Thread: Indexer question...

Is there a way to implement an Indexer on a class member and not the class itself?

This is strictly for code readability because I would like the idea of having an Indexer property of a variable within a class.

An Example would be

Code:

    public class MyClass
    {
        private List<PlayerClass> player = new List<PlayerClass>();

        public PlayerClass Player[int index]
        {
            get { return player[index]; }
            set { player.Add(value); }
        }    
    }

Then lets say I instantiate a MyClass object

MyClass blah = new MyClass();

I would be able to access the player list like so...

blah.Player[1]

without making the player list public. I would like to keep this member private so that I can use the properties to validate the input (even though that isn't shown in this example). I know I can do this with a method and keep my list private, I was just hoping i could use the indexer notation []. Any ideas?

You can do it with a secondary helper class.

E.g. MyClass.Player is a normal property that returns an object of another class called something like PlayerCollection (you could also make this class implement IEnumerable so that you can use it inside foreach) and then just have this second class support an indexer method.

Here, the second class would contain the actual List; MyClass just accesses it through its own Player object.

That still wont work. Lets use a baseball game for example. A baseball game would be a class and it has Innings, so those innings would be a class with seperate methods to access the top or botton of the inning, with scores, etc. Well a baseball game would have more than 1 inning, so I want to use a List<Innings> variable (keeping it private), but I want that variable to have a public indexer.... so...

BaseBallClass game = new BaseBallClass();

game.Inning[1].TopOf();
game.Innine[1].BottomOf(int runs);
game.Inning[4].BottomOf();

Code:

public class BaseBallClass
    {
        private List<Innings> inning = new List<Innings>();

        public Innings Inning   // Here I want public Innings Inning[int index] but it wont work.
        {
            get { return inning[index]; } // make sure its within range.
            set { inning.add(value); } // use the indexer for validation purposes.
        }

    }

    public class Innings
    {
        private int topInningRuns;
        private int bottomInningRuns;

        public int TopOf
        {
            //get ; set ; here
        }
    }

I think I am beginning to see that this can not be done unless I use public variables. Unless someone else has any ideas?

Windows Mixer Control in C# - CodeGuru

There is sample code for Audio Mixers... Take a look at the implementation of the MixerDetails class

The search term you're looking for is "indexed property". The following link has the information: Indexed Properties Tutorial (C#). In short, you can't use indexed properties (other than the one defined with 'this') in C#.

Originally Posted by pianorain

The search term you're looking for is "indexed property". The following link has the information: Indexed Properties Tutorial (C#). In short, you can't use indexed properties (other than the one defined with 'this') in C#.

Thanks, Yea, I've come to that conclusion, I have a specific implementation that I want to do, and its pretty much impossible. So I will just stick with accessor and mutator methods with my private List<>.

Originally Posted by stumon

That still wont work. Lets use a baseball game for example. A baseball game would be a class and it has Innings, so those innings would be a class with seperate methods to access the top or botton of the inning, with scores, etc. Well a baseball game would have more than 1 inning, so I want to use a List<Innings> variable (keeping it private), but I want that variable to have a public indexer.... so...

BaseBallClass game = new BaseBallClass();

game.Inning[1].TopOf();
game.Innine[1].BottomOf(int runs);
game.Inning[4].BottomOf();

Code:

public class BaseBallClass
    {
        private List<Innings> inning = new List<Innings>();

        public Innings Inning   // Here I want public Innings Inning[int index] but it wont work.
        {
            get { return inning[index]; } // make sure its within range.
            set { inning.add(value); } // use the indexer for validation purposes.
        }

    }

    public class Innings
    {
        private int topInningRuns;
        private int bottomInningRuns;

        public int TopOf
        {
            //get ; set ; here
        }
    }

I think I am beginning to see that this can not be done unless I use public variables. Unless someone else has any ideas?

No, here's what I'm suggesting:

Code:

    public class BaseBallClass
    {
        private InningCollection innings = new InningCollection();

        public InningCollection Innings
        {
            get { return innings; } 
            private set { } // only for your class's internal use
        }

    }

    public class InningCollection // Probably want to implement IEnumerable here too
    {

        private List<Inning> innings = new List<Inning>();

        public Inning this[int i]
        {
            get { return innings[i]; }
            set { innings[i] = value; }
        }

        public void Add(Inning i)
        {
            innings.Add(i);
        }

        // Other stuff, like IEnumerable members, etc.
    }


    public class Inning
    {
        private int topInningRuns;
        private int bottomInningRuns;

        public int TopOf
        {
            //get ; set ; here
        }
    }

Then you could do this:

Code:

    BaseBallClass game = new BaseBallClass();
    game.Innings[2].TopOf = 5;

That works because game.Innings returns an InningCollection, and then [2] references the second Inning in the InningCollection.

By making InningCollection support IEnumerable you can even do things like:

Code:

foreach (Inning i in game.Innings)
{
}

Essentially, the main class has a property that returns a collection class, and the collection class is indexable, making effectively an indexed named property. This is used a lot in Windows Forms, for example:
listbox1.Items[0] -- .Items returns a collection object that can be indexed, iterated over, etc.

Yes, at first it seems like it adds a lot of overhead, it adds an additional class to the mix, but that class consists of code you'd already need -- code to index and iterate your collection -- and it keeps all that code in one place, separated from everything else.

Thank you Cat. I see now where I was going wrong.

Originally Posted by Cat

Yes, at first it seems like it adds a lot of overhead, it adds an additional class to the mix, but that class consists of code you'd already need -- code to index and iterate your collection -- and it keeps all that code in one place, separated from everything else.

Exactly. Indexed properties have their failings: you can't enumerate over them and you can't ask for a count to know which indices are valid. With a separate class, you can define all that and the logic stays in one place.