NullReferenceException in ValidationAttribute.

[MikhailPikhota]

How i may a valigate attribute?
Some changes to your example and have a NullReferenceException in non-default language:

[Localized]
public class LogOnModel
{
    [Required]
    [ValidateUserNameUnique]
    [Display(Name = "User name")]
    public string UserName { get; set; }
...
}

public class ValidateUserNameUnique : ValidationAttribute
{
    protected override ValidationResult IsValid(object value, ValidationContext validationContext)
    {
        var userName = value as string;
        return ValidationResult.Success;
    }
}