C# Stopwatch to time an async/await method inaccurate

Question

I'm writing some performance tests, and want to be able to time a method that is asynchronous. The code looks like this, where action is a Func<Task<HttpResponseMessage>>:

var sw = new Stopwatch();
HttpResponseMessage response = null;

sw.Start();
response = await action().ConfigureAwait(continueOnCapturedContext: false);
sw.Stop();

The code compiles and runs ok, but the measured milliseconds are about 100 times higher than the request timings we see in Fiddler - Fiddler reports 200-300ms, but the stop watch reports ~30,000ms. Is there some catch about timing asynchronous methods? Is the solution to do the timing in the action itself (which would be annoying?)

Answer 1

This should work fine for measuring the true amount of time it takes for your asynchronous task to finish. You need to keep in mind that:

1. the time you are measuring with Fiddler is measuring the request only, and none of the time it takes for your code to process the response.
2. there is a significant difference in time here and you should easily be able to time your code yourself to see how long it takes to progress from a break point before your request to after your request. If this is closer to 30 seconds then your stop watch is probably accurate.
3. there isn't anything obvious to me that would make your time inaccurate.