How do I implement a dispatch table for a text adventure game?

Question

I am making a text adventure in C#, and someone suggested that I use a dispatch table instead of a switch statement.

Here's the switch statement code:

        #region Public Methods
        public static void Do(string aString)
        {
                if(aString == "")
                        return;

                string verb = "";
                string noun = "";

                if (aString.IndexOf(" ") > 0)
                {
                        string[] temp = aString.Split(new char[] {' '}, 2);
                        verb = temp[0].ToLower();
                        noun = temp[1].ToLower();
                }
                else
                {
                        verb = aString.ToLower();
                }

                switch(Program.GameState)
                {
                        case Program.GameStates.Playing:
                                if (IsValidInput(Commands, verb, true))
                                {
                                        switch(verb) //this is the switch statement
                                        {
                                                case "help":
                                                case "?":
                                                        WriteCommands();
                                                        break;
                                                case "exit":
                                                case "quit":
                                                        Program.GameState = Program.GameStates.Quit;
                                                        break;
                                                case "move":
                                                case "go":
                                                        MoveTo(noun);
                                                        break;
                                                case "examine":
                                                        Examine(noun);
                                                        break;
                                                case "take":
                                                case "pickup":
                                                        Pickup(noun);
                                                        break;
                                                case "drop":
                                                case "place":
                                                        Place(noun);
                                                        break;
                                                case "use":
                                                        Use(noun);
                                                        break;
                                                case "items":
                                                case "inventory":
                                                case "inv":
                                                        DisplayInventory();
                                                        break;
                                                case "attack":
                                                        //attack command
                                                        break;
                                        }
                                }
                                break;

                        case Program.GameStates.Battle:
                                if(IsValidInput(BattleCommands, verb, true))
                                {
                                        switch(verb) //this is the other switch statement
                                        {
                                                case "attack":
                                                        //attack command
                                                        break;
                                                case "flee":
                                                case "escape":
                                                        //flee command
                                                        break;
                                                case "use":
                                                        //use command
                                                        break;
                                                case "items":
                                                case "inventory":
                                                case "inv":
                                                        //items command
                                                        break;
                                        }
                                }
                                break;
                }
        }
        #endregion

How do I refactor this to use a dispatch table?

Answer 1

Simplest way would be to use a dictionary of delegates.

For example:

Dictionary<string, Action> dispatch = new Dictionary<string, Action>();

dispatch["help"] = new Action(() => Console.WriteLine("Hello"));
dispatch["dosomething"] = new Action(() =>
{
    // Do something else
    Console.WriteLine("Do Something");
});

// Call the 'help' command
dispatch["help"]();

For multiple different parameters it might be simplest to use a base Delegate and use dynamic Invoke.

Dictionary<string, Delegate> dispatch = new Dictionary<string, Delegate>();

dispatch["help"] = new Action(() => Console.WriteLine("Hello"));
dispatch["dosomething"] = new Action<string>(s => Console.WriteLine(s));

dispatch["help"].DynamicInvoke();
dispatch["dosomething"].DynamicInvoke("World");

And if using .NET 4 you can also use dynamic types to resolve at run time to reduce the clutter slightly of dynamic invoke.

Dictionary<string, dynamic> dispatch = new Dictionary<string, dynamic>();

dispatch["help"] = new Action(() => Console.WriteLine("Hello"));
dispatch["dosomething"] = new Action<string>(s => Console.WriteLine(s));

dispatch["help"]();
dispatch["dosomething"]("World");

Answer 2

Maybe he was refering to 'Double Dispatch', or the Visitor Pattern.

You could split up your code to improve a more 'dispatcher' style as follows:

public interface IGameState{
  void Help();
  void Question();
  void Attack();
}

public interface ICommand{
  bool IsValidFor(PlayingState state);
  bool IsValidFor(BattleState state);
  void Execute(IGameState state);
}

public class PlayingState : IGameState {
   public void Help(){ // Do Nothing }
   public void Question() { WriteCommands(); }

   private void WriteCommands(){ }
}

public class Battle : IGameState{
   public void Help(){ // Do Nothing }
   public void Question() { WriteCommands(); }
   public void Attack() { Roll(7); }

   private void Roll(int numDice){ }
}

public class CommandBuilder{
  public ICommand Parse(string verb){
    switch(verb){
       case "help":
         return new HelpCommand();
       case "?":
         return new QuestionCommand();
       case "attack":
         return new AttackCommand();
       default:
         return new UnknownCommand();
    }
  }
}

public class QuestionCommand(){
  bool IsValidFor(PlayingState  state){
     return true;
  }

  bool IsValidFor(BattleState state){
     return false;
  }

  void Execute(IGameState state){
     state.Question();
  }
}

public static void Do(string aString){
  var command = CommandBuilder.Parse(aString);
  if(command.IsValidFor(Program.GameStates))
     command.Execute(Program.Gamestates);
}

Answer 3

To take back @Jon Ericson's introduction :

A dispatch table is a data structure that associates an index (or key, read @pst's comment below) value to an action. It's a rather elegant replacement for a switch-type statement.

Concerning the implementation part, have a look on this question and particularly to this answer, which IMHO seems pretty correct but stays easy to understand.