MustNot and MustNotAsync API

Is your feature request related to a problem? Please describe.

When I want to apply MustNotAsync, I have to do something like this and I think we can make it neater

public Validator(ICameraRepository cameraRepository)
{
    RuleFor(e => e.SerialNumber)
        .Cascade(CascadeMode.Stop)
        .NotEmpty()
        .MustAsync(async (serialNumber, cancellationToken) => !(await cameraRepository.ContainsSerialNumberAsync(serialNumber, cancellationToken)))
}

Describe the solution you’d like

I suggest to add MustNot and MustNotAsync as new apis. It will be used like this

public Validator(ICameraRepository cameraRepository)
{
    RuleFor(e => e.SerialNumber)
        .Cascade(CascadeMode.Stop)
        .NotEmpty()
        .MustNotAsync(cameraRepository.ContainsSerialNumberAsync);
}

Describe alternatives you’ve considered

No response

Additional Context

No response

[dalefrancis88]

-1 for this feature. While it may seem simple, this is not something that should be the responsibility of the maintainer. This is the equivalent of asking to add ifNot to the base language. The maintainers work hard enough in their spare times to build out the base level features and adding trivial pieces such as this has a net negative on the cost/reward scale, it increases the burden for next to no value.

For those that find this you can implement this quite simply in two ways either wrap your predicate function in a Not function you import from a static class or implement your own extension. Both can be seen below.

public static class Functions
{
    public static Func<T, bool> Not<T>(Func<T, bool> predicate)
        => data => !predicate(data);
    public static Func<T, Task<bool>> Not<T>(Func<T, Task<bool>> predicate)
        => async data => !(await predicate(data));
}

public Validator(ICameraRepository cameraRepository)
{
    RuleFor(e => e.SerialNumber)
        .Cascade(CascadeMode.Stop)
        .NotEmpty()
        .MustAsync(Not(cameraRepository.ContainsSerialNumberAsync));
}

if if you want you can create your own extensions as seen belo

public static class ExtraValidatorExtensions
{
    public static IRuleBuilderOptions<T, TProperty> MustNot<T, TProperty>(this IRuleBuilder<T, TProperty> ruleBuilder, Func<TProperty, bool> predicate) =>
        ruleBuilder.Must(val => !predicate(val));
    public static IRuleBuilderOptions<T, TProperty> MustNotAsync<T, TProperty>(this IRuleBuilder<T, TProperty> ruleBuilder, Func<TProperty, CancellationToken, Task<bool>> predicate) => 
        ruleBuilder.MustAsync(async (val, cancel) => !(await predicate(val, cancel)));
}

[JeremySkinner]

Hi, thanks for the suggestion!

I’m not currently planning on adding any new validators to the core library, but this is something that I’ll consider for a future version sometime after 11.0 has shipped. However if you want to use these as methods in your own project you could implement them as extension methods that wrap the existing Must/MustAsync