TestHelper "broken?" after update to .NET Core 3.1.1

System Details

• FluentValidation version: 8.6.1 (was using 8.1.2 before)
• Web Framework version: ASP.NET Core 3.1.1

Issue Description

Runtime validation works as expected, but TestHelper - ShouldHaveValidationErrorFor does not get validation error.

    public class CreateTagParametersValidator : AbstractValidator<CreateTagParameters>
    {
        public CreateTagParametersValidator()
        {
            RuleFor(req => req.OrganizationId).NotEmpty();
            RuleFor(req => req.Request).NotNull();
            RuleFor(req => req.Request.TagName).NotEmpty().When(req => req.Request != null);
            RuleForEach(req => req.Request.Assets).SetValidator(new TagAssetInstanceValidator()).When(req => req.Request != null);
        }
    }

    public class TagAssetInstanceValidator : AbstractValidator<TagAssetInstance>
    {
        public TagAssetInstanceValidator()
        {
            RuleFor(t => t.AssetId).NotEmpty();
        }
    }

    using AutoFixture.Xunit2;

    [Theory, AutoData]
    public void Should_Have_Error_For_Null_AssetId(
            CreateTagParametersValidator validator,
            CreateTagParameters parameters)
        {
           // No validation error here !?
            validator.ShouldHaveValidationErrorFor(parameters, p => p.Request.Assets[0], p => p.Request.Assets[0].AssetId = null);
        }

[ErikEJ]

Thanks a lot for your help - we really appreciate your work!

[JeremySkinner]

But it worked in 8.1.2 ??

I’m afraid not - it was a false-positive. We’ve never supported indexers inside expression trees like this, but older versions of the testhelper (including 8.1.2) actually silently failed to run when an indexer was encountered (leading to a false positive). This was fixed last year in the 8.5 release (which is why you’re now correctly getting an error), and I also added the string-based overload to work around this limitation.

The lambda-based overloads only support simple property access expressions, and for anything more complex (including indexers), you’ll need to use the string overloads.