Question

Apparently it is possible to dynamically attach DataAnnotation attributes to object properties at runtime and as such achieve dynamic validation.

Can someone provide code sample on this?

Answer 1

MVC has a hook to provide your own ModelValidatorProvider. By default MVC 2 uses a sub class of ModelValidatorProvider called DataAnnotationsModelValidatorProvider that is able to use System.DataAnnotations.ComponentModel.ValidationAttribute attributes for validation.

The DataAnnotationsModelValidatorProvider uses reflection to find all the ValidationAttributes and simply loops through the collection to validate your models. All you need to do is override a method called GetValidators and inject your own attributes from whichever source you choose. I use this technique to do convention validations, the properties with DataType.Email attribute always gets passed through a regex, and use this technique to pull information from the database to apply more restrictive validations for “non-power” users.

The following example simply says “always make any FirstName properties required”:

 public class CustomMetadataValidationProvider : DataAnnotationsModelValidatorProvider
 {
    protected override IEnumerable<ModelValidator> GetValidators(ModelMetadata metadata, ControllerContext context, IEnumerable<Attribute> attributes)
    {
        //go to db if you want
        //var repository = ((MyBaseController) context.Controller).RepositorySomething;

        //find user if you need it
        var user = context.HttpContext.User;

        if (!string.IsNullOrWhiteSpace(metadata.PropertyName) && metadata.PropertyName == "FirstName")
            attributes = new List<Attribute>() {new RequiredAttribute()};

        return base.GetValidators(metadata, context, attributes);
    }
}

All you have to do is register the provider in your Global.asax.cs file:

    protected void Application_Start()
    {
        ModelValidatorProviders.Providers.Add(new CustomMetadataValidationProvider());

        AreaRegistration.RegisterAllAreas();

        RegisterRoutes(RouteTable.Routes);
    }

The end result:

with this model:

public class Person
{
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public DateTime Birthday { get; set; }
}