Unable to assert ThrowsExactly<OperationCancelledException> in async method

[alterius]

Description

When attempting to assert ThrowsExactly<OperationCancelledException> in an async method a TaskCancelledException is thrown instead.

[Test]
public void FluentAssertions_ThrowsCorrectException()
{
    var ex = new OperationCanceledException();

    Func<Task> action = async () => await ThrowException(ex);

    action.Should().ThrowExactly<OperationCanceledException>()
        .Which.Should().Be(ex);
}

[Test]
public void Nunit_ThrowsCorrectException()
{
    var ex = new OperationCanceledException();

    var result = Assert.Throws<OperationCanceledException>(async () => await ThrowException(ex));

    Assert.AreSame(ex, result);
}

public Task ThrowException(Exception ex)
{
    throw ex;
}

Expected behavior:

Both example tests should throw an OperationCancelledException and report passed.

Actual behavior:

The FluentAssertions test fails because a different TaskCancelledException is thrown.

Versions

FluentAssertions 5.4.1
NUnit 2.6.4

[jnyrup]

Some observations using NUnit 3.10.1 instead:

var result = Assert.Throws<OperationCanceledException>(async () => await ThrowException(ex));

throws

System.ArgumentException: ''async void' methods are not supported, please use 'async Task' instead'

and the dedicated ThrowsAsync

var result = Assert.ThrowsAsync<OperationCanceledException>(async () => await ThrowException(ex));

throws

NUnit.Framework.AssertionException: '  Expected: <System.OperationCanceledException>
  But was:  <System.Threading.Tasks.TaskCanceledException: A task was canceled.
   at NUnit.Framework.Internal.AsyncInvocationRegion.AsyncTaskInvocationRegion.WaitForPendingOperationsToComplete(Object invocationResult) in C:\src\nunit\nunit\src\NUnitFramework\framework\Internal\AsyncInvocationRegion.cs:line 113
   at NUnit.Framework.Assert.ThrowsAsync(IResolveConstraint expression, AsyncTestDelegate code, String message, Object[] args) in C:\src\nunit\nunit\src\NUnitFramework\framework\Assert.Exceptions.Async.cs:line 51>
'

When using another kind of exception, e.g. ArgumentNullException it works fine.

This special behavior for OperationCanceledException seems to stem from the source:

• https://referencesource.microsoft.com/#mscorlib/system/runtime/compilerservices/AsyncMethodBuilder.cs,661
• https://referencesource.microsoft.com/#mscorlib/system/threading/Tasks/FutureFactory.cs,557
• https://referencesource.microsoft.com/#mscorlib/system/threading/Tasks/FutureFactory.cs,1452

[jnyrup]

@alterius This issue seems to have been fixed.

Using Fluent Assertions 5.9.0 and NUnit 3.12 all these tests passes

[Test]
public void FluentAssertions_ThrowsCorrectException()
{
    var ex = new OperationCanceledException();

    Func<Task> action = () => ThrowException(ex);

    action.Should().ThrowExactly<OperationCanceledException>()
        .Which.Should().Be(ex);
}

[Test]
public async Task FluentAssertions_ThrowsCorrectException_async()
{
    var ex = new OperationCanceledException();

    Func<Task> action = () => ThrowException(ex);

    (await action.Should().ThrowExactlyAsync<OperationCanceledException>())
        .Which.Should().Be(ex);
}

[Test]
public void FluentAssertions_ThrowsCorrectException_await()
{
    var ex = new OperationCanceledException();

    Func<Task> action = async () => await ThrowException(ex);

    action.Should().ThrowExactly<OperationCanceledException>()
        .Which.Should().Be(ex);
}

[Test]
public async Task FluentAssertions_ThrowsCorrectException_async_await()
{
    var ex = new OperationCanceledException();

    Func<Task> action = async () => await ThrowException(ex);

    (await action.Should().ThrowExactlyAsync<OperationCanceledException>())
        .Which.Should().Be(ex);
}

private Task ThrowException(Exception ex)
{
    throw ex;
}