Excluding properties behind an array doesn't work in BeEquivalentTo on collections

Description

A bit of a weird title but basically if I call myCollection.Should().BeEquivalent(expected, cfg => cfg.Excluding(i => i.SomeArray[0].SomeProperty)), the property isn’t correctly excluded and I get the follow assertion error:

Expected property root[0].Items[0].Name to be “Foo”, but “Bar” differs near “Bar”

Complete minimal example reproducing the issue

var expected = new A[] {
	new A { Items = new List<B> { new B { Name = "Foo" } } }
};

var actual = new A[] {
	new A { Items = new List<B> { new B { Name = "Bar" } } }
};

actual.Should().BeEquivalentTo(
	expected,
	cfg => cfg.Excluding(a => a.Items[0].Name)
);

class A
{
	public string Name { get; set; }
	public List<B> Items { get; set; }
}

class B
{
	public string Name { get; set; }
}

Expected behavior:

The excluding rule to be used and the test to pass.

Actual behavior:

The excluding rule is ignored and the test fails.

Versions

• Which version of Fluent Assertions are you using? v6.6.0

• Which .NET runtime and version are you targeting? E.g. .NET framework 4.6.1 or .NET Core 2.1. .NET 6

Additional Information

I believe this is a regression between v5.5.3 and v6.6.0 - I haven’t (yet) nailed down which version but will update when I have.

Edit: The regression starts in v6.0.0. The last v5 has this behaving correctly.

Additionally, this only seems to affect properties behind the array in the excluded check. Properties on the same level as the array work as expected if excluded.

[dennisdoomen]

I initially thought it worked given we have this successful test, but that one is not using a collection at the root. Also, I can confirm the behavior is slightly different when the root is a collection.

[IT-VBFK]

Ahhm… this should be fixed with the next release. See #1782

The new syntax will be:

actual.Should().BeEquivalentTo(
	expected,
	cfg => cfg
                 .For(a => a.Items)
                 .Exclude(b => b.Name));