FluentValidation is requesting an IValidator<string>

Question

We're using fluent validation in a web API.

There are a couple of places where the endpoints accept view models which have a property of type string[].

I have a problem that whenever we use one of these endpoints, if the string array contains any elements, fluent validation requests an IValidator<string> from the validation factory (which is based on DI). So it seems fluent validation seems to want an IValidator<string> to use as a collection validator.

We haven't defined an IValidator<string> so we get a component not found exception from the DI container. Neither do we want a validator defined for a single string.

Why is fluent validation requesting a IValidator<string> for string collections, when it's not for normal string properties? Can I tell it not to?

Answer 1

That's my solution for problem of requesting validators that haven't registered in IoC container:

public class ServiceLocatorValidatorFactory : ValidatorFactoryBase
{
    public override IValidator CreateInstance(Type validatorType)
    {
        // to improve perfomance, you can reject frequently requested type before asking IoC container
        if (validatorType == typeof(string))
            return null;

        try
        {
            return ServiceLocator.Instance.GetService(validatorType) as IValidator;
        }
        catch (Exception e)
        {
            // additionally you can write information about requested type to the log
            return null;
        }
    }
}

Catching IoC exceptions helps me to filter unnecessary validators. Basically good idea is to log missed validator types to prevent spending long time to debug, when you forget to register validator that already exists.

Answer 2

Why is fluent validation requesting a IValidator for string collections

It's hard to say without seeing your validation configuration, but I'd imagine it's because FluentValidation is trying to resolve the type stored within your collection (in this case, a string).

As far as FluentValidation and your IoC container is concerned, a type is a type - which is why your IoC container is trying to resolve an instance of IValidator<string>.

Can I tell it not to?

You can tell it not to by checking the type upfront within your ValidatorFactory as suggested by Evgeny Levin (copied below).

public class ServiceLocatorValidatorFactory : ValidatorFactoryBase
{
    public override IValidator CreateInstance(Type validatorType)
    {
        if (validatorType == typeof(string))
            return null;
        ...
    }
}

Alternatively you can let your IoC container try resolve it and return null if unable to resolve the dependency using a method like TryGetInstance() (using StructureMap in this instance)

public override IValidator CreateInstance(Type validatorType)
{
    return this.serviceLocator.GetInstance(validatorType) as IValidator;
}