Custom Error Codes

A custom error code can also be associated with validation rules by calling the WithErrorCode method:

public class PersonValidator : AbstractValidator<Person> {
  public PersonValidator() {
    RuleFor(person => person.Surname).NotNull().WithErrorCode("ERR1234");        
    RuleFor(person => person.Forename).NotNull();

The resulting error code can be obtained from the ErrorCode property on the ValidationFailure:

var validator = new PersonValidator();
var result = validator.Validate(new Person());
foreach (var failure in result.Errors) {
  Console.WriteLine($"Property: {failure.PropertyName} Error Code: {failure.ErrorCode}");

The output would be:

Property: Surname Error Code: ERR1234
Property: Forename Error Code: NotNullValidator

ErrorCode and Error Messages

The ErrorCode is also used to determine the default error message for a particular validator. At a high level:

  • The error code is used as the lookup key for an error message. For example, a NotNull() validator has a default error code of NotNullValidator, which used to look up the error messages from the LanguageManager. See the documentation on localization.
  • If you provide an error code, you could also provide a localized message with the name of that error code to create a custom message.
  • If you provide an error code but no custom message, the message will fall back to the default message for that validator. You’re not required to add a custom message.
  • Using ErrorCode can also be used to override the default error message. For example, if you use a custom Must() validator, but you’d like to reuse the NotNull() validator’s default error message, you can call WithErrorCode("NotNullValidator") to achieve this result.