Former admin of DC# here (forgive the sourceforge hosting-- it was a long time ago! https://sourceforge.net/projects/dc-sharp/). Great write-up! This was a fascinating read- thank you for putting it together.

One issue to be mindful of- the HttpWebRequest.BeginGetResponse method does not honor timeouts, and you are on your own to timeout the attempt. Consider using HttpClient, if available in Mono / .NET Core. Otherwise, see MSDN for how to do this:

"In the case of asynchronous requests, it is the responsibility of the client application to implement its own time-out mechanism. The following code example shows how to do it." See: https://msdn.microsoft.com/en-us/library/system.net.httpwebr...

I'm not sure if you have access to the ThreadPool class. In a bug that Microsoft's library had, I used the TPL Task construct to resolve this. See the pull request here: https://github.com/Microsoft/ProjectOxford-ClientSDK/pull/83...