Migrating from MediatR

Caesar follows MediatR's programming model, so most code moves over by changing namespaces and the registration call.

Steps

1. Replace the packages: MediatR.Contracts becomes Caesar.Abstractions in the Application layer, and MediatR becomes Caesar in the host.
2. Replace using MediatR; with using Caesar;. Processors and exception handling types live in Caesar.Pipeline, and publishers in Caesar.NotificationPublishers.
3. Replace services.AddMediatR(cfg => ...) with services.AddCaesar(cfg => ...).
4. Check where the mediator is resolved. Caesar registers it as scoped by default, while MediatR registers it as transient: resolve it inside a scope, or set cfg.MediatorLifetime = ServiceLifetime.Transient. See Lifetimes.
5. Build. Open-generic handlers the container cannot close now fail at AddCaesar instead of at the first request; see Open-generic handlers.

Type map

MediatR	Caesar
using MediatR;	using Caesar; (processors and exception types live in Caesar.Pipeline)
services.AddMediatR(cfg => ...)	services.AddCaesar(cfg => ...)
MediatRServiceConfiguration	CaesarServiceConfiguration
IRequest, IRequest<T>, INotification, IStreamRequest<T>	same
IRequestHandler<,>, IRequestHandler<>, INotificationHandler<>, IStreamRequestHandler<,>	same
IPipelineBehavior<,>, RequestHandlerDelegate<T>	same; next optionally takes a CancellationToken
IRequestPreProcessor<>, IRequestPostProcessor<,>, IRequestExceptionHandler<,,>, IRequestExceptionAction<,>	same (Caesar.Pipeline)
ForeachAwaitPublisher, TaskWhenAllPublisher	same (Caesar.NotificationPublishers), plus ForeachAwaitContinueOnFailurePublisher
Unit, ISender, IPublisher, IMediator	same

Running both side by side

During a gradual migration a project can reference both packages. The container keeps them apart, since MediatR.IMediator and Caesar.IMediator are different types, but a file with both using MediatR; and using Caesar; gets CS0104 ambiguous-reference errors on names such as IRequest and Unit. Keep each file on one of them, or use an alias (using CaesarRequest = Caesar.IRequest;).

Handlers are not shared: Caesar only scans for its own interfaces, so a handler must be moved to Caesar's interfaces to be found. Third-party packages built on MediatR's interfaces need a Caesar equivalent.