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    <p>In Java, a <code>static</code> inner class has no implicit access to the members of its enclosing type. In C#, <em>all</em> nested types have no such access to their parent type's members; there is no modifier you need to add in C# to trigger this behavior.</p> <p>In C#, <code>static</code> classes are <code>abstract sealed</code>, so they cannot be created nor derived -- this is not the same meaning as in Java. Additionally, interfaces cannot contain type declarations of their own.</p> <p>Try something like this:</p> <pre><code>public interface IActivation { double Func(double inputput); } public class S1 : IActivation { public static readonly S1 Instance = new S1(); private S1() { } public double Func(double input) { if (input &gt; 0) return 1.0; return 0.0; } } </code></pre> <hr> <p>If your goal is to provide default implementations in some "readable" way (though I dispute that <code>IActivator.S1()</code> is inherently more readable...) then you could create a static factory class:</p> <pre><code>public static class Activator { public static S1 S1 { get { return S1.Instance; // Or you could do this if you make the S1 constructor public: // return new S1(); } } } </code></pre> <p>However, I dispute the claim that this is more readable or helpful. Visual Studio will, when constructing an object in the context of a particular type, display all of that type's subtypes. So if you do this (<code>|</code> represents the cursor):</p> <pre><code>IActivator foo = new | </code></pre> <p>You should get a neat list of all of the classes in your current scope that implement IActivotor.</p>
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    1. COTrue, that's one of the possibilities that I had considered, but it would break the "code readability" constraint (point 2.). Because then you'd have to use `protected IActivation activation = new S1();` . The original intent (this is my conjecture, as the code is not from me) is to have some form of intellisense provide you with alternatives should you feel to have to use another implementation. In your case everyone would have to guess what the other implementations would be...
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    2. COI see that you've updated your answer after my first comment, much appreciated. Maybe I should clarify a little: In the original programmer's design `IActivation activation = new IActivation.|` would list all available implementations of IActivation, and none other (e.g. implementations of IActivation2). In order to achieve the very same effect you'd need to put a factory pattern into use, which is quite the writing overhead imho. No offense meant, but I currently consider John's solution to be the most suited one, despite its drawbacks (which don't exist in the project).
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