--- a/Library/LibrarySystem/Core/Stock.cs	Tue Apr 03 23:32:40 2012 +0100
+++ b/Library/LibrarySystem/Core/Stock.cs	Wed Apr 04 19:20:20 2012 +0100
@@ -1,10 +1,24 @@
-using LibrarySystem.Interfaces;
+using System;
+using LibrarySystem.Interfaces;
 
 namespace LibrarySystem.Core
 {
     public class Stock : IStock
     {
+        public IBook Book { get; private set; }
         public bool IsAvailable { get; set; }
-        public IBook Book { get; set; }
+
+        private Stock()
+        {
+            
+        }
+
+        public static IStock Create(IBook book, bool isAvailable)
+        {
+            if (book == null)
+                throw new ArgumentNullException();
+
+            return new Stock() {Book = book, IsAvailable = isAvailable};
+        }
     }
 }