InterviewExample.cs

using System;

namespace TestApp
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Begin");

            var var1 = new MyClass("Name1","exe");
            var var2 = new MyClass("Name2", "txt");
            MyInterface var3 = var1;

            var1.FileExtension = "pdf";

            Console.WriteLine(var1.FileExtension);
            Console.WriteLine(var2.FileExtension);
            Console.WriteLine(var3.FileExtension);

            Console.ReadLine();
        }
    }

    public interface MyInterface
    {
        String FileName { get;  set; }
        String FileExtension { get; set; }
    }

    public class MyClass: MyInterface
    {
        public MyClass(string fileName, String fileExtension)
        {
            _fileName = fileName;
            _fileExtension = fileExtension;
        }

        private string _fileName;

        private string _fileExtension;

        public string FileName
        {
            get { return _fileName; }
            set { _fileName = value; }
        }

        public string FileExtension
        {
            get { return _fileExtension; }
            set { _fileExtension = value; }
        }
    }
}