CQRS with MediatR

Setup

dotnet add package MediatR
dotnet add package MediatR.Extensions.Microsoft.DependencyInjection

Commands

// Command
public record CreateProductCommand(string Name, decimal Price) : IRequest<int>;

// Handler
public class CreateProductCommandHandler : IRequestHandler<CreateProductCommand, int>
{
    private readonly IApplicationDbContext _context;

    public async Task<int> Handle(CreateProductCommand request, CancellationToken ct)
    {
        var product = new Product(request.Name, request.Price);
        _context.Products.Add(product);
        await _context.SaveChangesAsync(ct);
        return product.Id;
    }
}

Queries

// Query
public record GetProductByIdQuery(int Id) : IRequest<ProductDto>;

// Handler
public class GetProductByIdQueryHandler : IRequestHandler<GetProductByIdQuery, ProductDto>
{
    public async Task<ProductDto> Handle(GetProductByIdQuery request, CancellationToken ct)
    {
        return await _context.Products
            .Where(p => p.Id == request.Id)
            .Select(p => new ProductDto(p.Id, p.Name, p.Price))
            .FirstOrDefaultAsync(ct);
    }
}

Usage in Endpoint

app.MapPost("/api/products", async (CreateProductCommand command, IMediator mediator) =>
{
    var id = await mediator.Send(command);
    return Results.Created($"/api/products/{id}", new { id });
});

app.MapGet("/api/products/{id}", async (int id, IMediator mediator) =>
{
    var product = await mediator.Send(new GetProductByIdQuery(id));
    return product is null ? Results.NotFound() : Results.Ok(product);
});