“String Enum” implementation - comments?

Question

I have a need to represent a finite set of discrete string values:

• When these strings are used in code, they must belong to the valid set, or serious errors (or worse) can occur.
• As "magic strings" are a terrible code smell, I want to encapsulate the strings within a non-string type, so that I can require the use of my type instead of just a string and thus enforce at compile-time that a value is given that I know will work.
• No constants; if I change the string behind the scenes I should not have to recompile all usages of my type.
• The identifiers representing the valid values should not have to have any formulaic similarity to the actual string (no Enum.ToString or similar).
• Lastly, while I want to enforce the use of my type so I can restrict values to the ones I know are valid, the type must be easily convertible to a string so that the piece of code that really needs the string doesn't have to undergo a complicated or esoteric conversion.

What I came up with is a variation of a "multiton"; a class that cannot be instantiated in code, but has a finite number of static instances each representing a valid value of the type. They are implicitly convertible to strings and so the static members just drop in wherever a string is needed.

//abstract base; derive to create your specific set of values
public abstract class StringEnum
{
    protected StringEnum(string name)
    {
        this.name = name;
    }

    private readonly string name;

    public static implicit operator string(StringEnum sp)
    {
        return sp.name;
    }
}

//one possible derivation
public class CommandNames:StringEnum
{
    //unfortunately each derivation has to call back to the base constructor;
    //it would be nice not to have to do this, but meh
    protected CommandNames(string commName) : base(commName) { }

    //obviously the instance names don't have to match the real string value exactly
    public static readonly CommandNames DoSomething = new CommandNames("system638_Command_doSomething");
}

//example usage
public void ExecuteCommand(CommandNames name)
{
    //assume ExecuteByName expects a string
    CommandExecutor.ExecuteByName(name); //the StringEnum just drops in
}

...

//Only one of the static instances, with string values I control,
//can be passed into this function, thus safely encapsulating the inner
//objects' use of "magic strings".
ExecuteCommand(CommandNames.DoSomething);

Thoughts?

Answer 1

A few things don't add up for me regarding your usage. You want compile time absolution, but dynamic values while stating the critical nature of its use.

One option: You can just take care of this by having objects with inheritance structures and loading your "dynamic" values from an application configuration or db. Just have 3 layers of inheritance 1) StringEnum 2) Enum Declaration (like System638) and 3) actual values. Just wire up the inheritance. Mostly likely though, interfaces would be more appropriate for restrictions at the 1 & 2 layers.

Recommended (assuming the distributed smell of the use case): Build your functional class and bind it to a single DB table that ensures the integrity you are looking for. Just cache the values in memory for performance needs.

Recommended (given the critical nature of its use) Find a different way to design the system so that you can fully embed what you need in code. For example, a web service. Even if you need a local presence you can utilize a web service to control the pieces you need.

Answer 2

KISS! You are over-complicating things. Keep strings as strings. Keep the strings as readonly, not const. Generate the code at build time, possibly using a custom build task, which itself can be compiled in the same build. Here is a quick Python script to give you an idea:

# Python script to generate your code
print('public static class CommandNames')
print('{')
for name, value in open('commands.txt').readlines():
    print('    public static readonly string {0} = "{1}";'.format(name, value))
print('}')

You do not have to stay in Python as I mentioned previously. Powershell or a custom build task will do.

EDIT: There is a more idiomatic way to generate code with Visual Studio -

http://www.hanselman.com/blog/T4TextTemplateTransformationToolkitCodeGenerationBestKeptVisualStudioSecret.aspx

http://msdn.microsoft.com/en-us/library/ee847423.aspx