HttpClient.SendAsync 使用线程池而不是异步 IO

所以我一直在挖掘通过反射器实现HttpClient.SendAsync。我有意想要了解的是这些方法的执行流程，并确定调用哪个 API 来执行异步 IO 工作。

在探索了HttpClient内部的各种类之后，我看到它在内部使用从HttpMessageHandler派生的HttpClientHandler并实现了其SendAsync方法。

这是HttpClientHandler.SendAsync的实现：

protected internal override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
{
    if (request == null)
    {
        throw new ArgumentNullException("request", SR.net_http_handler_norequest);
    }
    this.CheckDisposed();
    this.SetOperationStarted();
    TaskCompletionSource<HttpResponseMessage> source = new TaskCompletionSource<HttpResponseMessage>();
    RequestState state = new RequestState 
    {
        tcs = source,
        cancellationToken = cancellationToken,
        requestMessage = request
    };
    try
    {
        HttpWebRequest request2 = this.CreateAndPrepareWebRequest(request);
        state.webRequest = request2;
        cancellationToken.Register(onCancel, request2);
        if (ExecutionContext.IsFlowSuppressed())
        {
            IWebProxy proxy = null;
            if (this.useProxy)
            {
                proxy = this.proxy ?? WebRequest.DefaultWebProxy;
            }
            if ((this.UseDefaultCredentials || (this.Credentials != null)) || ((proxy != null) && (proxy.Credentials != null)))
            {
                this.SafeCaptureIdenity(state);
            }
        }
        Task.Factory.StartNew(this.startRequest, state);
    }
    catch (Exception exception)
    {
        this.HandleAsyncException(state, exception);
    }
    return source.Task;
}

我觉得奇怪的是，上面使用Task.Factory.StartNew来执行请求，同时生成TaskCompletionSource<HttpResponseMessage>并返回它创建的Task。

为什么我觉得这很奇怪？ 好吧，我们继续讨论 I/O 绑定异步操作如何在幕后不需要额外的线程，以及它是如何与重叠的 IO 相关的。

为什么使用 Task.Factory.StartNew 来触发异步 I/O 操作？ 这意味着SendAsync不仅使用纯异步控制流来执行此方法，而且还在"背后">旋转 ThreadPool 线程来执行其工作。

// BeginGetResponse/BeginGetRequestStream have a lot of setup work to do before becoming async
// (proxy, dns, connection pooling, etc).  Run these on a separate thread.
// Do not provide a cancellation token; if this helper task could be canceled before starting then 
// nobody would complete the tcs.
Task.Factory.StartNew(startRequest, state);