浅谈一下c#多线程编程

• 概念
  • 线程：线程是操作系统能够进行运算调度的最小单位，被包含在进程之中，是进程中的实际运作单位。

  • 同步：一定要等任务执行完了，得到结果，才执行下一个任务。如果程序执行耗时操作时会阻塞线程。
• 应用场景
  • UI与I/O: UI发出I/O操作，I/O操作是费时任务

  • 计算密集型工作（CPU-bound）：多线程

  •  IO密集型工作（I/O-bound）：异步多线程机制

• 实例代码

            public partial class MainForm : Form
            {
                /// <summary>
                /// Http连接对象
                /// </summary>
                private readonly HttpClient httpClient = new HttpClient();
        
                private readonly List<string> DownloadUrlList = new List<string>
                {
                    "https://www.jd.com",
                    "https://www.tmall.com",
                    "http://book.dangdang.com",
                    "https://www.cnblogs.com",
                    "https://www.runoob.com",
                    "https://www.csdn.net",
                    "https://www.githubs.cn",
                    "https://cn.vuejs.org",
                    "https://msdn.itellyou.cn",
                    "https://www.iconfont.cn",
                    "https://www.baidu.com",
                    "https://cn.bing.com",
                    "https://docs.microsoft.com/zh-cn/windows",
                    "https://docs.microsoft.com/zh-cn",
                    "https://docs.microsoft.com/zh-cn/dotnet/csharp",
                    "https://www.12306.cn",
                    "https://hotels.ctrip.com",
                    "http://www.shooting.org.cn",
                    "https://www.issf-sports.org",
                    "https://www.cba.net.cn",
                    "https://uland.taobao.com",
                    "https://top.baidu.com"
                };
        
                public MainForm()
                {
                    InitializeComponent();
                }
        
                private void ReportResult(string result)
                {
                    txtReport.Text += result;
                }
        
                /// <summary>
                /// 同步下载按钮点击事件
                /// </summary>
                /// <param name="sender"></param>
                /// <param name="e"></param>
                private void btnSync_Click(object sender, EventArgs e)
                {
                    txtReport.Text = "";
                    Stopwatch stopwatch = Stopwatch.StartNew();
                    Download();
                    txtReport.Text += $"Elapsed time:{stopwatch.Elapsed}.{Environment.NewLine}";
                }
        
                /// <summary>
                /// 同步下载：下载时UI线程会卡顿、直到下载结束后才恢复
                /// </summary>
                private void Download()
                {
                    foreach (var url in DownloadUrlList)
                    {
                        HttpResponseMessage response = httpClient.GetAsync(url).Result;
                        string responsePayloadBytes = response.Content.ReadAsStringAsync().Result;
                        ReportResult($"Finish downloading data for {url}.Total bytes {responsePayloadBytes.Length}.{Environment.NewLine}");
                    }
                }
        
                /// <summary>
                /// 异步下载按钮点击事件
                /// </summary>
                /// <param name="sender"></param>
                /// <param name="e"></param>
                private async void btnAsync_Click(object sender, EventArgs e)
                {
                    txtReport.Text = "";
                    Stopwatch stopwatch = Stopwatch.StartNew();
                    await DownloadAsync();
                    txtReport.Text += $"Elapsed time:{stopwatch.Elapsed}.{Environment.NewLine}";
                }
        
                /// <summary>
                /// 异步下载：
                /// 1、程序运行时不影响UI线程
                /// 2、虽然不影响UI线程，但是下载速度并没有变化
                /// </summary>
                /// <returns></returns>
                private async Task DownloadAsync()
                {
                    foreach (var url in DownloadUrlList)
                    {
                        ReportResult(await DownloadWebSitePayloadAsync(url));
                    }
                }
        
                private async Task<string> DownloadWebSitePayloadAsync(string url)
                {
                    HttpResponseMessage response = await httpClient.GetAsync(url);
                    string responsePayloadBytes = await response.Content.ReadAsStringAsync();
                    return $"Finish downloading data for {url}.Total bytes {responsePayloadBytes.Length}.{Environment.NewLine}";
                }
        
                /// <summary>
                /// 异步多线程按钮点击事件
                /// </summary>
                /// <param name="sender"></param>
                /// <param name="e"></param>
                private async void btnAsyncMultithreading_Click(object sender, EventArgs e)
                {
                    txtReport.Text = "";
                    Stopwatch stopwatch = Stopwatch.StartNew();
                    await MultithreadingDownloadAsync();
                    txtReport.Text += $"Elapsed time:{stopwatch.Elapsed}.{Environment.NewLine}";
                }
        
                /// <summary>
                /// 异步多线程：UI不会卡顿且大幅提升程序性能
                /// </summary>
                /// <returns></returns>
                private async Task MultithreadingDownloadAsync()
                {
                    List<Task<string>> downloadTasks = new List<Task<string>>();
                    DownloadUrlList.ForEach(o => downloadTasks.Add(DownloadWebSitePayloadAsync(o)));
                    var results = await Task.WhenAll(downloadTasks);
                    foreach (var result in results)
                    {
                        ReportResult(result);
                    }
                }

    }
    总结：多线程虽然可以在一定程度上提升程序性能，但是使用时需要谨慎选择，如果使用不当的话会出现超预期的效果。