C# Extension Methods only visible and accessible within one class ("private")

Question

Is it possible, in C#, to create extension methods on a class but restrict visibility/accessibility within a class? (e.g. Extension Method A on class M is only accessible within class Z)

Example:

class A
{
     String foo = "";
     String bar = foo.MakeMillionaire("arg");
}

In above example I want the extension method "MakeMillionaire" extending the String class only to be visible and accessible within class A. Can I do this somehow by defining the extension method in a static class within class A?

Edit: Trying a regular nested class yields "Error: Extension methods must be defined in a top level static class".

Answer 1

Extension methods can only be defined in a static non-generic outer (non-nested) class.

What I usually do in such scenarios is make a separate static internal class in a different namespace in the same file, then include that namespace only in that file.

It would still be visible to other classes in that assembly; the only way to avoid that is to move the consuming class (class A in your example) to its own assembly, which you probably don't want to do.

Answer 2

Declare your Extension Methods in a separate namespace, and you can include that namespace in specific files that you want to use them. Then, declare ClassA (the class you want to use your extension methods in) in a separate file, and use that namespace at the top of ClassA.cs. That way, only that class will have access to those extension methods.

Edit:

Something like the following

namespace Extension {
    public static class ExtensionMethods {
        public static string EnumValue(this MyEnum e) {
            switch (e) {
                case MyEnum.First:
                    return "First Friendly Value";
                case MyEnum.Second:
                    return "Second Friendly Value";
                case MyEnum.Third:
                    return "Third Friendly Value";
            }
            return "Horrible Failure!!";
        }
    }
}

ClassA.cs:

using Extension;

public class ClassA{
    //Work your magic here, using the EnumValue Extension method
    //wherever you want
}

ClassB.cs

public class ClassB{
    //EnumValue is not a valid Extension Method here.
}