Validation

public class ProductDto
{
    public required int id { get; init; }
    public string? name { get; init; }
    public string? description { get; init; }
    public decimal price { get; init; }
}

public class ProductDtoValidator : AbstractValidator<ProductDto>
{
}

public class ProductDtoValidator : AbstractValidator<ProductDto>
{
    public ProductDtoValidator() {
        RuleFor(x => x.name).NotEmpty();
    }  
}

RuleFor(x => x.name).NotEmpty().NotNull();

public ProductDtoValidator() {
        RuleFor(x => x.name).NotEmpty().NotNull();
        RuleFor(x => x.price).GreaterThan(0);
        RuleFor(x => x.description).Length(5, 20);
}

builder.Services.AddEndpointsApiExplorer();
builder.Services.AddSwaggerGen();
builder.Services.AddDbContext<CourseDbContext>(options => options.UseInMemoryDatabase("CourseDb"));

builder.Services.AddCors(Action => Action.AddPolicy("aspnet-course",
    config => config.AllowAnyHeader().AllowAnyMethod().AllowAnyOrigin()));

// Resolve DI
builder.Services.AddScoped<IProductsRepository, ProductsRepository>();
builder.Services.AddScoped<IValidator<ProductDto>, ProductDtoValidator>();

var app = builder.Build();

static async Task<Results<Created<ProductDto>, ValidationProblem>> AddNewProduct(
  ProductDto product, 
  IProductsRepository repo, 
  IMapper mapper, 
  IValidator<ProductDto> validator) {
    var validationResult = validator.Validate(product);

    if (!validationResult.IsValid) {
        return TypedResults.ValidationProblem(validationResult.ToDictionary());
    }

    var mappedResult = mapper.Map<Product>(product);
    Product addProduct = await repo.AddNewProduct(mappedResult);
    var productResult = mapper.Map<ProductDto>(addProduct);

    return TypedResults.Created($"api/products/{addProduct.id}", productResult);
}

builder.Services.AddScoped<IValidator<ProductDto>, ProductDtoValidator>();

builder.Services.AddValidatorsFromAssembly(Assembly.GetExecutingAssembly());
builder.Services.AddEndpointsApiExplorer();
builder.Services.AddSwaggerGen();
builder.Services.AddAutoMapper(Assembly.GetExecutingAssembly());
builder.Services.AddDbContext<CourseDbContext>(options => options.UseInMemoryDatabase("CourseDb"));

builder.Services.AddCors(Action => Action.AddPolicy("aspnet-course",
    config => config.AllowAnyHeader().AllowAnyMethod().AllowAnyOrigin()));

// Resolve DI
builder.Services.AddScoped<IProductsRepository, ProductsRepository>();

var app = builder.Build();

public ProductDtoValidator() {
    RuleFor(x => x.name).NotEmpty().NotNull();
    RuleFor(x => x.price)
        .GreaterThan(0)
        .Custom((value, context) => {
            if(value % 2 != 0) {
                context.AddFailure("Price is not even");
            }
        });
    RuleFor(x => x.description).Length(5, 20);
}