defaultInterface.cs

    public interface IFileSystem
    {
        File CreateFile(string filename);
        void DeleteFile(string filename);
        File GetFile(string filename);

        public File GetOrCreateFile(string filename)
        {
            var file = GetFile(filename);

            if (file == null)
                file = CreateFile(filename);

            return file;
        }
    }

    public class WindowsFileSystem : IFileSystem
    {
        public File CreateFile(string filename)
        {
            throw new System.NotImplementedException();
        }

        public void DeleteFile(string filename)
        {
            throw new System.NotImplementedException();
        }

        public File GetFile(string filename)
        {
            throw new System.NotImplementedException();
        }

       
    }