Issue with base response type

I try to implement validation behavior in asp.net core 2.1 application. My controller:

[HttpPost]
public async Task<CommandResponse<Application>> Post([FromBody] CreateApplicationCommand command)
{
    var response = await _mediator.Send(command);
    return response;
}

ValidationBehavior:

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

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

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

        return failures.Any()
            ? Errors(failures)
            : next();
    }

    private static Task<TResponse> Errors(IEnumerable<ValidationFailure> failures)
    {
        var response = new CommandResponse();

        foreach (var failure in failures)
        {
            response.AddError(failure.ErrorMessage);
        }

        return Task.FromResult(response as TResponse);
    }
}

Response:

public class CommandResponse
{
    private readonly IList<string> _errorMessages = new List<string>();

    public void AddError(string errorMessage)
    {
        _errorMessages.Add(errorMessage);
    }

    public bool IsSucceed => !_errorMessages.Any();

    public IReadOnlyCollection<string> Errors => new ReadOnlyCollection<string>(_errorMessages);
}

public class CommandResponse<TModel> : CommandResponse where TModel : class
{
    public CommandResponse(TModel model)
    {
        Result = model;
    }

    public TModel Result { get; }
}

Container registration (Asp.Net Core DI with Scrutor):

public static void AddMediator(this IServiceCollection services)
{
    services.AddScoped<ServiceFactory>(p => p.GetService);
    services.AddScoped<IMediator, Mediator>();

    services.Scan(scan => scan
        .FromAssemblies(Assembly.GetExecutingAssembly())
        .AddClasses(classes => classes.AssignableTo(typeof(IRequestHandler<,>)))
            .AsImplementedInterfaces()
            .WithScopedLifetime()
        .AddClasses(classes => classes.AssignableTo(typeof(IValidator<>)))
            .AsImplementedInterfaces()
            .WithScopedLifetime());

    services.AddScoped(typeof(IPipelineBehavior<,>), typeof(ValidationBehavior<,>));
}

If CreateApplicationCommand is valid everything works fine. When validation failure occures I expect to receive an instance of CommandResponse class with error messages. However response in controller in this case is null. What I am doing wrong?

[vertonghenb]

@MaciejWierzchowski Fixed using an Activator

        private static Task<TResponse> Errors(IEnumerable<ValidationFailure> failures)
        {
            var response = Activator.CreateInstance<TResponse>();
            foreach (var failure in failures)
            {
                response.AddError(failure.ErrorMessage);
            }

            return Task.FromResult(response);
        }

Caveat : Your TResponse should have a default constructor

[lilasquared]

Yep, basically what @joaomatossilva said. A base class of that can be Success or Failure, and the Failure result has properties for validation failure messages or any other metadata needed. All handled by a pipeline behavior that injects IValidators similar to what you have above. All request responses are wrapped in the particular Result response type and we have custom interfaces to help simplify this as well when creating commands, queries and handlers.