C# Add extension method to instance of class?

Question

I basically want to inject an extension method to each copy of ArrayList, which would perform the following behavior:

ArrayList ourList = new ArrayList();
ourList.Add(randomarray or random arraylist);

It'd add the CONTENT of the given array, arraylist or stack to 'ourList' and not the array, arraylist or stack itself.

However, my problem is:

How do I inject an extension method ONLY into the instanced class? The following code adds the method to the ArrayList baseclass and any instanced copy, however I want it to be ONLY available when you access an instanced copy of the class.

    public static void Add(this ArrayList ourlist){

    }

    ArrayList.Add(); // Works, but shouldn't
    ArrayList result = new ArrayList();
    result.Add(); // Works

So, how do I manage this?

In short, you can't. All methods of a type, and all extension methods of a type, apply to all instances of that type. — p.s.w.g, Jul 15 '13 at 13:04
@DanielHilgarth ArrayList result = new ArrayList(); Result would be the instanced copy here. Not sure, if that's the official name though considering I'm self-taught. — Jan Berktold, Jul 15 '13 at 13:05
Here is a good explanation on why you cannot do this: http://stackoverflow.com/a/2731707/2000557 (If I understand what you are asking) — Gray, Jul 15 '13 at 13:05
`ArrayList.Add(); // Works, but shouldn't`. Please show some real code. Extension methods can't be used to add static methods to a class, so what you are saying is actually not possible, `ArrayList.Add();` will never compile, unless `ArrayList` is actually a *variable* of type `ArrayList`. — Daniel Hilgarth, Jul 15 '13 at 13:07
@JanGieseler: A call of `ArrayList.Add` *doesn't* work. It's not a static method. — Jon Skeet, Jul 15 '13 at 13:07
If you mean you want the extension method to only work with instances, not as a static method, then it should already behave that way. — Charles Bretana, Jul 15 '13 at 13:10

score 10 · Accepted Answer · answered Jul 15 '13 at 13:05

How do I inject an extension method ONLY into the instanced class?

If you're talking about adding an extension method to individual instances, you can't. There's no such concept in C#. An extension method "extends" the type, not individual instances of the type. How would you expect the compiler to know which instances had the extension applied to it at compile-time, if that knowledge is only available at execution-time?

If you're talking about calling an extension method like any other static method, that will always be feasible, and can't be prevented. So you could always call:

MyExtensionClass.Add(null, null);

If you're not talking about either of those things, it's not clear what you mean - but making a static call to ArrayList.Add as shown in your sample code really won't compile. Are you sure you haven't got a variable called ArrayList? (That would certainly mess things up.)

Furthermore, as ArrayList already has an Add(object) instance method, your new extension method would never be used anyway. (You should really move away from the non-generic collections as quickly as you can.)

`ArrayList.Add()` can work if static Extension class named as ArrayList, but with two parameters 'ArrayList.Add(list,value)' — Sriram Sakthivel, Jul 15 '13 at 13:15
@SriramSakthivel: At that point `ArrayList result = new ArrayList();` wouldn't have worked. The type name `ArrayList` can only mean one type in a particular context. — Jon Skeet, Jul 15 '13 at 13:24
yes you're correct. that would have worked if it was ArrayList.Add(list,value); and System.Collections.ArrayList result = new System.Collections.ArrayList() assuming no using directive imported for System.Collections — Sriram Sakthivel, Jul 15 '13 at 13:43

C# Add extension method to instance of class?

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