Why can't I use resources as ErrorMessage with DataAnnotations?

When you are using the ErrorMessage property only constant strings or string literal can be assigned to it.

Use the ErrorMessageResourceType and ErrorMessageResourceName instead to specity your resources.

[RegularExpression(    "^[a-zA-Z0-9_]*$",     ErrorMessageResourceType=typeof(Resources.RegistrationModel),    ErrorMessageResourceName= "UsernameError")]

Note that the resources must be public (can be set in the resource editor).

Please see this link: http://code.msdn.microsoft.com/Getting-Started-WCF-RIA-1469cbe2/sourcecode?fileId=19242&pathId=774666288 (link broken, but left for attribution purposes)

public sealed partial class RegistrationData {     [Key]     [Required(ErrorMessageResourceName = "ValidationErrorRequiredField", ErrorMessageResourceType = typeof(ErrorResources))]     [Display(Order = 0, Name = "UserNameLabel", ResourceType = typeof(RegistrationDataResources))]     [RegularExpression("^[a-zA-Z0-9_]*$", ErrorMessageResourceName = "ValidationErrorInvalidUserName", ErrorMessageResourceType = typeof(ErrorResources))]     [StringLength(255, MinimumLength = 4, ErrorMessageResourceName = "ValidationErrorBadUserNameLength", ErrorMessageResourceType = typeof(ErrorResources))]     public string UserName { get; set; } 

Try FluentModelMetaDataProvider.

Managed to use resources for error messages in strongly typed fashion.

Looks like this:

using System.Web.Mvc.Extensibility;namespace UI.Model{    public class StoreInputMetadata : ModelMetadataConfigurationBase<StoreInput>    {        public StoreInputMetadata()        {            Configure(m => m.Id)                .Hide();            Configure(model => model.Name)                .Required(Resources.Whatever.StoreIsRequired)                .MaximumLength(64, Resources.Whatever.StoreNameLengthSomething);        }    }}

What is the error message telling me?

An attribute argument must be a constant expression, typeof expression or array creation expression of an attribute parameter type.

It's already self explanatory. C# isn't dynamic language like Ruby where You can write classes that inherits random base class at runtime. :)

Here's what Skeet says about this.