Question

Why can’t I do like this?

[Required(ErrorMessage = "*")]
[RegularExpression("^[a-zA-Z0-9_]*$", ErrorMessage = Resources.RegistrationModel.UsernameError)]
public string Username { get; set; }

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.

Answer 1

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).