FluentValidation 8.1 is now available. This release contains several new features and bug fixes:

String Formatting in Message Placeholders

FluentValidation has always supported placeholders within error messages, such as {PropertyValue}:

RuleFor(x => x.Age).GreaterThan(18).WithMessage("Must be older than 18 years. You entered {PropertyValue} years.");

Now you can also specify standard .NET formatting strings as part of the placeholder, such as {PropertyValue:d} or {PropertyValue:p1} etc.

Overriding Indexers for Collection Rules

Collection rules built using RuleForEach now allow you to override the indexer. For example, imagine you defined a collection rule against an collection of Address lines:

RuleForEach(x => x.AddressLines).NotNull();

…Then the validator would generate validation failures for properties person.AddressesLines[0] and person.AddressLines[1] etc. The indexer would always be the numeric index within the collection surrounded by square brackets. This can now be overridden to use something different, or remove the indexer completely. Eg:

v.RuleForEach(x => x.AddressLines)
  .OverrideIndexer((x, collection, element, index) => "<" + index + ">")
  .NotNull()

This would use angle brackets rather than square brackets in the generated indexer.

Error code can now be used to select a different default error message.

You can now use error codes to point to a particular error message stored in the default language manager. For example, the default ‘NotNull’ validation message has a code of NotNullValidator and a message of '{PropertyName}' must not be empty. If you want to use this message from inside a different validator (eg for a custom validator), you would have to hard-code the message again:

RuleFor(x => x.Name).NotNull(); //defaults to '{PropertyName}' must not be empty.
// `Must` has its own default error message, so if you want to use the NotNull message you'd have to specify it explicitly
RuleFor(x => x.Name).Must(name => name != null).WithMessage("'{PropertyName}' must not be empty.");

However now you can select a message based on error code:

RuleFor(x +> x.Name).Must(name => name != null).WithErrorCode("NotNullValidator"); 

Test Helper improvements

Additional details are now shown in the output for ShouldNotHaveValidationErrorFor.

Conditional validation improvements

If you use the top-level When or Unless methods to wrap multiple rules, the condition check is now cached and only executed once.

// The IsStudnet check will now only be executed once.
When(x => x.IsStudent, () => {
   RuleFor(x => x.StudentNumber).NotNull();
   RuleFor(x => x.Courses).NotNull();
});

There’s also a new Otherwise method that can now be chanined onto a When or Unless to do its opposite, without having to set up a second When call with the opposite condition:

When(x => x.IsStudent, () => {
   RuleFor(x => x.StudentNumber).NotNull();
   RuleFor(x => x.Courses).NotNull();
}).Otherwise(() => {
   RuleFor(x => x.StudentNumber).Null();
});

Other minor changes

• MVC 5’s CustomizeValidatorAttribute now has the Skip property, like WebApi and AspNetCore, allowing auto-validation to be skipped for specific action parameters.
• Japanese language translation
• The overload of OverridePropertyName that takes an expression can now use an expression with any return value, not just strings.
• Minor wording changes to the default English error messages.