FluentValidation within MediatR pipeline using .NET Core's default DI container

I would greatly appreciate it if someone could help me to integrate FluentValidation into the MediatR pipeline using .NET Core’s default DI container.

I have the following classes which represent my request, my handler, and my FluentValidation Validator: `

public class SimpleRequestRequest : IRequest<string>
{
	public int Id { get; set; }
}

public class SimpleRequestRequestValidator : AbstractValidator<SimpleRequestRequest>
{

	public SimpleRequestRequestValidator(SimpleRequestRequest request)
	{
	
		RuleFor(m => m.Id).MustAsync(BeValidIdAsync).WithMessage("Id must be greater than 0, it is {PropertyValue}");
	}

	private Task<bool> BeValidIdAsync(int id, CancellationToken cancellationToken)
	{
		return Task.FromResult( id > 0);
	}
}

public class SimpleRequestHandler : IRequestHandler<SimpleRequestRequest, string>
{
	public Task<string> Handle(SimpleRequestRequest request, CancellationToken cancellationToken)
	{
		return Task.FromResult($"Request was for {request.Id}");
	}
}

`

In order to inject FluentValidation, I have created the following behavior class: `

public class ValidationBehavior<TRequest, TResponse> : IPipelineBehavior<TRequest, TResponse> where TRequest : IRequest<TResponse>
{
	private readonly IEnumerable<IValidator<TRequest>> _validators;

	public ValidationBehavior(IEnumerable<IValidator<TRequest>> validators)
	{
		_validators = validators;
	}

	public Task<TResponse> Handle(TRequest request, CancellationToken cancellationToken, RequestHandlerDelegate<TResponse> next)
	{
		var context = new ValidationContext(request);
		var failures = _validators
			.Select(v => v.Validate(context))
			.SelectMany(result => result.Errors)
			.Where(f => f != null)
			.ToList();

		if (failures.Count != 0)
		{
			throw new ValidationException(failures);
		}

		return next();
	}
}

`

Within my ASP.NET Core (2.1) WebApi, I have the following controller: `

[Route("api/[controller]")]
[ApiController]
public class TestController : ControllerBase
{
    private readonly IMediator _mediator;

    public TestController(IMediator mediator)
    {
	    _mediator = mediator;
    }

	// GET api/values/5
	[HttpGet("{id}")]
    public Task<string> Get(int id)
	{
		return _mediator.Send(new SimpleRequestRequest {Id = id});
    }

}

`

What I would like to have happen is when the controller calls _mediator.Send, I would like the Validation to run. I have this working in a non .NET core solution using the IRequestPreProcessor interface. I am just trying to update things so that I can do the same type of thing within .NET Core.

I believe my struggle is in registering the services. This is in the Startup.cs file. `

	public void ConfigureServices(IServiceCollection services)
	{
		services.AddMvc()
			.SetCompatibilityVersion(CompatibilityVersion.Version_2_1)
			.AddFluentValidation(fv=> fv.RegisterValidatorsFromAssemblyContaining(typeof(SimpleRequestRequestValidator)));
		
		services.AddMediatR(typeof(SimpleRequestHandler).Assembly);
		services.AddTransient(typeof(IPipelineBehavior<,>), typeof(ValidationBehavior<,>));
		AssemblyScanner.FindValidatorsInAssembly(typeof(SimpleRequestHandler).Assembly)
			.ForEach(result =>
				{
					Console.WriteLine("Here we are");
					//services.AddScoped(result.InterfaceType, result.ValidatorType);
					services.AddTransient(result.InterfaceType, result.ValidatorType);
				}
				);
	}

`

Within this, I don’t think I need to call AddFluentValidation on the AddMvc builder. I really am not trying to overwrite the validation on the Web-API controllers. I just want it on the MediatR portion.

The issue I am running into, however, is when I call the WebAPI controller. It gets to _mediator.Send, and it throws the following error: Unable to resolve service for type ‘MediatrSample.DLL.BLL.Requests.SimpleRequestRequest’ while attempting to activate ‘MediatrSample.DLL.BLL.Requests.SimpleRequestRequestValidator’.

It is like the DI container for .NET Core doesn’t know how to create the SimpleRequestRequest to pass into the Validator.

Any thoughts or ideas? I have found a number of articles that use things like AutoFac for the DI Container, but I am trying to use the default DI in .NET Core.

Ideally, I don’t want to have to register all validation and request classes. I would rather it discovers it automatically. I thought that is what I was doing with the AssemblyScanner, but apparently not.

Thank you!

[dbeylkhanov]

maybe it will be useful for other developers who will face the issue like me. So my issue was in collection type for IValidator<TRequest> validators, because I was using IList and was getting the error something like - Unable to resolve service for type 'System.Collections.Generic.IList1[FluentValidation.IValidator... Finally, after replacing the IList type on IEnumerable, this error was disappeared. It means that the container can be set up to resolve only for IEnumerable collection

[Lapeno94]

I know it is late to answer but u can use: services.AddValidatorsFromAssemblyContaining(typeof(yourCommandValidator));