FluentAssertions Defaults for type

Question

I'm testing complicated object graphs with FluentAssertions and I want to set defaults for ShouldBeEquivalentTo. The problem is that the object graph re-used the same base-type for a lot of the properties. I just want to set up my AssertionOptionss once per Type but I can't seem to find out how.

Example: I have this object graph:

public class Inner
{
    public string One { get; set; }
    public string Two { get; set; }
}

public class Outer : Inner
{
    public Inner Inner { get; set; }
}

I set up a Custom Extension:

public AndConstraint<InnerAssertions<T>> BeEquivalentTo(T expectation)
{
    return base.BeEquivalentTo(expectation,
        c => c
            .RespectingRuntimeTypes()
            .Excluding(r => r.Two));
        }

Now I want this test to pass:

new Outer
{
    One = "1",
    Two = "2",
    Inner = new Inner {One = "inner.1", Two = "inner.2"}
}.Should()
    .BeEquivalentTo(new Outer
        {
            One = "1",
            Two = "3",
            Inner =new Inner {One = "inner.1", Two = "inner.3"}
        });

This obviously fails on root.Inner.Two, but I hope this makes clear what I want to do.

Answer 1

One way to achieve this is by using an IMemberSelectionRule to exclude Inner.Two from equivalency comparisons

class ExcludeTwo : IMemberSelectionRule
{
    public bool IncludesMembers => false;

    public IEnumerable<IMember> SelectMembers(INode currentNode, IEnumerable<IMember> selectedMembers, MemberSelectionContext context) =>
        selectedMembers.Where(e => !(e.DeclaringType == typeof(Inner) && e.Name == nameof(Inner.Two)));
}

and then add it to the stat AssertionOptions with e.g. a ModuleInitializer.

[ModuleInitializer]
public static void SetDefaults()
{
    AssertionOptions.AssertEquivalencyUsing(e => e.Using(new ExcludeTwo()));
}

Answer 2

Another way to avoid this situation is to pass an anonymous type to BeEquivalentTo that only includes the properties that you care about in those specific tests.