Exclude EnumerableEquivalencyStep on top level Assertion

Question

I have this object from an external library that implements IEnumerable<T>. I just want to check the properties of this object and not the equivalence of the collection. I'm not interested in the stuff inside the IEnumerable. How can I exclude this from the assertion?

public class Example : IEnumerable<string>
{
    public string Id { get; set; }
    public string Name { get; set; }
    public IEnumerator<string> GetEnumerator() => new List<string> { Id }.GetEnumerator();
    IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
}

This is the test, which will fail on the IEnumerable.

(new Example {Id = "123", Name= "Test" } as object)
    .Should().BeEquivalentTo(new Example{Name = "Test"},
        o => o.Excluding(p => p.Id));

Doing this before the test this will work, but then I lose the ability to check any IEnumerable property on the class:

AssertionOptions.EquivalencyPlan.Remove<GenericEnumerableEquivalencyStep>();
AssertionOptions.EquivalencyPlan.Remove<EnumerableEquivalencyStep>();

Answer 1

One way to workaround this limitation is to add a dedicated equivalency step to handle Example.

using FluentAssertions;
using FluentAssertions.Equivalency;
using FluentAssertions.Equivalency.Steps;
using System.Collections;
using System.Collections.Generic;
using System.Runtime.CompilerServices;

[TestClass]
public class UnitTest1
{
    [ModuleInitializer]
    public static void SetDefaults()
    {
        AssertionOptions.EquivalencyPlan.Insert<ExampleEquivalencyStep>();
    }

    [TestMethod]
    public void TestMethod1()
    {
        object subject = new Example { Id = "123", Name = "Test" };
        var expected = new Example { Name = "Test" };
        subject.Should().BeEquivalentTo(expected, o => o.Excluding(p => p.Id));
    }
}

class ExampleEquivalencyStep : EquivalencyStep<Example>
{
    protected override EquivalencyResult OnHandle(Comparands comparands, IEquivalencyValidationContext context, IEquivalencyValidator nestedValidator) =>
        new StructuralEqualityEquivalencyStep().Handle(comparands, context, nestedValidator);
}

public class Example : IEnumerable<string>
{
    public string Id { get; set; }
    public string Name { get; set; }
    public IEnumerator<string> GetEnumerator() => new List<string> { Id }.GetEnumerator();
    IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
}