zol-img.com.cn/t_s1920x1080c5/g6/M00/06/00/ChMkKWHXqV-IJFpUAEJatCdq_LUAAXW8AA1PvwAQlrM029.jpg?downfile=1642227460763.jpg"; //文件下载测试URL
        //private string _testFileDownloadUrl = "https://down21.xiazaidb.com/app/xiaomanghe.apk"; //文件下载测试URL
        //private string _testFileDownloadUrl = "https://codeload.github.com/0611163/DBHelper/zip/refs/heads/master"; //文件下载测试URL
        //private string _testFileDownloadUrl = "http://down-ww5.537a.com/soft/3/ce/com.yhong.muchun_51982ada.apk"; //文件下载测试URL
        //private string _testFileDownloadUrl = "https://dl.360safe.com/netunion/20140425/360se+191727+n3f07b78190.exe"; //文件下载测试URL 大一点的文件
        public Form1()
        {
            InitializeComponent();
        }
        private void Form1_Load(object sender, EventArgs e)
        {
            //定时器会使用线程池中的一个线程
            _timer = new System.Timers.Timer();
            _timer.Interval = 100;
            _timer.Elapsed += _timer_Elapsed;
            _timer.Start();
        }
        #region Log
        private void Log(string log)
        {
            if (!this.IsDisposed)
            {
                string msg = DateTime.Now.ToString("mm:ss.fff") + " " + log + "\r\n\r\n";
                if (this.InvokeRequired)
                {
                    this.BeginInvoke(new Action(() =>
                    {
                        textBox1.AppendText(msg);
                    }));
                }
                else
                {
                    textBox1.AppendText(msg);
                }
            }
        }
        #endregion
        #region _timer_Elapsed
        private void _timer_Elapsed(object sender, System.Timers.ElapsedEventArgs e)
        {
            int workerThreads;
            int completionPortThreads;
            int maxWorkerThreads;
            int maxCompletionPortThreads;
            ThreadPool.GetAvailableThreads(out workerThreads, out completionPortThreads);
            ThreadPool.GetMaxThreads(out maxWorkerThreads, out maxCompletionPortThreads);
            this.BeginInvoke(new Action(() =>
            {
                label1.Text = "程序当前使用线程数:" + (maxWorkerThreads - workerThreads) + "  workerThreads:" + workerThreads.ToString() + "  completionPortThreads:" + completionPortThreads;
            }));
        }
        #endregion
        #region button1_Click 测试同步方法
        private void button1_Click(object sender, EventArgs e)
        {
            Task.Run(() => //下载文件是耗时操作,需要在线程中执行,否则界面卡住
            {
                Log("开始");
                DateTime dt = DateTime.Now;
                //输入参数
                string arg1 = "1";
                string arg2 = "2";
                string arg3 = "3";
                //方法调用
                string result1 = SyncFunction(arg1);
                string result2 = SyncFunction(arg2);
                string result3 = SyncFunction(arg3);
                //输出结果
                Log(result1);
                Log(result2);
                Log(result3);
                Log("结束,耗时:" + DateTime.Now.Subtract(dt).TotalSeconds.ToString("0.000"));
            });
        }
        #endregion
        #region button2_Click 测试异步方法(三个异步方法并行,按顺序输出)
        private async void button2_Click(object sender, EventArgs e)
        {
            Log("开始");
            DateTime dt = DateTime.Now;
            //输入参数
            string arg1 = "1";
            string arg2 = "2";
            string arg3 = "3";
            //方法调用
            var t1 = AsyncFunction(arg1);
            var t2 = AsyncFunction(arg2);
            var t3 = AsyncFunction(arg3);
            string result1 = await t1;
            string result2 = await t2;
            string result3 = await t3;
            //输出结果
            Log(result1);
            Log(result2);
            Log(result3);
            Log("结束,耗时:" + DateTime.Now.Subtract(dt).TotalSeconds.ToString("0.000"));
        }
        #endregion
        #region button3_Click 测试异步方法(三个异步方法顺序执行,按顺序输出)
        private async void button3_Click(object sender, EventArgs e)
        {
            Log("开始");
            DateTime dt = DateTime.Now;
            //输入参数
            string arg1 = "1";
            string arg2 = "2";
            string arg3 = "3";
            //方法调用
            string result1 = await AsyncFunction(arg1);
            string result2 = await AsyncFunction(arg2);
            string result3 = await AsyncFunction(arg3);
            //输出结果
            Log(result1);
            Log(result2);
            Log(result3);
            Log("结束,耗时:" + DateTime.Now.Subtract(dt).TotalSeconds.ToString("0.000"));
        }
        #endregion
        #region button4_Click 测试状态机(三个异步方法顺序执行,按顺序输出)(等价于button3_Click)
        private void button4_Click(object sender, EventArgs e)
        {
            List<object> resultList = new List<object>();
            //回调、状态机,不使用async await语法糖
            Action<int, object> awaitAction = null;
            awaitAction = (state, result) =>
            {
                //输入参数
                string arg1 = "1";
                string arg2 = "2";
                string arg3 = "3";
                //方法调用
                if (state == 0)
                {
                    Log("开始");
                    _dt1 = DateTime.Now;
                    AsyncFunctionWithCallback(arg1, r => awaitAction(1, r));
                    return;
                }
                if (state == 1)
                {
                    resultList.Add(result);
                    AsyncFunctionWithCallback(arg2, r => awaitAction(2, r));
                    return;
                }
                if (state == 2)
                {
                    resultList.Add(result);
                    AsyncFunctionWithCallback(arg3, r => awaitAction(3, r));
                    return;
                }
                //输出结果
                if (state == 3)
                {
                    resultList.Add(result);
                    foreach (object item in resultList)
                    {
                        Log(item != null ? item.ToString() : "null");
                    }
                    Log("结束,耗时:" + DateTime.Now.Subtract(_dt1).TotalSeconds.ToString("0.000"));
                    return;
                }
            };
            awaitAction(0, null);
        }
        #endregion
        #region button5_Click 测试状态机(三个异步方法并行,按顺序输出)(等价于button2_Click)
        private void button5_Click(object sender, EventArgs e)
        {
            object[] resultArray = new object[3];
            //是否已调用回调函数
            bool _completedCalled = false;
            //锁
            object _lock = new object();
            //输出结果
            Action<object[]> output = arr =>
            {
                if (arr.Count(a => a != null) == 3)
                {
                    lock (_lock)
                    {
                        if (!_completedCalled)
                        {
                            _completedCalled = true;
                            foreach (object item in arr)
                            {
                                Log(item != null ? item.ToString() : "null");
                            }
                            Log("结束,耗时:" + DateTime.Now.Subtract(_dt1).TotalSeconds.ToString("0.000"));
                        }
                    }
                }
            };
            //回调、状态机,不使用async await语法糖
            Action<int, object> awaitAction = null;
            awaitAction = (state, result) =>
            {
                //输入参数
                string arg1 = "1";
                string arg2 = "2";
                string arg3 = "3";
                //方法调用
                if (state == 0)
                {
                    Log("开始");
                    _dt1 = DateTime.Now;
                    AsyncFunctionWithCallback(arg1, r => awaitAction(1, r));
                    AsyncFunctionWithCallback(arg2, r => awaitAction(2, r));
                    AsyncFunctionWithCallback(arg3, r => awaitAction(3, r));
                    return;
                }
                //输出结果
                if (state == 1)
                {
                    resultArray[0] = result;
                    output(resultArray);
                    return;
                }
                if (state == 2)
                {
                    resultArray[1] = result;
                    output(resultArray);
                    return;
                }
                if (state == 3)
                {
                    resultArray[2] = result;
                    output(resultArray);
                    return;
                }
            };
            awaitAction(0, null);
        }
        #endregion
        #region button6_Click 测试状态机(三个异步方法并行,按顺序输出)(等价于button2_Click)(相对于button5_Click更优雅的实现)
        private void button6_Click(object sender, EventArgs e)
        {
            Log("开始");
            DateTime dt = DateTime.Now;
            //输入参数
            string arg1 = "1";
            string arg2 = "2";
            string arg3 = "3";
            //操作结果
            object result1 = null;
            object result2 = null;
            object result3 = null;
            //方法调用
            Await await = new Await();
            await.Add(() => AsyncFunctionWithCallback(arg1, r => await.Collect(r, out result1)))
                .Add(() => AsyncFunctionWithCallback(arg2, r => await.Collect(r, out result2)))
                .Add(() => AsyncFunctionWithCallback(arg3, r => await.Collect(r, out result3)))
                .Completed(resultList => //输出结果
                {
                    Log(result1 != null ? result1.ToString() : "null");
                    Log(result2 != null ? result2.ToString() : "null");
                    Log(result3 != null ? result3.ToString() : "null");
                    Log("结束,耗时:" + DateTime.Now.Subtract(dt).TotalSeconds.ToString("0.000"));
                })
                .Start();
        }
        #endregion
        #region button7_Click 不使用状态机,执行带回调的异步方法(基本等价于button1_Click)
        private void button7_Click(object sender, EventArgs e)
        {
            Task.Run(() =>
            {
                Log("开始");
                DateTime dt = DateTime.Now;
                //输入参数
                string arg1 = "1";
                string arg2 = "2";
                string arg3 = "3";
                //操作结果
                object result1 = null;
                object result2 = null;
                object result3 = null;
                //方法调用
                AsyncFunctionWithCallback(arg1, r => result1 = r);
                AsyncFunctionWithCallback(arg2, r => result2 = r);
                AsyncFunctionWithCallback(arg3, r => result3 = r);
                //等待三个操作结果全部返回,三个操作结果返回之前,始终占用一个线程
                while (result1 == null || result2 == null || result3 == null)
                {
                    Thread.Sleep(1);
                }
                //输出结果
                Log(result1 != null ? result1.ToString() : "null");
                Log(result2 != null ? result2.ToString() : "null");
                Log(result3 != null ? result3.ToString() : "null");
                Log("结束,耗时:" + DateTime.Now.Subtract(dt).TotalSeconds.ToString("0.000"));
            });
        }
        #endregion
        #region 同步方法
        /// <summary>
        /// 同步方法
        /// </summary>
        private string SyncFunction(string arg)
        {
            MemoryStream ms = HttpUtil.HttpDownloadFile(_testFileDownloadUrl);
            return "同步方法 arg=" + arg + " 下载文件的字节数组长度=" + ms.Length;
        }
        #endregion
        #region 异步方法
        /// <summary>
        /// 异步方法
        /// </summary>
        private async Task<string> AsyncFunction(string arg)
        {
            MemoryStream ms = await HttpUtil.HttpDownloadFileAsync(_testFileDownloadUrl);
            return "异步方法 arg=" + arg + " 下载文件的字节数组长度=" + ms.Length;
        }
        #endregion
        #region 带回调的异步方法
        /// <summary>
        /// 带回调的异步方法
        /// </summary>
        /// <param name="arg">传入参数</param>
        /// <param name="callback">回调</param>
        /// <param name="nextState">下一个状态</param>
        private void AsyncFunctionWithCallback(string arg, Action<object> callback)
        {
            HttpUtil.HttpDownloadFileAsyncWithCallback(_testFileDownloadUrl, new HttpUtilAsyncState() { State = 0 }, obj =>
            {
                if (obj is Exception)
                {
                    Exception ex = obj as Exception;
                    Log("错误:" + ex.Message);
                }
                else
                {
                    MemoryStream ms = obj as MemoryStream;
                    string result = "带回调的异步方法 arg=" + arg + " 下载文件的字节数组长度=" + ms.Length;
                    callback(result);
                }
            });
        }
        #endregion
        #region 用Task模拟的异步方法
        /*
        #region 异步方法
        /// <summary>
        /// 异步方法
        /// </summary>
        private Task<string> AsyncFunction(string arg)
        {
            return Task.Run<string>(() =>
            {
                Thread.Sleep(2000);
                return "异步方法 arg=" + arg;
            });
        }
        #endregion
        #region 带回调的异步方法
        /// <summary>
        /// 带回调的异步方法
        /// </summary>
        /// <param name="arg">传入参数</param>
        /// <param name="callback">回调</param>
        /// <param name="nextState">下一个状态</param>
        private void AsyncFunctionWithCallback(string arg, Action<object> callback)
        {
            Task.Run(() =>
            {
                if (arg == "1")
                {
                    Thread.Sleep(2000);
                }
                else
                {
                    Thread.Sleep(2000);
                }
                string result = "带回调的异步方法 arg=" + arg;
                callback(result);
            });
        }
        #endregion
        */
        #endregion
    }
}

异步工具类Await.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
namespace Utils
{
    /// <summary>
    /// 异步工具类
    /// 代替async await语法糖
    /// </summary>
    public class Await
    {
        /// <summary>
        /// 状态
        /// </summary>
        private int _state { get; set; }
        /// <summary>
        /// 异步操作集合
        /// </summary>
        private List<Action> _actionList = new List<Action>();
        /// <summary>
        /// 操作结果集合
        /// </summary>
        private List<object> _resultList = new List<object>();
        /// <summary>
        /// 完成后的回调
        /// </summary>
        private Action<List<object>> _completedCallback;
        /// <summary>
        /// 是否已调用回调函数
        /// </summary>
        private bool _completedCalled = false;
        /// <summary>
        /// 锁
        /// </summary>
        private object _lock = new object();
        /// <summary>
        /// 异步工具类 构造函数
        /// </summary>
        public Await()
        {
            _state = 0;
        }
        /// <summary>
        /// 添加一个异步操作
        /// </summary>
        public Await Add(Action action)
        {
            _actionList.Add(action);
            return this;
        }
        /// <summary>
        /// 开始执行
        /// </summary>
        public Await Start()
        {
            foreach (Action action in _actionList)
            {
                action();
            }
            return this;
        }
        /// <summary>
        /// 收集结果
        /// </summary>
        public Await Collect(object result, out object outResult)
        {
            outResult = result;
            _resultList.Add(result);
            lock (_lock)
            {
                _state++;
                if (_state == _actionList.Count && _completedCallback != null && !_completedCalled)
                {
                    _completedCalled = true;
                    _completedCallback(_resultList);
                }
            }
            return this;
        }
        /// <summary>
        /// 注册完成后的回调函数
        /// </summary>
        public Await Completed(Action<List<object>> completedCallback)
        {
            this._completedCallback = completedCallback;
            return this;
        }
    }
}

文件下载工具类HttpUtil.cs

下面的代码使用了三种方式实现文件下载

  1. 同步方式实现文件下载
  2. 异步方式实现文件下载
  3. 通过回调和状态机实现的异步下载
using Models;
using System;
using System.Collections.Generic;
using System.Collections.Specialized;
using System.IO;
using System.Linq;
using System.Net;
using System.Text;
using System.Threading;
using System.Threading.Tasks;
namespace Utils
{
    /// <summary>
    /// Http上传下载文件
    /// </summary>
    public class HttpUtil
    {
        #region HttpDownloadFile 下载文件
        /// <summary>
        /// 下载文件
        /// </summary>
        /// <param name="url">下载文件url路径</param>
        /// <param name="cookie">cookie</param>
        public static MemoryStream HttpDownloadFile(string url, CookieContainer cookie = null, WebHeaderCollection headers = null)
        {
            try
            {
                // 设置参数
                HttpWebRequest request = WebRequest.Create(url) as HttpWebRequest;
                request.Method = "GET";
                request.CookieContainer = cookie;
                if (headers != null)
                {
                    foreach (string key in headers.Keys)
                    {
                        request.Headers.Add(key, headers[key]);
                    }
                }
                //发送请求并获取相应回应数据
                HttpWebResponse response = request.GetResponse() as HttpWebResponse;
                //直到request.GetResponse()程序才开始向目标网页发送Post请求
                Stream responseStream = response.GetResponseStream();
                //创建写入流
                MemoryStream stream = new MemoryStream();
                byte[] bArr = new byte[10240];
                int size = responseStream.Read(bArr, 0, (int)bArr.Length);
                while (size > 0)
                {
                    stream.Write(bArr, 0, size);
                    size = responseStream.Read(bArr, 0, (int)bArr.Length);
                }
                stream.Seek(0, SeekOrigin.Begin);
                responseStream.Close();
                return stream;
            }
            catch (Exception ex)
            {
                throw ex;
            }
        }
        #endregion
        #region HttpDownloadFile 下载文件(异步)
        /// <summary>
        /// 下载文件
        /// </summary>
        /// <param name="url">下载文件url路径</param>
        /// <param name="cookie">cookie</param>
        public static async Task<MemoryStream> HttpDownloadFileAsync(string url, CookieContainer cookie = null, WebHeaderCollection headers = null)
        {
            try
            {
                // 设置参数
                HttpWebRequest request = WebRequest.Create(url) as HttpWebRequest;
                request.Method = "GET";
                request.CookieContainer = cookie;
                if (headers != null)
                {
                    foreach (string key in headers.Keys)
                    {
                        request.Headers.Add(key, headers[key]);
                    }
                }
                //发送请求并获取相应回应数据
                HttpWebResponse response = await request.GetResponseAsync() as HttpWebResponse;
                //直到request.GetResponse()程序才开始向目标网页发送Post请求
                Stream responseStream = response.GetResponseStream();
                //创建写入流
                MemoryStream stream = new MemoryStream();
                byte[] bArr = new byte[10240];
                int size = await responseStream.ReadAsync(bArr, 0, (int)bArr.Length);
                while (size > 0)
                {
                    stream.Write(bArr, 0, size);
                    size = await responseStream.ReadAsync(bArr, 0, (int)bArr.Length);
                }
                stream.Seek(0, SeekOrigin.Begin);
                responseStream.Close();
                return stream;
            }
            catch (Exception ex)
            {
                throw ex;
            }
        }
        #endregion
        #region HttpDownloadFile 下载文件(基于回调的异步)
        /// <summary>
        /// 下载文件
        /// </summary>
        /// <param name="url">下载文件url路径</param>
        /// <param name="cookie">cookie</param>
        public static void HttpDownloadFileAsyncWithCallback(string url, HttpUtilAsyncState state = null, Action<object> callback = null, CookieContainer cookie = null, WebHeaderCollection headers = null)
        {
            try
            {
                if (state.State == 0)
                {
                    // 设置参数
                    HttpWebRequest request = WebRequest.Create(url) as HttpWebRequest;
                    request.Method = "GET";
                    request.CookieContainer = cookie;
                    if (headers != null)
                    {
                        foreach (string key in headers.Keys)
                        {
                            request.Headers.Add(key, headers[key]);
                        }
                    }
                    //发送请求并获取相应回应数据
                    request.BeginGetResponse(asyncResult =>
                    {
                        HttpUtilAsyncState asyncState = asyncResult.AsyncState as HttpUtilAsyncState;
                        try
                        {
                            HttpWebResponse response = request.EndGetResponse(asyncResult) as HttpWebResponse;
                            asyncState.ResponseStream = response.GetResponseStream();
                            HttpDownloadFileAsyncWithCallback(null, asyncState, callback);
                        }
                        catch (Exception ex)
                        {
                            asyncState.Exception = ex;
                            asyncState.State = 2;
                            HttpDownloadFileAsyncWithCallback(null, asyncState, callback);
                        }
                    }, new HttpUtilAsyncState { Request = request, State = 1 });
                    return;
                }
                if (state.State == 1)
                {
                    byte[] bArr = new byte[10240];
                    state.ResponseStream.BeginRead(bArr, 0, (int)bArr.Length, asyncResult =>
                    {
                        HttpUtilAsyncState asyncState = asyncResult.AsyncState as HttpUtilAsyncState;
                        try
                        {
                            int size = state.ResponseStream.EndRead(asyncResult);
                            if (size > 0)
                            {
                                state.Stream.Write(bArr, 0, size);
                                HttpDownloadFileAsyncWithCallback(null, asyncState, callback);
                            }
                            else
                            {
                                asyncState.State = 2;
                                HttpDownloadFileAsyncWithCallback(null, asyncState, callback);
                            }
                        }
                        catch (Exception ex)
                        {
                            asyncState.Exception = ex;
                            asyncState.State = 2;
                            HttpDownloadFileAsyncWithCallback(null, asyncState, callback);
                        }
                    }, state);
                    return;
                }
                if (state.State == 2)
                {
                    if (state.Exception != null)
                    {
                        callback(state.Exception);
                        return;
                    }
                    else
                    {
                        state.Stream.Seek(0, SeekOrigin.Begin);
                        state.ResponseStream.Close();
                        callback(state.Stream);
                        return;
                    }
                }
            }
            catch (Exception ex)
            {
                throw ex;
            }
        }
        #endregion
    }
}

 

源码https://gitee.com/s0611163/AsyncAwaitDemo

原文地址https://www.cnblogs.com/s0611163/p/15835033.html

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