Getting Started

MonadicSharp.DI is a lightweight functional mediator for .NET — CQRS without MediatR's exception-based conventions, aligned with MonadicSharp's Result<T> primitives.

Install

dotnet add package MonadicSharp.DI

Requires: .NET 6.0+, MonadicSharp.

Register with DI

builder.Services.AddMonadicMediator(typeof(Program).Assembly);

This scans the assembly for all IQueryHandler and ICommandHandler implementations and registers them automatically.

Define a query

// Query definition
public record GetUserQuery(Guid UserId) : IQuery<User>;

// Handler
public class GetUserHandler(IUserRepository repo) : IQueryHandler<GetUserQuery, User>
{
    public Task<Result<User>> HandleAsync(GetUserQuery query, CancellationToken ct) =>
        repo.FindByIdAsync(query.UserId, ct);
}

Dispatch a query

public class UserController(IMediator mediator) : ControllerBase
{
    [HttpGet("{id}")]
    public async Task<IActionResult> GetUser(Guid id)
    {
        var result = await mediator.QueryAsync(new GetUserQuery(id));

        return result.Match(
            onSuccess: user  => Ok(user),
            onFailure: error => error.ToActionResult());
    }
}

Define a command

public record CreateOrderCommand(Guid UserId, List<OrderItem> Items) : ICommand<Order>;

public class CreateOrderHandler(
    IOrderRepository orders,
    IInventoryService inventory) : ICommandHandler<CreateOrderCommand, Order>
{
    public Task<Result<Order>> HandleAsync(CreateOrderCommand cmd, CancellationToken ct) =>
        inventory.ReserveAsync(cmd.Items, ct)
            .BindAsync(reserved => orders.CreateAsync(cmd.UserId, reserved, ct));
}

Add a pipeline behavior

// Validation behavior — runs before every command handler
public class ValidationBehavior<TRequest, TResult>
    : IPipelineBehavior<TRequest, TResult>
    where TRequest : ICommand<TResult>
{
    public async Task<Result<TResult>> HandleAsync(
        TRequest request,
        RequestHandlerDelegate<TResult> next,
        CancellationToken ct)
    {
        var validation = ValidateRequest(request);
        if (validation.IsFailure) return validation.Error;
        return await next();
    }
}

// Register
builder.Services.AddBehavior(typeof(ValidationBehavior<,>));

Standalone (no DI)

var mediator = new Mediator(handler => new GetUserHandler(userRepo));
var result = await mediator.QueryAsync(new GetUserQuery(userId));

Next steps

• CQRS Pattern — query vs command design decisions
• Pipeline Behaviors — validation, logging, caching
• Notifications — pub/sub events