How does System.Text.Encoding class work?

Question

In .NET, there is a class called System.Text.Encoding in mscorlib. But when you do System.Text.Encoding.ASCII, you can access the ASCII encoding class.

At first, I thiught this was a class in a class kind of thing:

class Encoding {
    class ASCII { ... }
    ...
}

But what is confusing is that you can also use System.Text.ASCIIEncoding to access the same functions.

Not being able to decompile .NET (as .NET Reflector costs money and I don't understand IL that well [so no ildasm]), I can't wrap my head around how the two classes are equal. Could it be something like this?

class ASCIIEncoding {
    internal ASCIIEncoding() { ... }
    /* static functions */
}
static class Encoding {
    public static readonly ASCIIEncoding ASCII = new ASCIIEncoding();
    ...
}

This seems like the most likely way, but the function prototype is

public static Encoding ASCII { get; }

Returning a class deriviated from Encoding doesn't make sense as Encoding is a class with the Encoding types (ASCII, UTF-(7/8/16/32), etc.), so that means that ASCIIEncoding would need those variables also, no?

Can anyone help clear up this confusion?

Answer 1

Returning a class deriviated from Encoding doesn't make sense as Encoding is a class with the Encoding types (ASCII, UTF-(7/8/16/32), etc.), so that means that ASCIIEncoding would need those variables also, no?

No, because Encoding.ASCII etc are static properties, presumably backed by static fields (although that's an implementation detail - the property could just create a new instance each time, or there could be a static internal field within ASCIIEncoding which it uses, etc).

It's just like this:

public class BaseClass
{
    private readonly static BaseClass firstDerived = new FirstDerived();
    private readonly static BaseClass secondDerived = new SecondDerived();

    public static BaseClass First
    { 
        get { return firstDerived; } 
    }

    public static BaseClass Second
    { 
        get { return secondDerived; } 
    }
}

public class FirstDerived : BaseClass {}
public class SecondDerived : BaseClass {}

That's complete code - albeit useless in terms of the derived classes actually doing anything. Make sure you understand every bit of how that works, and then just apply it to Encoding...