通过自定义DataAnnotationsModelMetadataProvider添加StringLengthAttribute

Question

I have a requirement to add StringLengthAttribute validation to a lot of models in the existing ASP.NET MVC 4 project and am trying to do this automatically through my own model metadata provider derived from DataAnnotationsModelMetadataProvider .

It works perfectly with RequiredAttribute and some other data annotation attributes (I get both client-side and server-side validation working), however validation for the string length is not added - find the minimal example below.

public class CustomModelMetadataProvider : DataAnnotationsModelMetadataProvider
{
    protected override ModelMetadata CreateMetadata(IEnumerable<Attribute> attributes,
                                                    Type containerType,
                                                    Func<object> modelAccessor,
                                                    Type modelType,
                                                    string propertyName)
    {
        StringLengthAttribute lengthAttribute = new StringLengthAttribute(256);
        attributes = attributes.Union(new[] { lengthAttribute });
        return base.CreateMetadata(attributes,
                                   containerType,
                                   modelAccessor,
                                   modelType,
                                   propertyName);
    }
}

So, looks like StringLengthAttribute it being handled in some special way. Any ideas on how to make it work or a better implementation idea?

Answer 1

After playing around, I couldn't get the StringLength attribute to fire either.

Not ideal, but i guess an alternative solution is to the use a global ModelValidatorProvider class. Granted you won't get the built in Javascript that is provided by the StringLengthAttribute and you'd be writing your own logic, but its a possible quick fix for a problem you can solve later?

    public class MyCustomModelValidatorProvider : ModelValidatorProvider
{
    public override IEnumerable<ModelValidator> GetValidators(ModelMetadata metadata, ControllerContext context)
    {
        return new List<ModelValidator>() { new MyCustomModelValidator(metadata, context) };
    }

    public class MyCustomModelValidator : ModelValidator
    {
        public MyCustomModelValidator(ModelMetadata metadata, ControllerContext context)
            : base(metadata, context)
        {   }

        public override IEnumerable<ModelValidationResult> Validate(object container)
        {
            var model = this.Metadata.Model; 
            if (model is string)
            {
                var value = model as string;

                if (String.IsNullOrEmpty(value) || value.Length > 256)
                {
                    var validationResult = new ModelValidationResult();
                    validationResult.Message = (this.Metadata.DisplayName ?? this.Metadata.PropertyName) 
                        + " needs to be no more then 256 characters";

                    return new List<ModelValidationResult>() { validationResult };
                }
            }

            return new List<ModelValidationResult>();
        }
    }

}

Add MyCustomModelValidator to the collection in Global.asax

        protected void Application_Start()
    {
        AreaRegistration.RegisterAllAreas();

        // Use LocalDB for Entity Framework by default
        Database.DefaultConnectionFactory = new SqlConnectionFactory(@"Data Source=(localdb)\v11.0; Integrated Security=True; MultipleActiveResultSets=True");

        RegisterGlobalFilters(GlobalFilters.Filters);
        RegisterRoutes(RouteTable.Routes);


        ModelValidatorProviders.Providers.Add(new MyCustomModelValidatorProvider());
    }