Question

Using interfaces won’t work because I want a single implementation. Using this solution would end in a lot of redundant code because I plan on having quite a few sub classes (composition vs inheritance). I’ve decided that a problem-specific design solution is what I’m looking for, and I can’t think of anything elegant.

Basically I want classes to have separate properties, and for those properties to be attached at design time to any sub class I choose. Say, I have class ‘ninja’. I would like to be able to make arbitrary sub classes such as ‘grayNinja’ where a gray ninja will always have a sword and throwing stars. Then possibly ‘redNinja’ who will always have a sword and a cape. Obviously swords, stars, and capes will each have their own implementation – and this is where I can’t use interfaces. The closest solution I could find was the decorator pattern, but I don’t want that functionality at runtime. Is the best solution an offshoot of that? Where inside the Black Ninja class constructor, I pass it through the constructors of sword and throwingStar? (those being abstract classes)

haven’t coded in a while and reading hasn’t gotten me too far – forgive me if the answer is simple.

Edit: Answered my own question. I can’t mark it as ‘answer’ until tomorrow. Please let me know if there’s a problem with it that I didn’t catch. All the reading this problem forced me to do has been awesome. Learned quite a bit.

Answer 1

Alright so mix-ins through extension methods are going to be my preferred route. I couldn’t figure out how to use dynamic proxies in vb.net (seemed to require libraries with lots of documentation that didn’t cover specifically what I needed). Dynamic proxies also seems to be a bit dirtier of a solution than using extension methods. Composition would have been what I defaulted to if the previous two didn’t work.

So one problem with extension methods, is that the code gets a little dirtier if you want to hold variables. Not much though. Another problem is that all the extension methods must be defined in modules, so the code might look a little goofy to a new eye. I will solve this by defining my interface and module with the corresponding extension method in the same file.

finally, here’s some sample vb.net code if you don’t want to see a full fledged example through the link.

Imports System.Runtime.CompilerServices 'for extension methods

Public Interface ISword
End Interface
Public Interface IThrowingStar
End Interface

Module ExtensionMethods

    <Extension()>
    Public Sub swingSword(ByVal hasASword As ISword)
        Console.WriteLine("Sword has been swung")
    End Sub

    <Extension()>
    Public Sub throwStar(ByVal hasAStar As IThrowingStar)
        Console.WriteLine("Star has been thrown")
    End Sub

End Module

Public Class RedNinja
    Inherits Ninja
    Implements IThrowingStar, ISword

    Public Sub New()
    End Sub

End Class

Public MustInherit Class Ninja

    private curHealth as Integer

    Public Sub New()
        curHealth = 100
    End Sub

    Public Function getHP() As Integer
        Return curHealth
    End Function

End Class

Module Module1

    Sub main()

        Console.WriteLine("Type any character to continue.")
        Console.ReadKey()

        Dim a As New RedNinja
        a.swingSword() 'prints "Sword has been swung"
        a.throwStar()  'prints "Star has been thrown"

        Console.WriteLine("End of program - Type any key to exit")
        Console.ReadKey()

    End Sub
End Module