I think this could be written as:

if (foundIndices.Count > 0)
{
    Assertion assertion = Execute.Assertion
        .BecauseOf(because, becauseArgs)
        .WithExpectation("Expected {context:collection} {0} not to contain equivalent of {1}{reason}, ", Subject, unexpected)
        .AddReportable("configuration", options.ToString());

    if (foundIndices.Count == 1)
    {
        assertion
            .FailWith("but found one at index {0}.", foundIndices[0])
            .Then
            .ClearExpectation();
    }
...
}

I tried similar approach earlier and it did not work unfortunately.

Firstly I was surprised that AddReportable is a void method, however I've extracted return value from .WithExpectation and just called AddReportable on it (that's not an issue of course).

The problem I faced is that AddReportable does not work when called outside new AssertionScope(). I'm not familiar with the codebase much, but quick look brought me to conclusion that "With" stuff is only attached on Dispose which seems to be not called on regular non-lazy assertion scope.

Am I missing something? What do you think?

I haven't used AddReportable much, so @dennisdoomen might be better to answer this than me.

Ok thanks.

BTW I added a test with using NotContainEquivalentOf inside AssertionScope: 05b5f66. I did it in a separate commit, because I don't know how this aligns with your testing approach. Looks like it works fine (if I understand "expected" correctly).

AddReportable was indeed only used by EquivalencyValidator to initialize an AssertionScope within which the entire equivalency engine runs.

AddReportable was indeed only used by EquivalencyValidator to initialize an AssertionScope within which the entire equivalency engine runs.