Message builder example

CustomMessageBuilder.cs

public static class Extensions {
  public static IRuleBuilderOptions<T, TProperty> Prefix<T, TProperty>(this IRuleBuilderOptions<T, TProperty> rule, string prefix) {
    return rule.Configure(cfg => {
      cfg.MessageBuilder = context => {
        // The MessageBuilderContext has lots of useful things on it, see https://github.com/JeremySkinner/FluentValidation/blob/master/src/FluentValidation/Internal/MessageBuilderContext.cs
        return prefix + " " + context.GetDefaultMessage();
      };
    });
}
  
// Usable like so:
  RuleFor(x => x.Surname).NotNull().Prefix("Foo");