IRequestPreProcessor not called before IPipeLineBehavior

Hi there,

I’m having some difficulties migrating my code to version 12.0.1.

I have a RequestPreProcessor defined as

public class RequestContextPreProcessor<TRequest> : IRequestPreProcessor<TRequest>  
where TRequest : notnull, IHomeControllerRequest

and a number of Behaviors which I register using the follwoing code:

services.AddMediatR(config => {
                        config.RegisterServicesFromAssemblies(PluginProvider.GetAssemblies());
                        config.AddOpenBehavior(typeof(TelemetryBehavior<,>));
                        config.AddOpenBehavior(typeof(LoggingBehavior<,>));
                        config.AddOpenBehavior(typeof(ErrorHandlingBehavior<,>));
                    })

I can’t get the RequestPreProcessor to execute. For some reason, it’s not being registered and injected as part of the pipeline. My code worked fine with version 11. I tried explicitly registering the RequestPreProcessor, but that did not work either.

Any suggestions?

Thanks Alexander

[AleRoe]

Hi @jbogard! Thanks for getting back and the explanations. Funny thing though is that I do have an IRequestPreProcessor implementation in the main assembly, but its Process() method does not get called (the class is being loaded though, because the constructor is called). The only way to get it to work is by manually adding the RequestPreProcessorBehavior<,> prior to any other custom behaviors.

So this works:

services.AddMediatR(config => {
                        config.RegisterServicesFromAssemblies(PluginProvider.GetAssemblies());
                        config.AddOpenBehavior(typeof(RequestPreProcessorBehavior<,>));
                        config.AddOpenBehavior(typeof(TelemetryBehavior<,>));
                        config.AddOpenBehavior(typeof(LoggingBehavior<,>));
                        config.AddOpenBehavior(typeof(ErrorHandlingBehavior<,>));
                    });

Also, my code worked fine with v11.

I can live with manually adding the RequestPreProcessorBehavior, but it does seem strange and not how it’s supposed to work. So I did some more testing:

And as it turns out, my IRequestPreProcessor is being called, but in the wrong order. It gets called after my custom IPipeLineBehaviors, instead of before. I verified this by first removing all my custom behaviors and then by adding only a single custom beviour that does not depend on the IRequestPreProcessor, and then checking the call-order using breakpoints.

So it seems that the built in pipeline discovery is adding the behaviors in the wrong order, at least when configuring AddMediatR() the way I do it (see my first post please).

Hope this helps. Kind regards Alexander

[jbogard]

I’m going to address this in the next release, to put request pre/post processors before behaviors.