Validation

See: https://event-driven.io/en/explicit_validation_in_csharp_just_got_simpler/

Example / end solution

public static class Validation
{
    public static Guid AssertNotEmpty(
        [NotNull] this Guid? value,
        [CallerArgumentExpression("value")] string? argumentName = null
    ) =>
        (value != null && value.Value != Guid.Empty)
            ? value.Value
            : throw new ArgumentOutOfRangeException(argumentName);


    public static string AssertNotEmpty(
        [NotNull] this string? value,
        [CallerArgumentExpression("value")] string? argumentName = null
    ) =>
        !string.IsNullOrWhiteSpace(value)
            ? value
            : throw new ArgumentOutOfRangeException(argumentName);


    public static string? AssertNullOrNotEmpty(
        this string? value,
        [CallerArgumentExpression("value")] string? argumentName = null
    ) =>
        value?.AssertNotEmpty(argumentName);

    public static string AssertMatchesRegex(
        [NotNull] this string? value,
        [StringSyntax(StringSyntaxAttribute.Regex)]
        string pattern,
        [CallerArgumentExpression("value")] string? argumentName = null
    ) =>
        Regex.IsMatch(value.AssertNotEmpty(), pattern)
            ? value
            : throw new ArgumentOutOfRangeException(argumentName);

    public static int AssertPositive(
        [NotNull] this int? value,
        [CallerArgumentExpression("value")] string? argumentName = null
    ) =>
        value?.AssertPositive() ?? throw new ArgumentOutOfRangeException(argumentName);

    public static int AssertPositive(
        this int value,
        [CallerArgumentExpression("value")] string? argumentName = null
    ) =>
        value > 0
            ? value
            : throw new ArgumentOutOfRangeException(argumentName);
}

public readonly record struct ProductId(Guid Value)
{
    public static ProductId From(Guid? productId) =>
        new(productId.AssertNotEmpty());
}

public readonly record struct SKU(string Value)
{
    public static SKU From(string? sku) =>
        new(sku.AssertMatchesRegex("[A-Z]{2,4}[0-9]{4,18}"));
}

public record RegisterProduct(
    ProductId ProductId,
    SKU SKU,
    string Name,
    string? Description
)
{
    public static RegisterProduct From(Guid? id, string? sku, string? name, string? description) =>
        new(
            ProductId.From(id),
            SKU.From(sku),
            name.AssertNotEmpty(),
            description.AssertNullOrNotEmpty()
        );
}

endpoints.MapPost(
        "api/products/",
        async (
            ProductsDBContext dbContext,
            RegisterProductRequest request,
            CancellationToken ct
        ) =>
        {
            var (sku, name, description) = request;
            var productId = Guid.NewGuid();

            var command = RegisterProduct.From(productId, sku, name, description);

            await Handle(
                dbContext.AddAndSave,
                dbContext.ProductWithSKUExists,
                command,
                ct
            );

            return Created($"/api/products/{productId}", productId);
        })
    .Produces(StatusCodes.Status201Created)
    .Produces(StatusCodes.Status400BadRequest);


// we're embracing here that we can expect anything from the UI
public record RegisterProductRequest(
    string SKU,
    string Name,
    string? Description
);