Question

I am trying to write a NotAttribute validator attribute for my MVC3 project. I want to use it like this:

[Not("00000000-0000-0000-0000-000000000000", typeof(Guid), ErrorMessage = "You must select a valid id.")]
public Guid ObjectId { get; set; }

Now I don’t want to tie the NotAttribute to a Guid, but the object should be parseable from a string; I want to look for a TryParse method first, then a Parse method; if both should fail, and the type is not already a string, I want to throw an error. So far, so good, no complications.

My class looks like this:

public class NotAttribute : ValidationAttribute, IClientValidatable
{
    public Type PropertyType { get; set; }
    public string NotEqualTo { get; private set; }
    public bool IgnoreWhitespace { get; set; }
    public bool CaseInsensitive { get; set; }

    public NotAttribute(string notEqualTo)
    {
        NotEqualTo = notEqualTo;
    }

    protected override ValidationResult IsValid(object value, ValidationContext validationContext)
    {
        if (PropertyType == null) PropertyType = typeof(string);
        if (PropertyType == typeof(string))
        {
            //this is the easy case.  We can compare easy enough
            var leftSide = value.ToStringOrEmpty();
            var rightSide = NotEqualTo ?? string.Empty;

            if (IgnoreWhitespace)
            {
                leftSide = leftSide.Trim();
                rightSide = rightSide.Trim();
            }

            if (CaseInsensitive)
            {
                leftSide = leftSide.ToUpperInvariant();
                rightSide = rightSide.ToUpperInvariant();
            }

            if (leftSide != rightSide) return null; // all is well
        }

        var tryParseMethod = PropertyType.GetMethod("TryParse",
            Reflection.BindingFlags.Static | Reflection.BindingFlags.Public, null,
            new Type[] {typeof(string), PropertyType.MakeByRefType()}, null);

        //This is where I get lost


        return base.IsValid(value, validationContext);
    }

    public IEnumerable<ModelClientValidationRule> GetClientValidationRules(ModelMetadata metadata, ControllerContext context)
    {
        throw new NotImplementedException();
    }
}

I’m pretty sure my tryParseMethod will return a reference to the object method that I could theoretically invoke, but here’s where I run into problems.

How would I invoke it, passing in a type determined at runtime? This seems to be screaming for a generics implementation, but I have no idea where I could inject that here. Also, once I have the value I need to compare against the “not” value, I need to somehow cast it at runtime, which also screams for generics.

How can I make this work? Am I doing this completely the wrong way?

Answer 1

It’s a static method, so you should just be able to invoke it without needing an instance of a class of the type PropertyType to invoke on (See the MSDN page). So, you can just call Invoke on tryParseMethod.

However, the TryParse method itself will need an instance of a class of the type PropertyType as it’s out parameter though, so you’ll need to use System.Activator.CreateInstance() for that.

So, presumably something like

var parsedObject = System.Activator.CreateInstance(PropertyType);
bool parseSuceeded = tryParseMethod.Invoke(null, new [] { value, parsedObject });

will do the job.