Cqrs Patterns

CQRS & MediatR Standards

This skill defines patterns for CQRS (Command Query Responsibility Segregation) using MediatR in .NET projects.

When to Use CQRS

Use CQRS When

• Read and write models have different shapes (list views vs edit forms)
• Write operations have complex validation or side effects
• You need audit trails on state changes
• Domain events should trigger downstream actions
• Multiple teams work on the same domain

Don't Use CQRS When

• Simple CRUD with identical read/write shapes
• No complex business rules
• Small domain with few entities
• Adding complexity outweighs the benefit

MediatR Setup

Registration

// Program.cs
builder.Services.AddMediatR(cfg => cfg.RegisterServicesFromAssembly(typeof(AppDbContext).Assembly));

Project Structure

Core/
  Commands/          # IRequest<TResult> command types
    CreateGoal.cs    # Contains Command record + Handler class
  Queries/           # IRequest<TResult> query types
    GetGoalById.cs   # Contains Query record + Handler class
  Events/            # INotification types for domain events
    GoalCreated.cs

Command Pattern

Command + Handler (Same File)

// Commands/CreateGoal.cs
public record CreateGoalCommand(
    string Name,
    GoalType Type,
    decimal TargetAmount,
    DateOnly? Deadline,
    string UserId
) : IRequest<ApiResponse<GoalDto>>;

public class CreateGoalHandler : IRequestHandler<CreateGoalCommand, ApiResponse<GoalDto>>
{
    private readonly AppDbContext _db;
    private readonly IPublisher _publisher;

    public CreateGoalHandler(AppDbContext db, IPublisher publisher)
    {
        _db = db;
        _publisher = publisher;
    }

    public async Task<ApiResponse<GoalDto>> Handle(
        CreateGoalCommand request, CancellationToken ct)
    {
        var goal = new Goal
        {
            Name = request.Name,
            Type = request.Type,
            TargetAmount = request.TargetAmount,
            Deadline = request.Deadline,
            UserId = request.UserId,
        };

        _db.Goals.Add(goal);
        await _db.SaveChangesAsync(ct);

        await _publisher.Publish(new GoalCreatedEvent(goal.Id, goal.UserId), ct);

        return ApiResponse.Ok(GoalDto.From(goal));
    }
}

Command Rules

• Commands represent intent: CreateGoal, ArchiveWorkflow, ApproveHumanTask
• Commands are records (immutable)
• One handler per command
• Commands return results (not void) — caller needs to know success/failure
• Commands can publish domain events via IPublisher

Query Pattern

// Queries/GetGoalById.cs
public record GetGoalByIdQuery(Guid GoalId, string UserId) : IRequest<GoalDto?>;

public class GetGoalByIdHandler : IRequestHandler<GetGoalByIdQuery, GoalDto?>
{
    private readonly AppDbContext _db;

    public GetGoalByIdHandler(AppDbContext db) => _db = db;

    public async Task<GoalDto?> Handle(GetGoalByIdQuery request, CancellationToken ct)
    {
        return await _db.Goals
            .AsNoTracking()
            .Where(g => g.Id == request.GoalId && g.UserId == request.UserId)
            .Select(g => new GoalDto(g.Id, g.Name, g.TargetAmount, g.CurrentAmount))
            .FirstOrDefaultAsync(ct);
    }
}

Query Rules

• Queries are read-only — NEVER modify state
• Use AsNoTracking() for all query handlers
• Project directly to DTOs — never return domain entities
• Filter by UserId / TenantId in the query (access control)

Domain Events

// Events/GoalCreated.cs
public record GoalCreatedEvent(Guid GoalId, string UserId) : INotification;

// Handlers can be in Infrastructure (for side effects)
public class GoalCreatedHandler : INotificationHandler<GoalCreatedEvent>
{
    private readonly IXpService _xpService;

    public GoalCreatedHandler(IXpService xpService) => _xpService = xpService;

    public async Task Handle(GoalCreatedEvent notification, CancellationToken ct)
    {
        await _xpService.AwardXpAsync(notification.UserId, XpAction.GoalCreated, ct);
    }
}

Event Rules

• Events are past tense: GoalCreated, WorkflowCompleted, HumanTaskApproved
• Events are notifications (INotification) — multiple handlers can respond
• Handlers should be idempotent (events may be replayed)
• Side-effecting handlers go in Infrastructure, not Core

Pipeline Behaviors

Validation Behavior

public class ValidationBehavior<TRequest, TResponse>
    : IPipelineBehavior<TRequest, TResponse>
    where TRequest : IRequest<TResponse>
{
    private readonly IEnumerable<IValidator<TRequest>> _validators;

    public ValidationBehavior(IEnumerable<IValidator<TRequest>> validators)
        => _validators = validators;

    public async Task<TResponse> Handle(
        TRequest request, RequestHandlerDelegate<TResponse> next, CancellationToken ct)
    {
        var failures = _validators
            .Select(v => v.Validate(request))
            .SelectMany(r => r.Errors)
            .Where(f => f != null)
            .ToList();

        if (failures.Any())
            throw new ValidationException(failures);

        return await next();
    }
}

Logging Behavior

public class LoggingBehavior<TRequest, TResponse>
    : IPipelineBehavior<TRequest, TResponse>
    where TRequest : IRequest<TResponse>
{
    public async Task<TResponse> Handle(
        TRequest request, RequestHandlerDelegate<TResponse> next, CancellationToken ct)
    {
        _logger.LogInformation("Handling {RequestType}", typeof(TRequest).Name);
        var response = await next();
        _logger.LogInformation("Handled {RequestType}", typeof(TRequest).Name);
        return response;
    }
}

Registration Order

// Pipeline behaviors execute in registration order (outermost first)
services.AddTransient(typeof(IPipelineBehavior<,>), typeof(LoggingBehavior<,>));
services.AddTransient(typeof(IPipelineBehavior<,>), typeof(ValidationBehavior<,>));
// Request → Logging → Validation → Handler → Validation → Logging → Response

Controller Integration

[ApiController]
[Route("api/[controller]")]
[Authorize]
public class GoalsController : ControllerBase
{
    private readonly ISender _sender;

    public GoalsController(ISender sender) => _sender = sender;

    [HttpPost]
    public async Task<IActionResult> Create([FromBody] CreateGoalRequest req, CancellationToken ct)
    {
        var command = new CreateGoalCommand(req.Name, req.Type, req.TargetAmount, req.Deadline, CurrentUserId);
        var result = await _sender.Send(command, ct);

        if (!result.Success) return BadRequest(result);
        return CreatedAtAction(nameof(GetById), new { id = result.Data!.Id }, result);
    }
}

Anti-Patterns

• Handler calling another handler via ISender.Send() — compose in the service layer instead
• Commands that return complex domain objects — return DTOs only
• Queries that modify state — strict read-only enforcement
• Monolithic handlers with 200+ lines — extract domain logic to services
• Missing CancellationToken propagation through the pipeline
• Publishing events before SaveChangesAsync — data might not persist if save fails