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copied!<p>An interface could be private within another class</p> <pre><code>public class MyClass { private interface IFoo { int MyProp { get; } } private class Foo : IFoo { public int MyProp { get; set; } } public static void Main(string[] args) { IFoo foo = new Foo(); return foo.MyProp; } } </code></pre> <p>in terms of utility it simply hides from other code, even within the same assembly, that said interface exists. The utility of this is not terribly high in my opinion.</p> <p><a href="http://msdn.microsoft.com/en-us/library/aa288461(VS.71).aspx" rel="noreferrer">Explicit interface implementation</a> is a different matter, has some very useful cases (especially when working with generics and older non generic interfaces) but I would not term it 'private interfaces' and would not say that the term is commonly used in that manner.</p> <p>Using the two techniques together you can do:</p> <pre><code>public class MyClass { private interface IFoo { int MyProp { get; } } public class Foo : IFoo { int IFoo.MyProp { get; set; } } public static void Main(string[] args) { IFoo foo = new Foo(); return foo.MyProp; } } public class HiddenFromMe { public static void Main(string[] args) { MyClass.Foo foo = new MyClass.Foo(); return foo.MyProp; // fails to compile } } </code></pre> <p>This allows you to expose the nested classes in some fashion while allowing the parent class to invoke methods on them that the outside world cannot. This is a potentially useful case but is not something I would wish to use very often. Certainly it's use in an interview smacks of being a boundary case the interviewer is using because they've seen it and though it was 'interesting'</p>

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