IRequestPostProcessor executed multiple times

I was originally using IPipelineBehavior to add pre and post hooks to a mediator pipeline but have then swapped out these to use IRequestPreProcessor and IRequestPostProcessor respectfully.

Unfortunately doing so causes my handlers to be executed twice. Contrary to the docs, I’m not registering them explicitly as they appear to be getting registered automatically.

MediatR is registered like so (ASP.NET Core 2.2):

            services.AddMediatR(typeof(PaymentsRegistry).Assembly);
            services.AddTransient(typeof(IPipelineBehavior<,>), typeof(LoggingMediatorBehavior<,>));
            services.AddTransient(typeof(IPipelineBehavior<,>), typeof(SequentialProcessingBehavior<,>));

Example of one my behaviours:

    internal class PaymentRequestedDispatcher : IRequestPreProcessor<AuthorizeCardPayment>
    {
        private readonly IMediator _mediator;
        private readonly ILogger _logger;

        public PaymentRequestedDispatcher(IMediator mediator, ILogger logger)
        {
            _mediator = mediator ?? throw new ArgumentNullException(nameof(mediator));
            _logger = logger ?? throw new ArgumentNullException(nameof(logger));
        }

        public async Task Process(AuthorizeCardPayment command, CancellationToken cancellationToken)
        {
            if (command is null)
                throw new ArgumentNullException(nameof(command));

            _logger.Debug("Executing Payment Requested Dispatcher for Payment {PaymentId}", command.PaymentId.ToString());

            var requested = new Requested(

            );

            await _mediator.Publish(requested, cancellationToken);
        }
    }

From my logs I am seeing all of the pre/post handlers getting executed twice:

2019-10-29 18:21:51 DBG Computing Hash of ApiKey "completed" in 37.5 ms
2019-10-29 18:21:51 DBG Retrieving ApiKeyDetails from Couchbase "completed" in 71.4 ms
2019-10-29 18:21:51 DBG Validating type "PaymentRequest<CardSource>" using "PaymentRequestWithSourceValidator<CardSource, CardSourceValidator>" "completed" in 1.4 ms
2019-10-29 18:21:52 DBG Storing card details in Vault "completed" in 134.3 ms
2019-10-29 18:21:52 DBG Executing Payment Requested Dispatcher for Payment "pay_p57sma5ssgkevhzcgccpanqxc4"
2019-10-29 18:21:52 DBG Inserting GetPayment Document in Couchbase "completed" in 18.6 ms
2019-10-29 18:21:52 DBG Creating new publisher for stream "Payment-pay_p57sma5ssgkevhzcgccpanqxc4"
2019-10-29 18:21:52 DBG Executing Payment Requested Dispatcher for Payment "pay_p57sma5ssgkevhzcgccpanqxc4"
2019-10-29 18:21:52 INF Found credentials using the AWS SDK's default credential search
2019-10-29 18:21:52 DBG Inserting GetPayment Document in Couchbase "completed" in 5.3 ms
2019-10-29 18:21:52 DBG Queueing event to batch for "Payment-pay_p57sma5ssgkevhzcgccpanqxc4"
2019-10-29 18:21:52 DBG Queueing event to batch for "Payment-pay_p57sma5ssgkevhzcgccpanqxc4"
2019-10-29 18:21:52 DBG Retrieving Merchant from Couchbase "completed" in 26.5 ms
2019-10-29 18:21:52 DBG Retrieving Acquirer service configuration from Couchbase "completed" in 17.3 ms
2019-10-29 18:21:52 DBG Generated acquirer transaction ID 41001 for payment transaction ID 5ea33cbf-98f0-440a-ab23-cef0c774e94a
2019-10-29 18:21:52 DBG Queueing event to batch for "Payment-pay_p57sma5ssgkevhzcgccpanqxc4"
2019-10-29 18:21:52 INF Processing "4" with acquirer: "cko-visa" "completed" in 184.1 ms
2019-10-29 18:21:52 DBG Writing PaymentRecord to Couchbase "completed" in 53.6 ms
2019-10-29 18:21:52 DBG Executing Authorization Result Dispatcher for Payment "pay_p57sma5ssgkevhzcgccpanqxc4"
2019-10-29 18:21:52 DBG Updating GetPayment Document in Couchbase "completed" in 40.6 ms
2019-10-29 18:21:52 DBG Queueing event to batch for "Payment-pay_p57sma5ssgkevhzcgccpanqxc4"
2019-10-29 18:21:52 DBG Executing Auto-Capture Handler for Payment "pay_p57sma5ssgkevhzcgccpanqxc4"
2019-10-29 18:21:52 DBG Retrieving URL for SQS queue "default" "completed" in 217.8 ms
2019-10-29 18:21:53 DBG Sending message "CapturePayment" to SQS "completed" in 45.7 ms
2019-10-29 18:21:53 DBG Handling "Enqueue" with Mediator "completed" in 285.3 ms
2019-10-29 18:21:53 DBG Queueing event to batch for "Payment-pay_p57sma5ssgkevhzcgccpanqxc4"
2019-10-29 18:21:53 DBG Executing Auto-Void Handler for Payment "pay_p57sma5ssgkevhzcgccpanqxc4"
2019-10-29 18:21:53 DBG Handling "AuthorizeCardPayment" with Mediator "completed" in 831.1 ms
2019-10-29 18:21:53 DBG Executing Authorization Result Dispatcher for Payment "pay_p57sma5ssgkevhzcgccpanqxc4"
2019-10-29 18:21:53 DBG Updating GetPayment Document in Couchbase "completed" in 1.3 ms
2019-10-29 18:21:53 DBG Executing Auto-Capture Handler for Payment "pay_p57sma5ssgkevhzcgccpanqxc4"
2019-10-29 18:21:53 DBG Queueing event to batch for "Payment-pay_p57sma5ssgkevhzcgccpanqxc4"
2019-10-29 18:21:53 DBG Sending message "CapturePayment" to SQS "completed" in 5.4 ms
2019-10-29 18:21:53 DBG Handling "Enqueue" with Mediator "completed" in 5.9 ms
2019-10-29 18:21:53 DBG Executing Auto-Void Handler for Payment "pay_p57sma5ssgkevhzcgccpanqxc4"
2019-10-29 18:21:53 DBG Queueing event to batch for "Payment-pay_p57sma5ssgkevhzcgccpanqxc4"
2019-10-29 18:21:53 DBG Handling "RequestPayment<FullCardSource>" with Mediator "completed" in 1078.5 ms
2019-10-29 18:21:53 DBG Handling "PaymentRequest<CardSource>" with Mediator "completed" in 1177.7 ms
2019-10-29 18:21:53 INF HTTP "POST" "/payments" responded 201 in 2871.4427 ms

Also am I right that the only way to control the order of behaviours is if I use IPipelineBehavior and register them explicitly?

[lilasquared]

if you are using AddMediatR, that function already adds the pre and post processors, you do not need to manually register them using AddTransient in the Core Package. Just remove the manual registration and they won’t get registered multiple times.

[levyhnunes]

I had the same problem, and it was precisely because I registered AddMediatR separately, this worked.

var assemblies = new System.Reflection.Assembly[]
{
 AppDomain.CurrentDomain.Load("Core"),
 AppDomain.CurrentDomain.Load("Notifications"),
 AppDomain.CurrentDomain.Load("Domain")
});

services.AddMediatR(assemblies);