FluentValidation multiple validators

Another possibility would be to override Validate:

public override ValidationResult Validate(ValidationContext<FooBar> context)
{
    var fooResult = new FooValidator().Validate(context.InstanceToValidate);
    var barResult = new BarValidator().Validate(context.InstanceToValidate);

    var errors = new List<ValidationFailure>();
    errors.AddRange(fooResult.Errors);
    errors.AddRange(barResult.Errors);

    return new ValidationResult(errors);
}

"Including Rules. You can include rules from other validators provided they validate the same type."

public class PersonValidator : AbstractValidator<Person> {
    public PersonValidator() {
        Include(new PersonAgeValidator());
        Include(new PersonNameValidator());
    }
}

https://docs.fluentvalidation.net/en/latest/including-rules.html

RuleFor is trying to create a property-level rule. You can additionally use the AddRule function to add a general-purpose rule.

Using this, I created a composite rule proof of concept. It takes in a set of other validators and runs them. The yield break code came straight from FluentValidator's DelegateValidator. I wasn't sure what to do with it so I grabbed that from the source. I didn't trace its full purpose, but everything seems to work as is :)

Code

public interface IFoo
{
    int Id { get; set; }
    string Name { get; set; }
}

public interface IBar
{
    string Stuff { get; set; }
}

public class FooValidator : AbstractValidator<IFoo>
{
    public FooValidator()
    {
        RuleFor(x => x.Id).NotEmpty().GreaterThan(0);
    }
}

public class BarValidator : AbstractValidator<IBar>
{
    public BarValidator()
    {
        RuleFor(x => x.Stuff).Length(5, 30);
    }
}

public class FooBar : IFoo, IBar
{
    public int Id { get; set; }
    public string Name { get; set; }
    public string Stuff { get; set; }
}

public class CompositeValidatorRule : IValidationRule
{
    private IValidator[] _validators;

    public CompositeValidatorRule(params IValidator[] validators)
    {
        _validators = validators;
    }

    #region IValidationRule Members
    public string RuleSet
    {
        get; set;
    }

    public IEnumerable<ServiceStack.FluentValidation.Results.ValidationFailure> Validate(ValidationContext context)
    {
        var ret = new List<ServiceStack.FluentValidation.Results.ValidationFailure>();

        foreach(var v in _validators)
        {
            ret.AddRange(v.Validate(context).Errors);
        }

        return ret;
    }

    public IEnumerable<ServiceStack.FluentValidation.Validators.IPropertyValidator> Validators
    {
        get { yield break; }
    }
    #endregion
}

public class FooBarValidator : AbstractValidator<FooBar>
{
    public FooBarValidator()
    {
        AddRule(new CompositeValidatorRule(new FooValidator(), new BarValidator()));
    }
}

Base Test Case:

    [TestMethod]
    public void TestValidator()
    {
        FooBarValidator validator = new FooBarValidator();
        var result = validator.Validate(new FooBar());

    }

I hope this helps.