CaseR 0.1.0

CaseR

CaseR is not another MediatR clone, but tries to solve the same problem in a different way (in context .NET 10).

The main task of this library is to model use cases in an application and separate cross-cutting concerns (like logging, caching, monitoring, transactions,...) and support vertical slice architecture in ASP.NET Core 8+ applications, even with support for AOT compilation.

I tried to solve mainly these problems:

• Interface IMediator is too generic, it's like injecting an IServiceProvider, so class dependencies or MinimalAPI don't make it clear what the class requires.
• In a project using a mediator, it is difficult to navigate because it is not easy to get to the handler implementation.
• MediatR is not type-safe in compile time. It is possible to call IMediator.Send() with a request for which there is no handler.
• The necessity to use IRequest<> and IResponse, we understand why these interfaces are in MediatR, but it bothers me a bit.

After a few projects where I used MediatR I realized a few things. Developers actually use MediatR to implement their use cases. There is no CQRS support, this arises naturally by having each HTTP request implemented in a separate class. It also doesn't directly implement the message queue either.

Therefore, I decided to create a library that uses the correct terminology for Use Case (and interactor from Clean Architecture).

Key Features

• Modeling use cases
• Built entirely on top of Dependency Injection
• Zero runtime reflection after registration
  • CaseR.SourceGenerator eliminate reflection at registration
• Direct code reference to business logic (no problem with trimming and orientation in codebase - F12 work)
• Compile-time type safety (using generic constraints and source generator)
• Interaction interceptor pipeline
• Supports keyed pipelines
• Supports domain events publisher

Get started

1. Install nugets

Install nugets into project using:

dotnet add package CaseR

dotnet add package CaseR.SourceGenerator

2. Register CaseR services

builder.Services.AddCaseR();
builder.Services.AddCaseRInteractors();

3. Create interactor

public record GetTodoInteractorRequest();

public record Todo(int Id, string? Title, DateOnly? DueBy = null, bool IsComplete = false);

public class GetTodoInteractor : IUseCaseInterceptor<GetTodoInteractorRequest, Todo[]>
{
    public GetTodoInteractor()
    {
        
    }

    public ValueTask<Todo[]> InterceptExecution(GetTodoInteractorRequest request, CancellationToken cancellationToken)
    {
        Todo[] sampleTodos = new Todo[] 
        {
                new Todo(1, "Walk the dog"),
                new Todo(2, "Do the dishes", DateOnly.FromDateTime(DateTime.Now)),
                new Todo(3, "Do the laundry", DateOnly.FromDateTime(DateTime.Now.AddDays(1))),
                new Todo(4, "Clean the bathroom"),
                new Todo(5, "Clean the car", DateOnly.FromDateTime(DateTime.Now.AddDays(2)))
        };

        return new ValueTask<Todo[]>(sampleTodos);
    }
}

4. Use use case in minimal API

var todosApi = app.MapGroup("/todos");
            todosApi.MapGet("/", async (IUseCase<GetTodoInteractor> getTodoInteractor,
                CancellationToken cancellationToken) =>
            {
                WebAppExample.Todo.UseCases.Todo[] todos = await getTodoInteractor.Execute(new GetTodoInteractorRequest(), cancellationToken);
                return todos;
            });

Domain events

1. Create domain event

public record GetTodoItemInteractorEvent(int Id) : IDomainEvent;

2. Create domain event handlers

public class GetTodoItemInteractorEventHandler : IDomainEventHandler<GetTodoItemInteractorEvent>
{
    public ValueTask Handle(GetTodoItemInteractorEvent domainEvent, CancellationToken cancellationToken)
    {
        //...
        return ValueTask.CompletedTask;
    }
}

3. Publish event

Inject IDomainEventPublisher into your interactor or handler and use it to publish events.

IDomainEventPublisher domainEventPublisher

Publish event:

 await this.domainEventPublisher.Publish(new GetTodoItemInteractorEvent(request), cancellationToken);

Use Case interceptors

Interceptors are used to create a pipeline that wraps the interactor call.

public class ElapsedTimeInterceptor<TRequest, TResponse> : IUseCaseInterceptor<TRequest, TResponse>
{
    private readonly ILogger<ElapsedTimeInterceptor<TRequest, TResponse>> logger;

    public ElapsedTimeInterceptor(ILogger<ElapsedTimeInterceptor<TRequest, TResponse>> logger)
    {
        this.logger = logger;
    }

    public async ValueTask<TResponse> InterceptExecution(IUseCaseInteractor<TRequest, TResponse> useCaseInteractor, TRequest request, UseCasePerformDelegate<TRequest, TResponse> next, CancellationToken cancellationToken)
    {
        long timestamp = Stopwatch.GetTimestamp();
        try
        {
            return await next(request).ConfigureAwait(false);
        }
        finally
        {
            TimeSpan elapsedTime = Stopwatch.GetElapsedTime(timestamp);
            this.logger.LogCritical("Elapsed {ElapsedTime}ms in {InteractorName}.", elapsedTime.TotalMilliseconds, useCaseInteractor.GetType().Name);
        }
    }
}

And register interceptors in registration:

builder.Services.AddCaseR(options =>
  {
    options.AddGenericInterceptor(typeof(ElapsedTimeInterceptor<,>));
    // register another interceptors
  });
builder.Services.AddCaseRInteractors();

Or use keyed piplines:

builder.Services.AddKeyedCaseR("GrpcPipeline", options =>
  {
    options.AddGenericInterceptor(typeof(AnotherInterceptor<,>));
    options.AddGenericInterceptor(typeof(nestedLogInterceptor<,>));
  });

builder.Services.AddKeyedCaseR("MinimalApiPipeline", options =>
  {
    options.AddGenericInterceptor(typeof(AnotherInterceptor<,>));
    options.AddGenericInterceptor(typeof(nestedLogInterceptor<,>));
  });

Usage:

var todosApi = app.MapGroup("/todos");
            todosApi.MapGet("/", async ([FromKeyedServices("MinimalApiPipeline")] IUseCase<GetTodoInteractor> getTodoInteractor,
                CancellationToken cancellationToken) =>
            {
                WebAppExample.Todo.UseCases.Todo[] todos = await getTodoInteractor.Execute(new GetTodoInteractorRequest(), cancellationToken);
                return todos;
            });

CaseR

The CaseR library contains all the logic and can be used without a source generator.

However, it has two limitations:

• Registering interactors is done using reflection builder.Services.AddCaseRInteractors(typeof(Program));,
• In the execute method, generic parameters must be passed when calling await useCase.Execute<TInteractor, TRequest, TResponse>(request, cancellationToken);.

CaseR.SourceGenerator

The CaseR.SourceGenerator library is a source generator that generates the necessary code for the CaseR library to work without reflection and typed Execute method.