🐾 Implementing CQRS with MediatR in My Final Project “Dog Walk” (.NET)

Today I want to talk about the pattern I implemented in my Final Web Development Project, “Dog Walk” — the CQRS pattern (Command Query Responsibility Segregation) using MediatR in .NET.

But… why follow this pattern?

📊 Clear separation between read and write operations
🔄 Improved scalability and maintainability
🎯 Better performance on specific operations
🧪 Easier testing and debugging

Key implementation highlights:

Use of MediatR to mediate commands and queries
Specific DTOs for each operation
Independent Handlers with single responsibility
Integration with Unit of Work and Repository Pattern
Validation through Behaviors

🔧 Example: Command & CommandHandler

CreateArticuloCommand.cs

public class CreateArticuloCommand : IRequest<Guid>
{
    public CreateArticuloDto ArticuloDto { get; set; }
}

CreateArticuloCommandHandler.cs

public class CreateArticuloCommandHandler : IRequestHandler<CreateArticuloCommand, Guid>
{
    private readonly IUnitOfWork _unitOfWork;

    public CreateArticuloCommandHandler(IUnitOfWork unitOfWork)
    {
        _unitOfWork = unitOfWork;
    }

    public async Task<Guid> Handle(CreateArticuloCommand request, CancellationToken cancellationToken)
    {
        var dto = request.ArticuloDto;

        var articulo = new Articulo(
            Guid.NewGuid(),
            dto.Nombre,
            dto.Descripcion,
            Dinero.Create(dto.Precio),
            dto.Stock,
            (CategoriaArticulo)dto.Categoria
        );

        if (dto.Imagenes != null && dto.Imagenes.Count > 0)
        {
            foreach (var urlImagen in dto.Imagenes)
            {
                var imagen = new ImagenArticulo(
                    Guid.NewGuid(),
                    articulo.Id,
                    urlImagen,
                    dto.Imagenes.IndexOf(urlImagen) == 0
                );

                articulo.AgregarImagen(imagen);
            }
        }

        await _unitOfWork.Articulos.AddAsync(articulo);
        await _unitOfWork.SaveChangesAsync();

        return articulo.Id;
    }
}

🔍 Example: Query & QueryHandler

GetAllArticulosQuery.cs

public class GetAllArticulosQuery : IRequest<ResultadoPaginadoDto<ArticuloDto>>
{
    public int PageNumber { get; set; } = 1;
    public int PageSize { get; set; } = 10;
    public string SearchTerm { get; set; }
    public string SortBy { get; set; }
    public bool Ascending { get; set; } = true;
    public CategoriaArticulo? Categoria { get; set; }
}

GetAllArticulosQueryHandler.cs

public class GetAllArticulosQueryHandler : IRequestHandler<GetAllArticulosQuery, ResultadoPaginadoDto<ArticuloDto>>
{
    private readonly IUnitOfWork _unitOfWork;

    public GetAllArticulosQueryHandler(IUnitOfWork unitOfWork)
    {
        _unitOfWork = unitOfWork;
    }

    public async Task<ResultadoPaginadoDto<ArticuloDto>> Handle(GetAllArticulosQuery request, CancellationToken cancellationToken)
    {
        Expression<Func<Articulo, bool>> predicate = a => true;

        if (!string.IsNullOrEmpty(request.SearchTerm))
        {
            var term = request.SearchTerm.ToLower();
            predicate = a =>
                EF.Functions.Like(a.Nombre.ToLower(), $"%{term}%") ||
                EF.Functions.Like(a.Descripcion.ToLower(), $"%{term}%");
        }

        if (request.Categoria.HasValue)
        {
            predicate = a => a.Categoria == request.Categoria.Value;
        }

        Expression<Func<Articulo, object>> orderBy = request.SortBy?.ToLower() switch
        {
            "precio" => a => a.Precio.Cantidad,
            "stock" => a => a.Stock,
            _ => a => a.Nombre
        };

        var (articulos, totalItems) = await _unitOfWork.Articulos.GetPagedAsync(
            predicate,
            orderBy,
            request.Ascending,
            request.PageNumber,
            request.PageSize
        );

        var articulosDto = articulos.Select(a => new ArticuloDto
        {
            Id = a.Id,
            Nombre = a.Nombre,
            Descripcion = a.Descripcion,
            Precio = a.Precio.Cantidad,
            Stock = a.Stock,
            Categoria = a.Categoria.ToString(),
            ImagenPrincipal = a.Imagenes?.FirstOrDefault()?.UrlImagen ?? string.Empty,
            FechaCreacion = a.CreadoEn
        }).ToList();

        return new ResultadoPaginadoDto<ArticuloDto>
        {
            Items = articulosDto,
            TotalItems = totalItems,
            TotalPaginas = (int)Math.Ceiling(totalItems / (double)request.PageSize),
            PaginaActual = request.PageNumber,
            ElementosPorPagina = request.PageSize
        };
    }
}

✅ Benefits achieved:

Cleaner, more maintainable code

Easier and more direct testing

Better performance on read operations

Greater scalability and flexibility

If you’re building with .NET and want to scale your application with clean responsibilities, CQRS with MediatR is a solid and elegant solution.