How to handle exceptions thrown by Tasks in xUnit .net's Assert.Throws<T>?

Question

The following asynchronous xUnit.net test with a lambda marked with the async modifier fails by reporting that no exception was thrown:

    [Theory, AutoWebData]     public async Task SearchWithNullQueryThrows(         SearchService sut,         CancellationToken dummyToken)     {         // Fixture setup         // Exercise system and verify outcome         Assert.Throws<ArgumentNullException>(async () =>             await sut.SearchAsync(null, dummyToken));         // Teardown     } 

To make sure that an ArgumentNullException is actually thrown I explicitly used a try-catch block. It worked, however the resulting code is not clean (compared to the first test):

[Theory, AutoWebData] public async Task SearchWithNullQueryThrows(     SearchService sut,     CancellationToken dummyToken) {     // Fixture setup     var expected = typeof(ArgumentNullException);     Type actual = null;     // Exercise system     try     {         await sut.SearchAsync(null, dummyToken);     }     catch (ArgumentNullException e)     {         actual = e.GetType();     }     // Verify outcome     Assert.Equal(expected, actual);     // Teardown } 

Why the Assert.Throws<T> with the lambda marked with the async modifier fails?

Answer 1

Update

This has been solved in xUnit 2, with the addition of Assert.ThrowsAsync.

---

I am suspecting that Assert.Throws is not async-aware. I recommend raising this issue with the xUnit team, suggesting a ThrowsAsync be added.

An async delegate in this case is returning Task or Task<T>, and the ArgumentNullException is not thrown out of the delegate directly; instead, it is placed on the Task (Task.Exception.InnerException). Assert.Throws is expecting the exception to be thrown out of the delegate directly, not placed on a property of the return value.

You can create your own AssertEx.ThrowsAsync as such:

public static async Task ThrowsAsync<TException>(Func<Task> func) {   var expected = typeof(TException);   Type actual = null;   try   {     await func();   }   catch (Exception e)   {     actual = e.GetType();   }   Assert.Equal(expected, actual); } 

which can be used as such:

[Theory, AutoWebData] public async Task SearchWithNullQueryThrows(     SearchService sut,     CancellationToken dummyToken) {     // Fixture setup     // Exercise system and verify outcome     await AssertEx.ThrowsAsync<ArgumentNullException>(async () =>         await sut.SearchAsync(null, dummyToken));     // Teardown } 

I use a similar approach in MSTest.