Question

Basically, what the title says. I have several properties that combine together to really make one logical answer, and i would like to run a server-side validation code (that i write) which take these multiple fields into account and hook up to only one validation output/error message that users see on the webpage.

I looked at scott guthries method of extending an attribute and using it in your dataannotations declarations, but, as i can see, there is no way to declare a dataannotations-style attribute on multiple properties, and you can only place the declarations (such as [Email], [Range], [Required]) over one property :(.

i have looked at the PropertiesMustMatchAttribute in the default mvc 2.0 project that appears when you start a new project, this example is as useful as using a pair of pins to check your motor oil – useless!

i have tried this method, however, creating a class level attribute, and have no idea how to display the error from this in my aspx page. i have tried html.ValidationMessage(“ClassNameWhereAttributeIsAdded”) and a variety of other things, and it has not worked. and i should mention, there is NOT ONE blog post on doing validation at this level – despite this being a common need in any project or business logic scenario!

can anyone help me in having my message displayed in my aspx page, and also if possible a proper document or reference explaining validation at this level?

Answer 1

Now that you’ve looked at Data Annotations and arrived to the conclusion that they are not adapted to your scenario I would suggest you looking at FluentValidation, its integration with ASP.NET MVC and the way you would unit test your validation logic – you won’t be disappointed (I really have nothing against Data annotations, they are great for blog posts and tutorials but once you are confronted to real world applications you quickly realize the limits).

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UPDATE:

As requested in the comments section here’s an example of using the FluentValidation framework with one server-side validation function accessing multiple properties (please don’t do this as it is ugly and there’s a better way):

class AuthInfo
{
    public string Username { get; set; }
    public string Password { get; set; }
    public string ConfirmPassword { get; set; }
}

class AuthInfoValidator : AbstractValidator<AuthInfo>
{
    public override ValidationResult Validate(AuthInfo instance)
    {
        var result = base.Validate(instance);
        if (string.IsNullOrEmpty(instance.Username))
        {
            result.Errors.Add(new ValidationFailure("Username", "Username is required"));
        }
        if (string.IsNullOrEmpty(instance.Password))
        {
            result.Errors.Add(new ValidationFailure("Password", "Password is required"));
        }
        if (string.IsNullOrEmpty(instance.ConfirmPassword))
        {
            result.Errors.Add(new ValidationFailure("ConfirmPassword", "ConfirmPassword is required"));
        }
        if (instance.Password != instance.ConfirmPassword)
        {
            result.Errors.Add(new ValidationFailure("ConfirmPassword", "Passwords must match"));
        }
        return result;
    }
}

The more natural way to do this is the following (it is also immune to property rename as it contains no magic strings):

class AuthInfoValidator : AbstractValidator<AuthInfo>
{
    public AuthInfoValidator()
    {
        RuleFor(x => x.Username)
            .NotEmpty()
            .WithMessage("Username is required");

        RuleFor(x => x.Password)
            .NotEmpty()
            .WithMessage("Password is required");

        RuleFor(x => x.ConfirmPassword)
            .NotEmpty()
            .WithMessage("ConfirmPassword is required");

        RuleFor(x => x.ConfirmPassword)
            .Equal(x => x.Password)
            .WithMessage("Passwords must match");
    }
}