public class Librarian {
  
  // librarian sort strategy storage
  protected ISortStrategy sortStrategy;

  public void SetSortType(ISortStrategy sortStrategy) {
    this.sortStrategy = sortStrategy;
  }
  
  public void SortBooks(List<Books> books) {
    if (sortStrategy != null)
      sortStrategy.Sort(List<Books> books);
  }
  
}

// sort interface
public interface ISortStrategy {
  void Sort(List<Books> books);
}

public class SortAlpha : ISortStrategy {
  public void Sort(List<Books> books) {
    // alphabetical sort
  }
}

public class SortDate : ISortStrategy {
  public void Sort(List<Books> books) {
    // publish date sort
  }
}

// example use
public class Program {
 
 public static void Main(string[] args) {
   
   var books = new List<Books>();
   var lib = new Librarian();
   
   lib.SetSortType(new SortAlpha());
   lib.SortBooks(books); // sorts the books alphabetically
   
   lib.SetSortType(new SortDate());
   lib.SortBooks(books); // sorts the books by publication date
   
 }
  
}