Exception returned by Should.ThrowAsync<T> has InnerException property equal to null

[Eduardo Serrano]

I was trying to do an assert on the inner exception returned by an async method and was using the Should.ThrowAsync<T> for that.

The test below is a representation of the problem I encountered. The local method ThrowExceptionWithInnerException is simulating an async method in my code that throws an exception with an inner exception.
When I run the test below I get a NullReferenceException because the expectedException.InnerException is null.

[Fact]
public async Task Demo()
{
    async Task ThrowExceptionWithInnerException()
    {
        await Task.Delay(1);
        var innerException = new InvalidOperationException("inner");
        var exception = new TaskCanceledException("outer", innerException);
        throw exception;
    }

    var expectedException = await Should.ThrowAsync<TaskCanceledException>(ThrowExceptionWithInnerException());
    expectedException.InnerException.ShouldBeOfType<InvalidOperationException>(); // test fails with: "System.NullReferenceException : Object reference not set to an instance of an object." because expectedException.InnerException is null
}

As a workaround for these cases I've been doing:

[Fact]
public async Task DemoWorkaround()
{
    async Task ThrowExceptionWithInnerException()
    {
        await Task.Delay(1);
        var innerException = new InvalidOperationException("inner");
        var exception = new TaskCanceledException("outer", innerException);
        throw exception;
    }

    try
    {
        await ThrowExceptionWithInnerException();
    }
    catch (TaskCanceledException expectedException)
    {
        expectedException.InnerException.ShouldBeOfType<InvalidOperationException>();
    }
}

I searched the docs and existing issues (open and closed) but didn't find anything about this. Is my expectation that the InnerException property should NOT be null incorrect?

[Adam Knights]

I'm new to this project, but thought I would have a dig in the code... Ultimately this line is the culprit. Because your outer exception is a TaskCanceledException the NET method ContinueWith gives a task where IsFaulted is false and isCanceled is true. Infact, if we place a debugger in the code we can see there is no original exception, and therefore no inner exception:

If the test has an outer exception of any other type, say like:

[Fact]
public async Task DemoWorks()
{
    async Task ThrowExceptionWithInnerException()
    {
        await Task.Delay(1);
        var innerException = new InvalidOperationException("inner");
        var exception = new ArgumentException("outer", innerException);
        throw exception;
    }

    var expectedException = await Should.ThrowAsync<ArgumentException>(ThrowExceptionWithInnerException());
    expectedException.InnerException.ShouldBeOfType<InvalidOperationException>();
}

Then it works, and we can see with a debugger is hits the earlier lines:

---

Given it's something fundamental with ContinueWith I couldn't see an easy fix, but maybe the main project people will comment further :).