HttpClient.SendAsync using the thread-pool instead of async IO?

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this.startRequest is a delegate that points to StartRequest which in turn uses HttpWebRequest.BeginGetResponse to start async IO. HttpClient is using async IO under the covers, just wrapped in a thread-pool Task.

That said, note the following comment in SendAsync

// 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);

This works around a well-known problem with HttpWebRequest: Some of its processing stages are synchronous. That is a flaw in that API. HttpClient is avoiding blocking by moving that DNS work to the thread-pool.

Is that good or bad? It is good because it makes HttpClient non-blocking and suitable for use in a UI. It is bad because we are now using a thread for long-running blocking work although we expected to not use threads at all. This reduces the benefits of using async IO.

Actually, this is a nice example of mixing sync and async IO. There is nothing inherently wrong with using both. HttpClient and HttpWebRequest are using async IO for long-running blocking work (the HTTP request). They are using threads for short-running work (DNS, …). That’s not a bad pattern in general. We are avoiding most blocking and we only have to make a small part of the code async. A typical 80-20 trade-off. It is not good to find such things in the BCL (a library) but in application level code that can be a very smart trade-off.

It seems it would have been preferable to fix HttpWebRequest. Maybe that is not possible for compatibility reasons.

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