using collection of strings in a switch statement

Question

I'm trying to find a solution for this problem. This is my example code:

class Program
{
  private string Command;

  private static string[] Commands = { "ComandOne", "CommandTwo", "CommandThree", "CommandFour" };


  static void Main(string[] args)
  {
    Command = args[0];
    switch(Command)
    {
      case Commands[0]: //do something 
        break;
      case Commands[1]: //do something else
        break;
      case Commands[2]: //do something totally different
        break;
      case Commands[3]: //do something boring
        break;
      default: //do your default stuff
        break;
    }
  }

  void DifferentMethod()
  {
    foreach(string c in Commands)
    {
      //do something funny
    }
  }
}

This code doesn't work because the string values in the switch are not constants. I want to write easy maintainable code.
I like to use something like an array because I need to use the same values somewhere else in a loop.
With int-values an enum would be perfect, but I didn't find a small solution for the same thing with strings.

Answer 1

Convert Commands into an enum:

enum Commands { ComandOne, CommandTwo, CommandThree, CommandFour }

Switch statement should look like:

static void Main(string[] args)
{
    Command = (Commands)Enum.Parse(typeof(Commands), args[0]);
    switch(Command)
    {
        case Commands.CommandOne: 
            //do something 
            break;
        case Commands.CommandTwo: 
            //do something else
            break;
        ...
        default:
            // default stuff
    }
}

And your last method should look like:

void DifferentMethod()
{
    foreach(var c in Enum.GetValues(typeof(Commands)))
    {
        string s = c.ToString(); 
        //do something funny
    }
}

Answer 2

An easy fix in your specific example:

switch(Array.IndexOf(Commands, Command))
{
    case 0: ...  
    case 1: ...

    default: //unknown command. Technically, this is case -1
} 

Other alternatives:

1. Inline the strings.

   switch(Command) { case "CommandOne": ... case "CommandTwo": ... }

2. Use an enumeration instead, as KirkWoll says. This is probably the cleanest solution.

3. In more complex scenarios, using a lookup such as Dictionary<string, Action> or Dictionary<string, Func<Foo>> might provide better expressibility.

4. If the cases are complex, you could create an ICommand interface. This will require mapping the command-string to the right concrete-implementation, for which you use simple constructs (switch / dictionaries) or fancy reflection (find ICommand implementations with that name, or with a certain attribute decoration).

Answer 3

Just yesterday i created a solution for it. In your case enums are better but here is my solution for general non-const switch-case situation.

usages:

    static string DigitToStr(int i)
    {
        return i
            .Case(1, "one")
            .Case(2, "two")
            .Case(3, "three")
            .Case(4, "four")
            .Case(5, "five")
            .Case(6, "six")
            .Case(7, "seven")
            .Case(8, "eight")
            .Case(9, "nine")
            .Default("");
    }

        int a = 1, b = 2, c = 3;
        int d = (4 * a * c - b * 2);
        string res = true
            .Case(d < 0, "No roots")
            .Case(d == 0, "One root")
            .Case(d > 0, "Two roots")
            .Default(_ => { throw new Exception("Impossible!"); });

        string res2 = d
            .Case(x => x < 0, "No roots")
            .Case(x => x == 0, "One root")
            .Case(x => x > 0, "Two roots")
            .Default(_ => { throw new Exception("Impossible!"); });

        string ranges = 11
            .Case(1, "one")
            .Case(2, "two")
            .Case(3, "three")
            .Case(x => x >= 4 && x < 10, "small")
            .Case(10, "ten")
            .Default("big");

definition:

class Res<O, R>
{
    public O value;
    public bool succ;
    public R result;

    public Res()
    {

    }

    static public implicit operator R(Res<O, R> v)
    {
        if (!v.succ)
            throw new ArgumentException("No case condition is true and there is no default block");
        return v.result;
    }
}

static class Op
{
    static public Res<O, R> Case<O, V, R>(this Res<O, R> o, V v, R r)
    {
        if (!o.succ && Equals(o.value, v))
        {
            o.result = r;
            o.succ = true;
        }
        return o;
    }

    static public Res<O, R> Case<O, V, R>(this O o, V v, R r)
    {
        return new Res<O, R>()
        {
            value = o,
            result = r,
            succ = Equals(o, v),
        };
    }

    static public Res<O, R> Case<O, R>(this Res<O, R> o, Predicate<O> cond, R r)
    {
        if (!o.succ && cond(o.value))
        {
            o.result = r;
            o.succ = true;
        }
        return o;
    }

    static public Res<O, R> Case<O, R>(this O o, Predicate<O> cond, R r)
    {
        return new Res<O, R>()
        {
            value = o,
            result = r,
            succ = cond(o),
        };
    }

    private static bool Equals<O, V>(O o, V v)
    {
        return o == null ? v == null : o.Equals(v);
    }

    static public R Default<O, R>(this Res<O, R> o, R r)
    {
        return o.succ
            ? o.result
            : r;
    }

    static public R Default<O, R>(this Res<O, R> o, Func<O, R> def)
    {
        return o.succ ? o.result : def(o.value);
    }
}

Answer 4

You could eliminate the switch statement altogether by creating IYourCommand objects and loading them into a Dictionary<string, IYourCommand>.

class Program
{
  private Dictionary<string, IYourCommand> Command = new Dictionary<string, IYourCommand>
    {
       { "CommandOne",   new CommandOne()   },
       { "CommandTwo",   new CommandTwo()   },
       { "CommandThree", new CommandThree() },
       { "CommandFour",  new CommandFour()  },
    };

  public static void Main(string[] args)
  {
    if (Command.ContainsKey(args[0]))
    {
      Command[args[0]].DoSomething();
    }
  }
}

public interface IYourCommand
{
  void DoSomething();
}

Answer 5

I generally dislike strings for this sort of thing - it's too easy to get into trouble with misspellings, different casings and the like - but presumably that's why you want to use a variable instead of string literal. If the enum solutions aren't suitable, using consts should accomplish your goal.

EDIT: Oct 28, 2013 to fix an incorrect assignment

class Program
{
    private string Command;

    const string command1 = "CommandOne";
    const string command2 = "CommandTwo";
    const string command3 = "CommandThree";
    const string command4 = "CommandFour";

    private static string[] Commands = { command1, command2, command3, command4 };

    static void Main(string[] args)
    {
        string Command = args[0];
        switch (Command)
        {
            case command1: //do something 
                break;
            case command2: //do something else
                break;
            case command3: //do something totally different
                break;
            case command4: //do something boring
                break;
            default: //do your default stuff
                break;
        }
    }

    void DifferentMethod()
    {
        foreach (string c in Commands)
        {
            //do something funny
        }
    }
}

Answer 6

As you said, only constant expressions are allowed in a switch. You would normally do this by defining an enum and use that in your switch.

class Program
{
  private enum Command
  {
    CommandOne = 1,
    CommandTwo = 2,
    CommandThree = 3
  }

  static void Main(string[] args)
  {
    var command = Enum.Parse(typeof(Commands), args[0]);
    switch(command )
    {
      case Command.CommandOne: //do something 
        break;
      case Command.CommandTwo: //do something else
        break;
      case Command.CommandThree: //do something totally different
        break;
      default: //do your default stuff
        break;
    }
  }
}

Use Enum.GetValues to enumerate through enum values in DifferentMethod.

Answer 7

Define a Dictionary<string, enum> and map the input command to the appropriate value before entering the switch. If match is not found, then default processing happens.

Answer 8

You can do it the other way around and reach your objective.

Use Enum and its GetNames call to get a string array to loop through.

Enum.GetNames(typeof (*YOURENUM*));

For more info. http://msdn.microsoft.com/en-us/library/system.enum.getnames.aspx

Answer 9

Great answers here and probably answer your question better than what I'm going to mention...

Depending on how complicated your logic is based, you may consider using a strategy pattern like this:

Refactoring a Switch statement

or

The Strategy Template Pattern

Again, most likely more complicated than your solution asked, just throwing it out there...