CQRS + MediatR Guidelines

Project-Agnostic CQRS Guidelines

Placeholders use {Placeholder} syntax - see docs/TEMPLATE_GLOSSARY.md.

Purpose

Provides best practices for implementing CQRS (Command Query Responsibility Segregation) using MediatR in .NET Clean Architecture projects. Ensures consistent, testable, and maintainable application logic.

When This Skill Activates

Triggered by:

Keywords: "command", "query", "handler", "mediatr", "cqrs", "validation", "validator"
Intent patterns: "create feature", "add endpoint", "implement use case"
File patterns: **/*Command.cs, **/*Query.cs, **/*Handler.cs, **/*Validator.cs
Content patterns: IRequest, IRequestHandler, AbstractValidator

CQRS Pattern Overview

mermaid

graph TD
    subgraph Presentation Layer
        Controller -- "Sends IRequest" --> MediatR[MediatR]
    end

    subgraph Application Layer
        MediatR --> Handler[Handler]
        Handler --> Validator[Validator]
        Handler --> Mapper[AutoMapper]
        Handler --> Repository[Repository]
        Repository --> Domain[Domain Entities]
    end

    subgraph Infrastructure Layer
        Repository --> DB[Database]
    end

    style MediatR fill:#fbe5c5,stroke:#333
    style Handler fill:#e5c5fb,stroke:#333
    style Validator fill:#e5fbe5,stroke:#333
    style Mapper fill:#fbe5e5,stroke:#333
    style Repository fill:#c5fbe5,stroke:#333
    style Domain fill:#c5fbc5,stroke:#333
    style DB fill:#c5c5fb,stroke:#333

Key Principles

Separation: Commands (write operations) and Queries (read operations) are distinct.
Single Responsibility: Each handler processes one specific request (command or query).
Class Requests: Commands and Queries are defined as classes.
Validation: FluentValidation is used at the Application boundary for input validation.
Thin Controllers: Controllers should be minimal, primarily responsible for receiving HTTP requests, sending them to MediatR, and returning HTTP responses.
CancellationToken: Always pass CancellationToken to all asynchronous methods to enable cancellation.
Repository Returns Entities: Repositories must return domain entities, and handlers are responsible for mapping these entities to DTOs.
Validators Use Manual Instantiation: Validators are instantiated manually within handlers, NOT injected via Dependency Injection.

Resources

For more detailed examples, refer to the resources/ folder within this skill.

Resource	Description
command-patterns.md	Command structure, naming, handlers
query-patterns.md	Query structure, pagination, projections
handler-patterns.md	Handler implementation, DI, error handling
validation-integration.md	FluentValidation patterns and manual integration
complete-examples.md	End-to-end feature examples for CQRS

Quick Reference

1. Command (Write Operation)

Commands represent actions that modify the application's state.

csharp

// File: {Project}.Application/Features/{Entities}/Requests/Commands/Create{Entity}Command.cs
namespace {Project}.Application.Features.{Entities}.Requests.Commands;

using MediatR;
using {Project}.Application.DTOs.{Entity};
using {Project}.Application.Responses;

public class Create{Entity}Command : IRequest<BaseCommandResponse<{IdType}>>
{
    public Create{Entity}Dto {Entity}Dto { get; set; } = null!; // Command wraps a DTO
}

For more details, see command-patterns.md.

2. Query (Read Operation)

Queries represent requests for data and should not alter the application's state.

csharp

// File: {Project}.Application/Features/{Entities}/Requests/Queries/Get{Entity}ListRequest.cs
namespace {Project}.Application.Features.{Entities}.Requests.Queries;

using System.Collections.Generic;
using {Project}.Application.DTOs.{Entity};
using MediatR;

public class Get{Entity}ListRequest : IRequest<List<{Entity}ListDto>>
{
    // Can include filter properties if needed
}

For more details, see query-patterns.md.

3. Handler (Processing Logic)

Handlers contain the business logic to execute a command or query.

csharp

// File: {Project}.Application/Features/{Entities}/Handlers/Commands/Create{Entity}CommandHandler.cs
namespace {Project}.Application.Features.{Entities}.Handlers.Commands;

using System.Linq;
using System.Threading;
using System.Threading.Tasks;
using AutoMapper;
using {Project}.Application.Contracts.Persistence;
using {Project}.Application.DTOs.{Entity}.Validators;
using {Project}.Application.Features.{Entities}.Requests.Commands;
using {Project}.Application.Responses;
using MediatR;

public class Create{Entity}CommandHandler : IRequestHandler<Create{Entity}Command, BaseCommandResponse<{IdType}>>
{
    private readonly I{Entity}Repository _{entity}Repository;
    private readonly IMapper _mapper;
    // ... other dependencies needed for validation or business logic

    public Create{Entity}CommandHandler(I{Entity}Repository {entity}Repository, IMapper mapper /* ... */)
    {
        _{entity}Repository = {entity}Repository;
        _mapper = mapper;
    }

    public async Task<BaseCommandResponse<{IdType}>> Handle(Create{Entity}Command request, CancellationToken cancellationToken)
    {
        var response = new BaseCommandResponse<{IdType}>();

        // ✅ CRITICAL: Manual validator instantiation with dependencies
        var validator = new Create{Entity}DtoValidator(/* repositories */);
        var validationResult = await validator.ValidateAsync(request.{Entity}Dto, cancellationToken);

        if (!validationResult.IsValid)
        {
            response.Success = false;
            response.Message = "{Entity} creation failed.";
            response.Errors = validationResult.Errors.Select(e => e.ErrorMessage).ToList();
            return response;
        }

        var {entity} = _mapper.Map<{Entity}>(request.{Entity}Dto);
        {entity}.ViewCount = 0; // Set properties not coming from DTO
        {entity} = await _{entity}Repository.Create({entity});

        response.Success = true;
        response.Id = {entity}.Id;
        response.Message = "{Entity} created successfully.";
        return response;
    }
}

For more details, see handler-patterns.md.

4. Validation (Manual Integration)

FluentValidation is used for input validation, with validators instantiated manually within handlers.

csharp

// File: {Project}.Application/DTOs/{Entity}/Validators/Create{Entity}DtoValidator.cs
namespace {Project}.Application.DTOs.{Entity}.Validators;

using FluentValidation;
using {Project}.Application.Contracts.Persistence;

public class Create{Entity}DtoValidator : AbstractValidator<Create{Entity}Dto>
{
    public Create{Entity}DtoValidator(I{RelatedEntity1}Repository {relatedEntity1}Repository /* ... */)
    {
        // ... inject repositories needed for FK validation

        RuleFor(x => x.Title)
            .NotEmpty().WithMessage("Title is required")
            .MaximumLength(200);

        RuleFor(x => x.{RelatedEntity1}Id)
            .NotEmpty().WithMessage("{RelatedEntity1} is required")
            .MustAsync(async (id, cancellation) => await {relatedEntity1}Repository.Exists(id))
            .WithMessage("{RelatedEntity1} not found");
    }
}

For more details, see validation-integration.md.

Do's

✅ DO separate Commands (write) and Queries (read).
✅ DO use classes for Commands/Queries (not records).
✅ DO pass CancellationToken to all asynchronous methods.
✅ DO use repositories that return domain entities (not DTOs).
✅ DO perform DTO mapping in handlers using AutoMapper.
✅ DO instantiate validators manually within handlers.
✅ DO keep controllers thin, delegating to MediatR.
✅ DO use BaseCommandResponse<{IdType}> for command responses (except bool for Delete).

Don'ts

❌ DON'T return entities directly from query handlers.
❌ DON'T put business logic in controllers.
❌ DON'T use IRequest without a response type.
❌ DON'T use .Result or .Wait() in asynchronous code.
❌ DON'T mutate state in query handlers.
❌ DON'T throw exceptions for business validation failures; return them in the BaseCommandResponse.
❌ DON'T inject validators via Dependency Injection.

Related Documentation:

docs/ARCHITECTURE.md - Overall system architecture.
clean-architecture-rules - Dependency enforcement.

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SKILL.md

CQRS + MediatR Guidelines

Purpose

When This Skill Activates

CQRS Pattern Overview

Key Principles

Resources

Quick Reference

1. Command (Write Operation)

2. Query (Read Operation)

3. Handler (Processing Logic)

4. Validation (Manual Integration)

Do's

Don'ts