方法 MustAsync() 与 WhenAsync() FluentValidation 之间的差异

Question

I implement an API with .net 5 and I use the NuGet FluentValidation. I try to understand what is the difference between the two methods

MustAsync()

WhenAsync()

and when must I use each of them. It is better if you can provide an example.

Answer 1

The WhenAsync() method: Specifies an asynchronous condition limiting when the validator should run. The validator will only be executed if the result of the lambda returns true.

The MustAsync() method: Defines an asynchronous predicate validator on the current rule builder using a lambda expression to specify the predicate. Validation will fail if the specified lambda returns false. Validation will succeed if the specified lambda returns true.

You could go to the method definition and check the used of these methods.

Both of the above methods are used to define asynchronous rules, for example, the following code will check if a user ID already exists:

public class CustomerValidator : AbstractValidator<Customer> {
  SomeExternalWebApiClient _client;

  public CustomerValidator(SomeExternalWebApiClient client) {
    _client = client;

    RuleFor(x => x.Id).MustAsync(async (id, cancellation) => {
      bool exists = await _client.IdExists(id);
      return !exists;
    }).WithMessage("ID Must be unique");
  }
}

Then, invoke the validator by calling ValidateAsync method.

var validator = new CustomerValidator(new SomeExternalWebApiClient());
var result = await validator.ValidateAsync(customer);

More detail information, refer the following articles:

Asynchronous Validation

FluentValidation