HttpClient - task was cancelled - How to get the exact error message?

The default HttpClient.Timeout value is 100 seconds (00:01:40). If you do a timestamp in your catch block you will notice that tasks begin to get canceled at exactly that time. Apparently there is a limited number of HTTP requests you can do per second, others get queued. Queued requests get canceled on timeout. Out of all 600k of tasks I personally got only 2500 successful, others got canceled.

I also find it unlikely, that you will be able to run the whole 600000 of tasks. Many network drivers let through high number of requests only for a small time, and reduce that number to a very low value after some time. My network card allowed me to send only 921 requests within 36 seconds and dropped that speed to only one request per second. At that speed it will take a week to complete all the tasks.

If you are able to bypass that limitation, make sure you build the code for 64-bit platform as the app is very hungry for memory.

Don't dispose the instance of HttpClient you're using. Weird but fixed for me this problem.