C# Task Parallel Library 第一次慢

Question

尝试并行执行多个 HTTP Get 请求，每个任务一个。如果我通过 Internet Explorer 执行 Gets，它们几乎会立即返回，但是当通过任务调用代码时，我第一次启动它们时它们会花几秒钟时间返回，但第二次运行它们会如我所料返回。所以要么我正在做一些阻塞的事情，要么由于某种原因线程没有启动？这是我第一次尝试使用 TPL。

这是基本的查找类:

public class Lookup
{
  public string Name { get; set; }
  public string URL { get; set; }

  public Lookup(string Name, string URL)
  {
    this.Name = Name;
    this.URL = URL;
  }

  public LookupReturn DoLookup()
  {
    LookupReturn d = new LookupReturn();
    d.Name = this.Name;
    d.URL = this.URL;

    WebRequest wrGETURL;
    wrGETURL = WebRequest.Create(this.URL);

    Stream objStream;
    objStream = wrGETURL.GetResponse().GetResponseStream();

    StreamReader objReader = new StreamReader(objStream);

    string sLine = objReader.ReadToEnd();
    d.Result = sLine;
    return d;
  }
}

返回类型很简单:

public class LookupReturn
{
  public string Name { get; set; }
  public string Result { get; set; }
  public string URL { get; set; }
}

因此尝试并行运行它 - 我是从 Winforms GUI 进行测试，但最终将在 WCF 服务中进行。

public partial class Form1 : Form
{
  private List<Lookup> Lookups = new List<Lookup>();

  private async void btnRunLookups_Click(object sender, EventArgs e)
  {
    Lookups.Add(new Lookup("Name1", "http://geturl1 "));
    Lookups.Add(new Lookup("Name2", "http://geturl2 ")); 
    // More lookups…  

    int workerThreads, complete;
    ThreadPool.GetMinThreads(out workerThreads, out complete);

    ThreadPool.SetMinThreads(100, complete);

    btnRunLookups.Visible = false;
    List <Task<LookupReturn>> lookupTasks = new List<Task<LookupReturn>>();
    foreach(Lookup dl in Lookups)
    {
      lbLookups.Items.Add("Starting task for " + dl.URL);

      Task<LookupReturn> t = new Task<LookupReturn>(() => dl.DoLookup() );
      lookupTasks.Add(t);
      t.Start();
    }

    //await Task.WhenAny(
    //  Task.WhenAll(lookupTasks.ToArray<Task<LookupReturn>>()), 
    //  Task.Delay(3000)
    //  );

    // This takes a good few seconds the first time
    await Task.WhenAll(lookupTasks.ToArray<Task<LookupReturn>>());

    // Now I need to see which ones completed and returned a result
    foreach (var x in lookupTasks)
    {
      if (x.IsCompleted)
      {
        lbLookups.Items.Add(x.Result);
      }
      else
      {
        // lbLookups.Items.Add("Not finished " + x.Result.Name);
      }
    }
    btnRunLookups.Visible = true;
  }

Answer 1

Other people注意到 HttpWebRequest 的第一个请求可能需要很长时间，因为它正在寻找代理信息。尝试将其 Proxy 属性设置为 null。

wrGETURL = WebRequest.Create(this.URL);
wrGETURL.Proxy = null;