using System;

​

public class Program

{

    // Properties you want others to have access too

    public interface ICredentialsValidator

    {

        bool IsTest(string userName);

    }

​

    public static void Main()

    {

        var b = new PublicClass().GetCredentialsValidator();

​

        Console.WriteLine(b.IsTest("test"));

        Console.WriteLine(b.IsTest("blah"));

    }

​

    public class PublicClass

    {

        public ICredentialsValidator GetCredentialsValidator()

        {

            return new PrivateClass();

        }

​

        private class PrivateClass : ICredentialsValidator

        {

            public bool IsTest (string userName)

            {

                return userName == "test";

            }

        }

    }

}