Done.

DesignPatternsSolution/Execise.Tests/UnitTests.cs

@@ -10,6 +10,9 @@ public class UnitTests
         [TestMethod]
         public void LibraryShouldBeSingleton()
         {
+            Library lib1 = Library.getLib();
+            Library lib2 = Library.getLib();
+            Assert.ReferenceEquals(lib1, lib2);
             //check that when you create a Library instance second time, 
             //you get exactly the same instance as for a very first time
         }
@@ -18,21 +21,31 @@ public void LibraryShouldBeSingleton()
         [TestMethod]
         public void BookShouldRegister()
         {
+            Book book = new Book("Gary", "Program coding", 2017, 1, new Exercise.RegistrationApi());
+            Assert.AreNotEqual(-1, RegistrationRepository.Register(book));
         }
 
         //test that an customer was registered successfully by checking the returned Id value is not -1
         [TestMethod]
         public void CustomerShouldRegister()
         {
+            Customer customer = new Customer("Gagandeep", "Winnipeg", new Exercise.RegistrationApi());
+            Assert.AreNotEqual(-1, RegistrationRepository.Register(customer));
         }
 
         //test that a book can be borrowed
         [TestMethod]
         public void CanBorrowBook()
         {
+            Book book1 = new Book("Gary2", "Program coding", 2017, 200, new Exercise.RegistrationApi());
+            BookBorrowable bookborrow = new BookBorrowable(book1, new Decorator());
+
+            bookborrow.Borrow();
+            Assert.AreEqual(199, book1.AvailableAmount);
+
             //create a borrowable book with available amount more than one. 
             //Run BorrowOne method of the BookBorrowable instance. Check that total amount was reduced by one.
-            
-        }       
+
+        }
     }
 }

DesignPatternsSolution/Exercise/BookBorrowable.cs

@@ -1,7 +1,28 @@
 namespace Exercise
 {
-    public class BookBorrowable 
+    public class BookBorrowable : Book
     {
-
+        private Book book;
+        private Decorator decorator;
+
+        public BookBorrowable(Book book, Decorator decorator)
+            : base(book.Author, book.NameOrTitle, book.YearCreated, book.AvailableAmount, new Exercise.RegistrationApi())
+        {
+            this.book = book;
+            this.decorator = decorator;
+        }
+
+        public void Borrow()
+        {
+            decorator.Borrow(book);
+        }
+    }
+
+    public class Decorator
+    {
+        public void Borrow(Book book)
+        {
+            book.AvailableAmount--;
+        }
     }
 }

DesignPatternsSolution/Exercise/Item.cs

@@ -2,7 +2,9 @@
 {
     public abstract class Item : LibObject
     {
-        public Item(int amount, int year)
+        //public Item(int amount, int year)
+        public Item(int amount, int year, IRegistrationApi registrationApi)
+            : base(registrationApi)
         {
             AvailableAmount = amount;
             ObjType = ObjectType.Item;
@@ -12,7 +14,9 @@ public Item(int amount, int year)
 
     public class Book : Item
     {
-        public Book(string author, string title, int year, int amount) : base(amount, year)
+        //public Book(string author, string title, int year, int amount) : base(amount, year)
+        public Book(string author, string title, int year, int amount, IRegistrationApi registrationApi)
+            : base(amount, year, registrationApi)
         {
             NameOrTitle = title;
             Author = author;

DesignPatternsSolution/Exercise/LibObject.cs

@@ -1,12 +1,22 @@
 namespace Exercise
 {
-    public abstract class LibObject
+    public abstract class LibObject : IRegistarable
     {
+        private IRegistrationApi registrationApi;
+        protected LibObject(IRegistrationApi registrationApi)
+        {
+            this.registrationApi = registrationApi;
+        }
         public int ObjectId { get; set; }
         public int AvailableAmount { get; set; }
         public string NameOrTitle { get; set; }
         public ObjectType ObjType { get; set; }
         public int YearCreated { get; set; }
+
+        public RegisteredObject GetRegistrationInfo()
+        {
+            return registrationApi.GetRegistrationInfo(this);
+        }
     }
 
     public enum ObjectType

DesignPatternsSolution/Exercise/Library.cs

@@ -4,14 +4,23 @@ public class Library
     {
         //implement Singleton to make sure only one library will exist
 
-
 
+        static Library lib = new Library();
+
+        private Library() { }
         //Implement Register method by utilizing RegistrationRepository (complete missing parts)
+
+        public static Library getLib()
+        {
+            return lib;
+        }
         public int Register()
         {
             // return RegistrationRepository.Register();
 
             //dummy, just to compile
+
+
             return 0;
         }
     }

DesignPatternsSolution/Exercise/Person.cs

@@ -7,7 +7,9 @@ public class Customer : LibObject
         public string Address { get; set; }
         public DateTime RegisteredAt { get; set; }
 
-        public Customer(string name, string addr)
+        //public Customer(string name, string addr)
+        public Customer(string name, string addr, IRegistrationApi registrationApi)
+            : base(registrationApi)
         {
             NameOrTitle = name;
             Address = addr;

DesignPatternsSolution/Exercise/Registration.cs

@@ -3,6 +3,31 @@
 using System.Linq;
 namespace Exercise
 {
+
+
+    public interface IRegistrationApi
+    {
+        RegisteredObject GetRegistrationInfo(LibObject libObject);
+    }
+
+    public class RegistrationApi : IRegistrationApi
+    {
+        public RegistrationApi()
+        {
+        }
+
+        public RegisteredObject GetRegistrationInfo(LibObject libObject)
+        {
+            return new RegisteredObject
+            {
+                Info = libObject.NameOrTitle,
+                AvailableAmount = libObject.AvailableAmount,
+                Id = libObject.ObjectId
+            };
+        }
+    }
+
+
     public interface IRegistarable
     {
         RegisteredObject GetRegistrationInfo();