Reusing validation rules

Hey @JeremySkinner

You’ve said repeatedly that FV is just for UI validation, but I got it working for domain validation as well.

However I need to duplicate rules again and again - e.g. if I want to validate a Product entity’s Name, then I need a validator and a validation rule for the entity itself, and for the create action, and the edit action, etc. This is not a unique problem, every large system has it.

I want to define a shared set of validation rules for a Product, and reuse them in various places. For example, to validate the create action’s CreateProductViewModel:

public class CreateProductViewModelValidator : AbstractValidator<CreateProductViewModel>
{

  public CreateProductValidator()
  {
    var productRules = new ProductRules<CreateProductViewModel>();
    productRules.Name(v => v.Name);
    productRules.Code(v => v.Code);
    // ...more rules...
  }

}

So instead of redefining rules, I just pick and choose from the shared rules.

Implementation of the shared rules class, which I copied from AbstractValidator.RuleFor():

public class ProductRules<T>
{

  // validation rule for "Name"
  public IRuleBuilderOptions<T, string> ruleForName(Expression<Func<T, string>> expression)
  {
    // first create the rule builder
    var rule = PropertyRule.Create(expression, () => CascadeMode.Continue);
    var ruleBuilder = new RuleBuilder<T, string>(rule);

    // now define the rule
    ruleBuilder.NotNull().NotEmpty().Length(1, 20);
    return ruleBuilder;
  }

  // validation rule for "Code"
  public IRuleBuilderOptions<T, int> ruleForCode(Expression<Func<T, int>> expression)
  {
    var rule = PropertyRule.Create(expression, () => CascadeMode.Continue);
    var ruleBuilder = new RuleBuilder<T, int>(rule);
    
    ruleBuilder.GreaterThan(0).LessThan(2000);
    return ruleBuilder;
  }

  // ...more rules...
}

So I can validate a Product in various places, e.g. Product, CreateProductViewModel, EditProductViewModel, CreateDiscountedProductViewModel, DeleteObsoleteProductCommand, etc. etc. Although the “shape” of each entity/viewmodel/DTO will differ, their properties are almost identical - so each has its own validator, but I can pick and choose which rules to run without repeating myself.

Have I broken FV somehow?

• The validator just includes a rule defined in another class instead of defining it itself, but I think the expression caching etc. will still work as normal?
• I used PropertyRule and RuleBuilder which you’ve defined in the “internal” FluentValidation.Internal namespace. So my code can break whenever FV is updated because I’m (ab)using internal methods. Is there a non-internal way to create those objects?

[JeremySkinner]

You may have already thought about this already and discounted it for some reason, but could you not just do this with extension methods & generics to make it a lot simpler?

public static class ProductRules {
  public static IRuleBuilderOptions<T, string> Name<T>(this IRuleBuilder<T,string> ruleBuilder) {
    return ruleBuilder.NotNull().NotEmpty().Length(1, 20);
  }
}

…and then use this from inside your CreateProductViewModelValidator like:

RuleFor(x => x.Name).Name();

…that way you don’t need to mess with PropertyRule or AddRule directly. Also makes it much easier to read and takes advantage of the existing fluent interface.

[granthoff1107]

@grokky1 I’m implementing a validator based off of entity state, Thanks, You just save me a day of troubles.