Dynamic method dispatch based on value of variable

Question

Hi there,

Long switch statments are often frowned upon. The solution is to use polymorphism. However what if the thing I'm switching on is not a type code? What I would like to do is replace the switch statement with something like this...

public void HandleString(string s = "Hello")
{
 ...
}

public void HandleString(string s = "Goodbye")
{
 ...
}

...
HandleString("Hello"); // results in the first method being called.

This would replace the following...

string s = "Hello";

switch(s)
{
   case "Hello":
   ...
   break;
   case "Goodbye":
   ...
   break;
   default;
   break;
}

Any ideas? In theory I think you could do away with 'if/switch' statements altogether and just call methods that are automatically bound based on the value of an expression.

Answer 1

If you have a large number of options, and high possibility that there will be more in the future - or you just need to system to be easily extensible - then you can always use an explicit dispatch table:

Dictionary<string, Action<string>> actions =
    new Dictionary<string, Action<string>>()
    {
        { "Hello", HandleHello },
        { "Goodbye", HandleGoodbye }
    };

private static void HandleHello(string s) { ... }

private static void HandleGoodbye(string s) { ... }

...

actions[s](s);

You can also provide a way to extend the table by allowing external clients of your API to register their own handler for a given string.

Answer 2

There are languages that implement that sort of semantics. One that I'm familiar with is the compiler generator tool called Elegant from Phillips.

In a language like this, a simple factorial algorithm might look like:

fact (value : Int) : Int
    conditions value < 0
{
    { "Illegal input value\n" } astype Message
    return 0
}

fact (value = 0) : Int
{
    return 0
}

fact (value = 1) : Int
{
    return 1
}

fact (value : Int) : Int
{
    return value * fact(value - 1);
}