一次最多限制 3 个线程

请查看数据并行性与任务并行性。您可以在此处使用并行 LINQ：

var URLsToProcess = new List<string>
                    {
                        "http://www.microsoft.com",
                        "http://www.stackoverflow.com",
                        "http://www.google.com",
                        "http://www.apple.com",
                        "http://www.ebay.com",
                        "http://www.oracle.com",
                        "http://www.gmail.com",
                        "http://www.amazon.com",
                        "http://www.yahoo.com",
                        "http://www.msn.com"
                    };
Console.WriteLine("waiting now");
string[] tURLs = URLsToProcess
    .AsParallel()
    .WithDegreeOfParallelism(3)
    .Select(uri => this.DownloadStringAsTask(new Uri(uri)).Result)
    .ToArray();
Console.WriteLine("download all done");
foreach (string t in tURLs)
{
    Console.WriteLine(t);
}

您可以使用 SemaphoreSlim 类来限制一次执行的线程数。 班级课程 {

    static void Main(string[] args)
    {
        List<string> URLsToProcess = new List<string>()
        {
            "http://www.microsoft.com",
            "http://www.stackoverflow.com",
            "http://www.google.com",
            "http://www.apple.com",
            "http://www.ebay.com",
            "http://www.oracle.com",
            "http://www.gmail.com",
            "http://www.amazon.com",
            "http://www.yahoo.com",
            "http://www.msn.com"
        };
        SemaphoreSlim ss = new SemaphoreSlim(3); //limit 3 threads at a time
        List<Task<string>> tURLs = new List<Task<string>>();

        foreach (string url in URLsToProcess)
        {
            //Task<string> t = DownloadStringAsTask(new Uri(url));
            //tURLs.Add(t);
            tURLs.Add((Task<string>)Task.Run(() =>
            {
                DownloadStringAsTask(new Uri(url));
                ss.Release();
            }));
        }
        Console.WriteLine("waiting now");
        Task.WaitAll(tURLs.ToArray());
        Console.WriteLine("download all done");
        foreach (Task<string> t in tURLs)
            Console.WriteLine(t.Result);
        Console.ReadLine();
    }

    static Task<string> DownloadStringAsTask(Uri address)
    {
        TaskCompletionSource<string> tcs =
          new TaskCompletionSource<string>();
        WebClient client = new WebClient();
        client.DownloadStringCompleted += (sender, args) =>
        {
            if (args.Error != null)
                tcs.SetException(args.Error);
            else if (args.Cancelled)
                tcs.SetCanceled();
            else
                tcs.SetResult(args.Result);
        };
        client.DownloadStringAsync(address);
        return tcs.Task;
    }
}