SmartEnum VS Result

I agree SmartEnum is my go-to for this

If we knew more about these methods and what kinds of parameters they take we could maybe make some suggestions. There are some patterns like smart enum that are sometimes just the thing.

In C# I am not able to add constructors, properties, and methods. although there is a library to do this https://github.com/Ardalis/SmartEnum but wouldn't be nice if we have this feature built-in to C#.

You can use the SmartEnum library.

Using the Nuget Package Ardalis.Result for mapping result objects to HTTP Response Codes and associated objects; FastEndpoint has its own way of approaching this problem, which seems to be to hang everything off an "Endpoint" and optionally using Swagger to redundantly describe the route's behavior; this seems like a step backwards from more advanced Web frameworks in other languages. The advantage of Ardalis.Result should be clear - each closed form of Ardalis.Result can have its own knowledge of what it does, and therefore automatically describe the abstract transformations it does on a request. In functional programming, this "abstract transformation" is typically called a monad transformer or Arrow. Once you have a type that describes the abstract transformation, plugging that into Swagger should be straight-forward.

You could also use the Ardalis Guard Clause, Ardalis Result and Fluent Validation

I use this generic package. https://github.com/ardalis/Result

For you're Result.cs , look at [Result pattern]https://github.com/ardalis/Result) as an example.