How to override "context" with AssertionScope

[egil]

I love creating domain specific assertion extensions, but sometimes run into issues where I cannot get my validation messages to format nicely.

E.g., using (.NET 6 rc.1, FluentAssertions 6.1.x, xunit 2.4.x):

using FluentAssertions;
using FluentAssertions.Execution;
using FluentAssertions.Primitives;
using Xunit;
using Xunit.Sdk;

namespace ScopeAssertions;

public class Foo
{
    public string? Name { get; set; }

    public object? Content { get; set; }
}

public class FooAssertions : ReferenceTypeAssertions<Foo, FooAssertions>
{
    public FooAssertions(Foo instance) : base(instance) { }

    protected override string Identifier => "foo";

    public AndConstraint<FooAssertions> WithContent<T>(T content, string because = "", params object[] becauseArgs)
    {
        using (new AssertionScope($"content of {Identifier}"))
        {
            Subject.Content.Should().BeAssignableTo<T>().And.BeEquivalentTo(content, because, becauseArgs);
        }

        return new(this);
    }
}

public class UnitTest1
{
    [Fact]
    public void Test()
    {
        var target = new Foo { Name = "Foo", Content = "Bar" };

        var sut = new FooAssertions(target);

        sut.Invoking(x => x.WithContent("Baz"))
            .Should()
            .Throw<XunitException>()
            .WithMessage(@"Expected content of foo to be ""Baz"", but ""Bar"" differs near ""r"" (index 2).");
    }
}

Here I expect my custom assertion scope's context string to be used, but it does not seem to be. Running the test produces this output:

 ScopeAssertions.UnitTest1.Test
   Source: UnitTest1.cs line 36
   Duration: 79 ms

  Message: 
Expected exception message to match the equivalent of                                        ADDED LINE BREAK FOR READABILITY
  "Expected content of foo to be "Baz", but "Bar" differs near "r" (index 2).", but          ADDED LINE BREAK FOR READABILITY
  "Expected Subject.Content to be "Baz", but "Bar" differs near "r" (index 2).
" does not.


  Stack Trace: 
XUnit2TestFramework.Throw(String message)
TestFrameworkProvider.Throw(String message)
CollectingAssertionStrategy.ThrowIfAny(IDictionary`2 context)
ExceptionMessageAssertion.Execute(IEnumerable`1 messages, String expectation, String because, Object[] becauseArgs)
ExceptionAssertions`1.WithMessage(String expectedWildcardPattern, String because, Object[] becauseArgs)
UnitTest1.Test() line 42

So, how do I control the failure message in this particular case?

[dennisdoomen]

You need to add the [CustomAssertion] attribute to your WithContent message.

[egil]

Do you mean like this?

    [CustomAssertion]
    public AndConstraint<FooAssertions> WithContent<T>(T content, string because = "", params object[] becauseArgs)...

That results in this message:

    Expected exception message to match the equivalent of "Expected content of OK result to be "BAR", but "FOO" differs near "FOO" (index 0).", 
    but "Expected Subject.Value to be "BAR", but "FOO" differs near "FOO" (index 0).
" does not.

[dennisdoomen]

Ah, now I see what you're doing. The BeEquivalentTo call will override the context (that you tried to set through the AssertionScope) with the path within the graph it is comparing.

[egil]

Is there any way to influence that? Alternatively, can I get the equivalent to BeEquivalentTo by building out a Execute.Assertion expression myself?

[dennisdoomen]

Not really. The best you could do is to call DiscardFailures on the assertion scope, right after the BeEquivalentTo call. It'll give you the failure messages it was going to throw at the end of the using block. Then you could replace the Subject.Value with something you need and do an Execute.Assertion to throw your final message.

[egil]

Does this look correct to you:

public AndConstraint<FooAssertions> WithContent<T>(T content, string because = "", params object[] becauseArgs)
{
    using (new AssertionScope($"content of {Identifier}"))
    {
      Subject.Content.Should().BeAssignableTo<T>().And.BeEquivalentTo(content, because, becauseArgs);
      var error = scope.Discard().SingleOrDefault();
      if (error is not null)
      {
        var fixedErrorMessage = error.Replace("Subject.Value", $"content of {Identifier}", StringComparison.InvariantCulture);
        Execute.Assertion.FailWith(fixedErrorMessage);
      }
    }

    return new(this);
}

In particular, is it safe to assume that there is only going to be one error in this case, and thus using the SingleOrDefault filter method?

[egil]

Hmm nope, if the BeAssignableTo fails, then there are more than one. I guess in this case FirstOrDefault is the right thing to use.

[dennisdoomen]

Yes, it's an entire wall of text listing the differences as well as the options that were used (both defaults as well as options provided as a parameter)