Methods inside enum in C#

You can write extension methods for enum types:

enum Stuff
{
    Thing1,
    Thing2
}

static class StuffMethods
{

    public static String GetString(this Stuff s1)
    {
        switch (s1)
        {
            case Stuff.Thing1:
                return "Yeah!";
            case Stuff.Thing2:
                return "Okay!";
            default:
                return "What?!";
        }
    }
}

class Program
{


    static void Main(string[] args)
    {
        Stuff thing = Stuff.Thing1;
        String str = thing.GetString();
    }
}

You can write an extension method for your enum:

How to: Create a New Method for an Enumeration (C# Programming Guide)

Another option is to use the Enumeration Class created by Jimmy Bogard.

Basically, you must create a class that inherits from his Enumeration. Example:

public class EmployeeType : Enumeration
{
    public static readonly EmployeeType Manager 
        = new EmployeeType(0, "Manager");
    public static readonly EmployeeType Servant 
        = new EmployeeType(1, "Servant");
    public static readonly EmployeeType Assistant
        = new EmployeeType(2, "Assistant to the Regional Manager");

    private EmployeeType() { }
    private EmployeeType(int value, string displayName) : base(value, displayName) { }

    // Your method...
    public override string ToString()
    {
        return $"{value} - {displayName}!";
    }
}

Then you can use it like an enum, with the possibility to put methods inside it (among another things):

EmployeeType.Manager.ToString();
//0 - Manager
EmployeeType.Servant.ToString();
//1 - Servant
EmployeeType.Assistant.ToString();
//2 - Assistant to the Regional Manager

You can download it with NuGet.

Although this implementation is not native in the language, the syntax (construction and usage) is pretty close to languages that implement enums natively better than C# (Kotlin for example).