done.

DesignPatternsSolution/Execise.Tests/UnitTests.cs

@@ -12,18 +12,27 @@ public void LibraryShouldBeSingleton()
         {
             //check that when you create a Library instance second time, 
             //you get exactly the same instance as for a very first time
+            var first = Library.getLibrary();
+            var second = Library.getLibrary();
+
+            Assert.AreEqual(first, second);
         }
 
         //test that an book was registered successfully by checking the returned Id value is not -1
         [TestMethod]
         public void BookShouldRegister()
         {
+            int x = RegistrationRepository.Register(new Book("Bakul", "No title", 2015, 19));
+
+            Assert.AreNotEqual(-1, x);
         }
 
         //test that an customer was registered successfully by checking the returned Id value is not -1
         [TestMethod]
         public void CustomerShouldRegister()
         {
+            int x = RegistrationRepository.Register(new Customer("Bakul", "320 street not available"));
+            Assert.AreNotEqual(-1, x);
         }
 
         //test that a book can be borrowed
@@ -32,7 +41,10 @@ public void CanBorrowBook()
         {
             //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.
-
+            Book b = new Book("Bakul", "No title", 2015, 200);
+            BookBorrowable bb = new BookBorrowable(b, new Decorator());
+            bb.Borrow();
+            Assert.AreNotEqual(200, b.AvailableAmount);
         }       
     }
 }

DesignPatternsSolution/Exercise/BookBorrowable.cs

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

DesignPatternsSolution/Exercise/Item.cs

@@ -10,13 +10,17 @@ public Item(int amount, int year)
         }
     }
 
-    public class Book : Item
+    public class Book : Item, IRegistarable
     {
         public Book(string author, string title, int year, int amount) : base(amount, year)
         {
             NameOrTitle = title;
             Author = author;
         }
         public string Author { get; set; }
+        public RegisteredObject GetRegistrationInfo()
+        {
+            return new RegisteredObject() { AvailableAmount = AvailableAmount, Info = NameOrTitle };
+        }
     }
 }

DesignPatternsSolution/Exercise/Library.cs

@@ -4,7 +4,11 @@ public class Library
     {
         //implement Singleton to make sure only one library will exist
 
-
+        static Library library = new Library();
+
+        private Library() { }
+
+        public static Library getLibrary() { return library; }
 
         //Implement Register method by utilizing RegistrationRepository (complete missing parts)
         public int Register()

DesignPatternsSolution/Exercise/Person.cs

@@ -2,7 +2,7 @@
 
 namespace Exercise
 {
-    public class Customer : LibObject
+    public class Customer : LibObject , IRegistarable
     {
         public string Address { get; set; }
         public DateTime RegisteredAt { get; set; }
@@ -14,7 +14,13 @@ public Customer(string name, string addr)
             RegisteredAt = DateTime.Now;
             ObjType = ObjectType.Person;
         }
-
+        public RegisteredObject GetRegistrationInfo()
+        {
+            return new RegisteredObject()
+            {
+                Info = NameOrTitle
+            };
+        }
         public void SetId(int id)
         {
             ObjectId = id;