Error code catalogs

What [ErrorCodeCatalog] emits: three nested classes, three extensions, and the fallback for a value that is not a declared member.

What you get

Three nested classes, one per shape:

// The code as a compile-time constant.
OrderErrorCodeCatalog.Names.NotFound   // "OrderErrorCode.NotFound"

// The code as an ErrorCode.
OrderErrorCodeCatalog.Codes.NotFound   // ErrorCode { Value = "OrderErrorCode.NotFound" }

// An Error carrying that code.
OrderErrorCodeCatalog.Errors.NotFound("no order with that id")

Names gives you a const string, so you can use it anywhere C# wants a constant — a case label, an attribute argument, a switch on a code that arrived over the wire.

And three extension methods, for when you are holding a value rather than naming a member:

OrderErrorCode errorCode = Classify(order);

string asName = errorCode.ToErrorCodeName();
ErrorCode asErrorCode = errorCode.ToErrorCode();
Error asError = errorCode.ToError("no order with that id");

The nesting is what keeps your member names usable as-is. A member called NotFoundCode becomes Names.NotFoundCode, and nothing collides.

Errors.NotFound(message) and ToError(message) build the Error for you, so they inherit how Error treats a message: it is trimmed, and a blank one is replaced by your configured fallback rather than rejected. Neither throws on a blank message, so pass a real one.

A value that is not a declared member

Casting an arbitrary integer to an enum is legal C#, so (OrderErrorCode)99 is a value you can be handed. The three extensions apply the same scheme to it:

((OrderErrorCode)99).ToErrorCodeName(); // "OrderErrorCode.99"

Reusing a code across two enums

Two attributed enums with the same name in different namespaces generate the same code for every member name they share. Ordering.OrderErrorCode.NotFound and Shipping.OrderErrorCode.NotFound both generate "OrderErrorCode.NotFound", and nothing reading the code can tell the two errors apart. So can two differently named enums that share a format: "order.{member:kebab}" on both OrderErrorCode and ShipmentError makes NotFound collide.

WM2018 reports that. It is a suggestion, not a warning — see WM2018.