InvalidCastException when using ForEach rule

FluentValidation version

10.0.4

ASP.NET version

.NET Core 3.1

Summary

I have a rulebuilder for a IList typed property with three rules: notEmpty, foreach and mustAsync. The problem is that the foreach rule returns a rulebuilder with out type ‘IEnumerable<TElement>’ but the next rule seems to expect the original property type IList since there is an invalid cast exception from …IList… to …IEnumerable…

This rule did not give any exception in FluentValidation version 9.3.0.

Steps to Reproduce

The code Dto to validate

    public class SaveRegistrationRequestForHardwareUnit : CommandWithTransactionToggle<SuccessOrFailureDto>
    {
        //other props
        public IList<KeyValuePair<Guid,string>> HardwareUnitRoles { get; set; }
    }

Validator

public class SaveRegistrationRequestForHardwareUnitValidator : AbstractValidator<SaveRegistrationRequestForHardwareUnit>
{
    private const string ValidRoleConfigMessage = "ValidRoleConfigMessage";

    public SaveRegistrationRequestForHardwareUnitValidator(IValidator<UserDto> userDtoValidator)
    {
        //Other rules

        RuleFor(r => r.HardwareUnitRoles).Cascade(CascadeMode.Stop)
            .NotEmpty()
            .ForEach(r => r
                .Must(kv => HardwareUnitUserRoles.IsValid(kv.Value))
                    .WithMessage(m => Resources.ValidationMessages.hardwareunit_role_notvalid)
                .Must(kv => kv.Value != HardwareUnitUserRoles.Owner)
                    .WithMessage(m => Resources.ValidationMessages.registrationrequest_hardwareUnitRole_notallowed)
            )
            .MustAsync((roles, token) => ValidRoleConfigAsync(roles))//GIVES EXCEPTION
                .WithMessage(dto => ValidationErrorMessages.GetErrorMessage(ValidRoleConfigMessage));
    }

    private async Task<bool> ValidRoleConfigAsync(IEnumerable<KeyValuePair<Guid, string>> hardwareUnitRoles)
    {
        //logic
    }
}

Exception:

System.InvalidCastException: Unable to cast object
of type 'FluentValidation.Internal.RuleBuilder`2[Foo.Commands.Registration.SaveRegistrationRequestForHardwareUnit,System.Collections.Generic.IList`1[System.Collections.Generic.KeyValuePair`2[System.Guid,System.String]]]'
to type 'FluentValidation.Internal.RuleBuilder`2[Foo.Commands.Registration.SaveRegistrationRequestForHardwareUnit,System.Collections.Generic.IEnumerable`1[System.Collections.Generic.KeyValuePair`2[System.Guid,System.String]]]'.

[JeremySkinner]

I’ve published the 10.1 release to nuget with this fix.

[JeremySkinner]

I’ve committed a change in 6e825d33 which will allow this to work in 10.x without needing to introduce breaking changes (adds an additional interface, which can be removed again in v11). Well, technically there is one breaking change to the return type of one undocumented public method (DefaultValidatorOptions.Configurable), but I think this is worth it to get the fix in place. I’ll try and get this pushed out to nuget over the weekend.

With this fix, the underlying problem will still be present if the user calls the Configure method following a call to ForEach, but for all other scenarios this will fix the issue. The use of Configure will need to wait until 11.

I’ll let you know once this is pushed to nuget so you can give it a try.