BeEquivalentTo fails when comparing lists containing lists

[michaelestermann]

Description

When we updated fluentassertions to the latest version 5.5.0, we encountered an issue with the BeEquivalentTo assertion.
When we compare a list of objects, which contains another list with the same items but different order, the test will fail, even though strict ordering is disabled.

Repro

To reproduce this issue, I've created a repro repo which can be found here:
https://github.com/michaelestermann/OrderingRepro

Expected behavior:

Expected behaviour would be, that the lists are the same, even though the order of the lists are not the same.

Actual behavior:

Since the update, the test fails, because of the of the mismatching ordering of the list

Versions

We use the latest FluentAssertions 5.5.0 along with .NET Core SDK 2.1.500

[dennisdoomen]

Hmm, that is very weird. Comparisons of collections never take into account the order, expect for byte[].

[jnyrup]

Here's a minimal example:

[Fact]
public void ListInList_MustBeEquivalent()
{
    var items = new[]
    {
        new int[0],
        new int[] { 42 }
    };

    items.Should().BeEquivalentTo(
        new int[] { 42 },
        new int[0]
    );
}

It hits an Exception in GetBestResultSets() when calling set.Values.Min(r => r.Length), where set is an empty Dictionary.

Here's the stack when hitting that exception:

at FluentAssertions.Equivalency.AssertionResultSet.GetBestResultSets()
at FluentAssertions.Equivalency.AssertionResultSet.SelectClosestMatchFor(Object key)
at FluentAssertions.Equivalency.EnumerableEquivalencyValidator.LooselyMatchAgainst[T](IList`1 subjects, T expectation, Int32 expectationIndex)
at FluentAssertions.Equivalency.EnumerableEquivalencyValidator.AssertElementGraphEquivalencyWithLooseOrdering[T](Object[] subjects, T[] expectations)
at FluentAssertions.Equivalency.EnumerableEquivalencyValidator.AssertElementGraphEquivalency[T](Object[] subjects, T[] expectations)
at FluentAssertions.Equivalency.EnumerableEquivalencyValidator.Execute[T](Object[] subject, T[] expectation)
at lambda_method(Closure )
at System.RuntimeMethodHandle.InvokeMethod(Object target, Object[] arguments, Signature sig, Boolean constructor)
at System.Reflection.RuntimeMethodInfo.UnsafeInvokeInternal(Object obj, Object[] parameters, Object[] arguments)
at System.Delegate.DynamicInvokeImpl(Object[] args)
at System.Delegate.DynamicInvoke(Object[] args)
at FluentAssertions.Equivalency.GenericEnumerableEquivalencyStep.Handle(IEquivalencyValidationContext context, IEquivalencyValidator parent, IEquivalencyAssertionOptions config)
at FluentAssertions.Equivalency.EquivalencyValidator.AssertEqualityUsing(IEquivalencyValidationContext context)
at FluentAssertions.Equivalency.EnumerableEquivalencyValidator.TryToMatch[T](Object subject, T expectation, Int32 expectationIndex)
at FluentAssertions.Equivalency.EnumerableEquivalencyValidator.LooselyMatchAgainst[T](IList`1 subjects, T expectation, Int32 expectationIndex)
at FluentAssertions.Equivalency.EnumerableEquivalencyValidator.AssertElementGraphEquivalencyWithLooseOrdering[T](Object[] subjects, T[] expectations)
at FluentAssertions.Equivalency.EnumerableEquivalencyValidator.AssertElementGraphEquivalency[T](Object[] subjects, T[] expectations)
at FluentAssertions.Equivalency.EnumerableEquivalencyValidator.Execute[T](Object[] subject, T[] expectation)
at lambda_method(Closure )
at System.RuntimeMethodHandle.InvokeMethod(Object target, Object[] arguments, Signature sig, Boolean constructor)
at System.Reflection.RuntimeMethodInfo.UnsafeInvokeInternal(Object obj, Object[] parameters, Object[] arguments)
at System.Delegate.DynamicInvokeImpl(Object[] args)
at System.Delegate.DynamicInvoke(Object[] args)
at FluentAssertions.Equivalency.GenericEnumerableEquivalencyStep.Handle(IEquivalencyValidationContext context, IEquivalencyValidator parent, IEquivalencyAssertionOptions config)
at FluentAssertions.Equivalency.EquivalencyValidator.AssertEqualityUsing(IEquivalencyValidationContext context)
at FluentAssertions.Equivalency.EquivalencyValidator.AssertEquality(EquivalencyValidationContext context)
at FluentAssertions.Collections.CollectionAssertions`2.BeEquivalentTo[TExpectation](IEnumerable`1 expectation, Func`2 config, String because, Object[] becauseArgs)
at FluentAssertions.Collections.CollectionAssertions`2.BeEquivalentTo(Object[] expectations)
at ListInList_MustBeEquivalent()

I just checked #911 caused the change.
If I revert the addition of

.Then
.ClearExpectation();

to AssertCollectionsHaveSameCount it works again.

[dennisdoomen]

Will be part of 5.5.1