Localized Attribute parameters in C#

Question

In C#, attribute parameters require to be a constant expression, typeof or array creation expression.

Various libraries, like for example Castle validator, allow specifying passing what seems like localized error messages to attribute constructor:

//this works
[ValidateNonEmpty("Can not be empty")]

//this does not compile
[ValidateNonEmpty(Resources.NonEmptyValidationMessage)]

Is there any way how to approach this problem and localize these arguments?

In case there is no workaround for this when using Castle Validator, is there a validation library similar to Castle Validator that allows localization of validation messages?

EDIT: I found how Data Annotations validation library approaches this problem. Very elegant solution: http://haacked.com/archive/2009/12/07/localizing-aspnetmvc-validation.aspx

Answer 1

It works out of the box:

    [ValidateNonEmpty(
        FriendlyNameKey = "CorrectlyLocalized.Description",
        ErrorMessageKey = "CorrectlyLocalized.DescriptionValidateNonEmpty",
        ResourceType = typeof (Messages)
        )]
    public string Description { get; set; }

Answer 2

We had a similar issue, although not with Castle. The solution we used was simply to define a new attribute which was derived from the other one, and which used the constant string as a lookup to the resources manager, and fell back to the key string itself if none was found.

[AttributeUsage(AttributeTargets.Class
  | AttributeTargets.Method
  | AttributeTargets.Property
  | AttributeTargets.Event)]
public class LocalizedIdentifierAttribute : ... {
  public LocalizedIdentifierAttribute(Type provider, string key)
    : base(...) {
    foreach (PropertyInfo p in provider.GetProperties(
      BindingFlags.Static | BindingFlags.NonPublic)) {
      if (p.PropertyType == typeof(System.Resources.ResourceManager)) {
        ResourceManager m = (ResourceManager) p.GetValue(null, null);

        // We found the key; use the value.
        return m.GetString(key);
      }
    }

    // We didn't find the key; use the key as the value.
    return key;
  }
}

Usage is something like:

[LocalizedIdentifierAttribute(typeof(Resource), "Entities.FruitBasket")]
class FruitBasket {
  // ...
}

Then each locale-specific resource file can define its own Entities.FruitBasket entry, as needed.