cqrs-mediatr-setup

CQRS + MediatR Setup

Wire up CQRS in an ASP.NET Core app: command/query separation, handlers, and the cross-cutting pipeline behaviors that make CQRS pay off (validation, logging, transactions).

Input — point it at your project

Works on a target, not pasted snippets: a file, folder, project/solution, or GitHub URL (git clone --depth 1 <url> then scan). If the project exists, read one feature first and match its conventions. If MediatR is being phased out in your stack, offer the thin custom-dispatcher variant.

Packages

dotnet add package MediatR
dotnet add package FluentValidation.DependencyInjectionExtensions

builder.Services.AddMediatR(c => c.RegisterServicesFromAssembly(typeof(Program).Assembly));
builder.Services.AddValidatorsFromAssembly(typeof(Program).Assembly);

Command / query + handler

public record CreateOrderCommand(Guid CustomerId, List<OrderLine> Lines) : IRequest<Result<Guid>>;

public class CreateOrderHandler(IOrderRepository repo, IUnitOfWork uow)
    : IRequestHandler<CreateOrderCommand, Result<Guid>>
{
    public async Task<Result<Guid>> Handle(CreateOrderCommand cmd, CancellationToken ct)
    {
        var order = Order.Create(cmd.CustomerId, cmd.Lines);
        await repo.AddAsync(order, ct);
        await uow.SaveChangesAsync(ct);
        return Result.Success(order.Id);
    }
}

Queries are the same shape with IRequest<TDto> and a read-only handler (AsNoTracking, projection).

Pipeline behaviors (where CQRS earns its keep)

public class ValidationBehavior<TReq, TRes>(IEnumerable<IValidator<TReq>> validators)
    : IPipelineBehavior<TReq, TRes> where TReq : notnull
{
    public async Task<TRes> Handle(TReq req, RequestHandlerDelegate<TRes> next, CancellationToken ct)
    {
        foreach (var v in validators)
        {
            var r = await v.ValidateAsync(req, ct);
            if (!r.IsValid) throw new ValidationException(r.Errors); // or map to a Result
        }
        return await next();
    }
}
builder.Services.AddTransient(typeof(IPipelineBehavior<,>), typeof(ValidationBehavior<,>));

Add LoggingBehavior and a TransactionBehavior (open transaction around commands only) the same way.