Generic Handler implementation only handles single generic argument and breaks existing implementations

[hisuwh]

Support for Generic Handlers has been added in v12.3.0 in this pr: #1013

However, this is not opt-in and only supports a single generic argument.

So a setup like this is incompatible:

public class MyRequest<T1, T2> : IRequest<T2>

public class MyRequestHandler<T1, T2> : IRequestHandler<MyRequest<T1, T2>, T2>

Also rather than simply not matching these requests this throws an exception on startup meaning this can't even fallback to existing code I have to handle these more complex generic handlers.

I suggest:

• if it cannot find/close a handler it should ignore it (as per standard behaviour) - and not throw an exception!
• there should be an option to opt-out
• and/or this should be extended to handle multiple generic arguments

[hisuwh]

@jbogard any chance of a 12.2.1 to fix https://github.com/jbogard/MediatR/pull/989

If and when I get some time I can look at the above as I have implemented generic handlers in a few projects.

[jbogard]

Gah. Care to submit a PR?

[zachpainter77]

@jbogard I have a fix for the above issue.. My only question is what limitations should be present? What I mean is, should we let the user define handlers that have 10 type parameters, each with no constraint? You can see how this could be problematic for the user, right? My solution is to allow more than one generic type parameter but throw errors if the max number of generic type params is exceeded OR the max number of concrete class combinations is exceed OR if a timeout occurs. My question is.. what should those limitations be?

I'm also trying to figure out a way to test these scenarios since even defining a class that should throw an exception does so for all tests that call AddMediatR.

So, any discussion around this is welcome. I think I'm just going to create a PR and let discussion take place there.

[hisuwh]

Is there a way to define a sensible limit internally, but then allow for this to be configurable?

[zachpainter77]

yes, I think there is a way to do this.. I will set up some configuration options with default values that can be overridden when calling AddMediatR.. I will have a PR coming today. We can probably discuss what is sensible in terms of default limitations in that thread.