What is a private interface?

Question

In an interview a while ago for a .NET position the interviewer asked me "what would you use a private interface for?".

I asked him did he mean the difference between implicit vs explicit interface implementation to which he answered no.

So I'm wondering:

1. What he meant?
2. What you would use a private interface for?

Answer 1

See this link. "No," is a pretty poor answer if he was looking to find out what you knew. Sounds like someone who just wants to show how much they know.

Answer 2

An interface could be private within another class

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;
    }
}

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.

Explicit interface implementation 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.

Using the two techniques together you can do:

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
    }
}

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'

Answer 3

Just like an inner class (which is also private) you can use a private interface in an existing class.

Answer 4

I googled around a bit and found this article explaining how private interfaces can be used to provide different interfaces to different clients. This is C++ story.

I don't think this can be applied to C# tho, because the same effect IMO can be achieved with explicit interfaces and clients that cast host to appropriate interface.

Maybe somebody else can see something I missed there....

I also found this at MSDN:

Interface methods have public accessibility, which cannot be changed by the implementing type. An internal interface creates a contract that is not intended to be implemented outside the assembly that defines the interface. A public type that implements a method of an internal interface using the virtual modifier allows the method to be overridden by a derived type that is outside the assembly. If a second type in the defining assembly calls the method and expects an internal-only contract, behavior might be compromised when, instead, the overridden method in the outside assembly is executed. This creates a security vulnerability.