New Post: Custom Validator With Arguments

Yes, this is perfectly possible. All the built-in validators (such as equal/length etc) are built as custom property validators under the covers. Essentially, you just take parameters as constructor arguments for your custom validator. Here's a snippet from the FluentValidation code for Length:


This is the extension method that gives the syntax:

public static class Extensions {
  public static IRuleBuilderOptions<T, string> Length<T>(this IRuleBuilder<T, string> ruleBuilder, int min, int max) {
    return ruleBuilder.SetValidator(new LengthValidator(min, max));
  }
}

...and the underlying validator:

public class LengthValidator : PropertyValidator, ILengthValidator {
        public int Min { get; private set; }
        public int Max { get; private set; }

        public LengthValidator(int min, int max) : this(min, max, () => Messages.length_error) {
        }

        public LengthValidator(int min, int max, Expression<Func<string>> errorMessageResourceSelector) : base(errorMessageResourceSelector) {
            Max = max;
            Min = min;

            if (max != -1 && max < min) {
                throw new ArgumentOutOfRangeException("max", "Max should be larger than min.");
            }
        }

        protected override bool IsValid(PropertyValidatorContext context) {
            if (context.PropertyValue == null) return true;

            int length = context.PropertyValue.ToString().Length;

            if (length < Min || (length > Max && Max != -1)) {
                context.MessageFormatter
                    .AppendArgument("MinLength", Min)
                    .AppendArgument("MaxLength", Max)
                    .AppendArgument("TotalLength", length);

                return false;
            }

            return true;
        }
    }

...so essentially the custom arguments are passed to the custom validator through its constructor. If you'd like to see more examples, check out the implementation in the FV code:

https://github.com/JeremySkinner/FluentValidation/tree/master/src/FluentValidation/Validators

Jeremy