Running xUnit tests on Teamcity using async method

Question

I made the following xUnit test which is using a HttpClient to call a status api method on a webserver.

[Fact]
public void AmIAliveTest()
{
    var server = TestServer.Create<Startup>();

    var httpClient = server.HttpClient;
    var response = httpClient.GetAsync("/api/status").Result;

    response.StatusCode.Should().Be(HttpStatusCode.OK);
    var resultString = response.Content.ReadAsAsync<string>().Result;

    resultString.Should().Be("I am alive!");
}

This test is running fine locally. But when I commit the code and try to run the same test on the TeamCity build server, it runs forever. I even have to kill the xunit runner process because stopping the build will not stop this process.

However when I write the test like this

[Fact]
public async void AmIAliveTest()
{
   var server = TestServer.Create<Startup>();

   var httpClient = server.HttpClient;
   var response = await httpClient.GetAsync("/api/status");

   response.StatusCode.Should().Be(HttpStatusCode.OK);
   var resultString = await response.Content.ReadAsAsync<string>();

   resultString.Should().Be("I am alive!");
}

It runs fine locally and also on TeamCity.

My concern is now that I forget to write the test like the second variant and that once in a while the teamcity build is hanging.

Can anybody explain to me why xUnit running on the teamcity buildserver is not running the test correctly in the first place? And is there a solution for this to solve this?

Answer 1

Can anybody explain to me why xUnit running on the teamcity buildserver is not running the test correctly in the first place?

First, I'd check your xUnit versions - you should be running the recently-released 2.0. I suspect your local version may be out of date.

The core problem is in this line:

var resultString = response.Content.ReadAsAsync<string>().Result;

I suspect you're running into a deadlock situation that I describe on my blog. HttpClient has some methods on some platforms that do not properly use ConfigureAwait(false), and is thus subject to this deadlock. xUnit 2.0 installs a single-threaded SynchronizationContext into all its unit tests, which provides the other half of the deadlock scenario.

The proper solution is to replace Result with await, and to change the return type of your unit test method from void to Task.

Answer 2

Your Tests are broken.

xUnit needs the Task return to be able to figure out that a test failed and more to the point, it is the handle where the exception bubbles back up to xUnit.

By having a public async void, you have an orphaned Task that exceptioned. The resultant exception of course isn't handled, and of course therefore blows up the entire Process. Therefore your test run stops.

You can fix it by writing all your tests like this...

[Fact]
public async Task AmIAliveTest()
{
   var server = TestServer.Create<Startup>();

   var httpClient = server.HttpClient;
   var response = await httpClient.GetAsync("/api/status");

   response.StatusCode.Should().Be(HttpStatusCode.OK);
   var resultString = await response.Content.ReadAsAsync<string>();

   resultString.Should().Be("I am alive!");
}