Add Iterator Design Pattern implementation

Implemented the Iterator pattern with `Book`, `BookCollection`,
`BookIterator`, `IBookCollection`, and `IIterator` classes and
interfaces. Updated `Program.cs` to demonstrate the pattern
with a collection of books and an iterator.

Updated `README.md` to document the Iterator pattern, its
purpose, usage example, and test locations. Added `IteratorTests`
to verify correct iteration over a `BookCollection`.

Ensured adherence to the Iterator pattern principles by
providing sequential access to collection elements without
exposing the underlying representation.

Author: froghramar

DesignPatterns/Iterator/Book.cs

@@ -0,0 +1,15 @@
+namespace DesignPatterns.Iterator;
+
+public sealed class Book
+{
+    public string Title { get; }
+    public string Author { get; }
+
+    public Book(string title, string author)
+    {
+        Title = title ?? string.Empty;
+        Author = author ?? string.Empty;
+    }
+
+    public override string ToString() => $"{Title} by {Author}";
+}

DesignPatterns/Iterator/BookCollection.cs

@@ -0,0 +1,10 @@
+namespace DesignPatterns.Iterator;
+
+public class BookCollection : IBookCollection
+{
+    private readonly List<Book> _books = new();
+
+    public void Add(Book book) => _books.Add(book);
+
+    public IIterator CreateIterator() => new BookIterator(_books);
+}

DesignPatterns/Iterator/BookIterator.cs

@@ -0,0 +1,20 @@
+namespace DesignPatterns.Iterator;
+
+public class BookIterator : IIterator
+{
+    private readonly List<Book> _books;
+    private int _position = 0;
+
+    public BookIterator(List<Book> books)
+    {
+        _books = books ?? new List<Book>();
+    }
+
+    public bool HasNext() => _position < _books.Count;
+
+    public Book Next()
+    {
+        if (!HasNext()) throw new InvalidOperationException();
+        return _books[_position++];
+    }
+}

DesignPatterns/Iterator/IBookCollection.cs

@@ -0,0 +1,6 @@
+namespace DesignPatterns.Iterator;
+
+public interface IBookCollection
+{
+    IIterator CreateIterator();
+}

DesignPatterns/Iterator/IIterator.cs

@@ -0,0 +1,7 @@
+namespace DesignPatterns.Iterator;
+
+public interface IIterator
+{
+    bool HasNext();
+    Book Next();
+}

DesignPatterns/Program.cs

@@ -11,6 +11,7 @@
 using DesignPatterns.Flyweight;
 using DesignPatterns.Proxy;
 using DesignPatterns.Chain;
+using DesignPatterns.Iterator;
 
 Console.WriteLine("Factory pattern demo:");
 
@@ -98,6 +99,18 @@
 nested.Add(new CircleGraphic());
 Console.WriteLine(nested.Draw());
 
+// Iterator demo
+Console.WriteLine();
+Console.WriteLine("Iterator pattern demo:");
+var books = new BookCollection();
+books.Add(new Book("Title1", "Author1"));
+books.Add(new Book("Title2", "Author2"));
+
+var it = books.CreateIterator();
+while (it.HasNext())
+{
+    Console.WriteLine(it.Next());
+}
 
 // Chain of Responsibility demo
 Console.WriteLine();

README.md

@@ -3,7 +3,7 @@ Design Patterns Examples
 This solution demonstrates simple design pattern examples in C# targeting .NET 10 (C# 14).
 
 Projects
-- `DesignPatterns` - Console demo showing Factory, Singleton, Builder, Prototype, Abstract Factory, Adapter, Bridge, Composite, Decorator, Facade, Flyweight, Proxy, Chain of Responsibility, and Command patterns.
+- `DesignPatterns` - Console demo showing Factory, Singleton, Builder, Prototype, Abstract Factory, Adapter, Bridge, Composite, Decorator, Facade, Flyweight, Proxy, Chain of Responsibility, Command, and Iterator patterns.
   - Factory implementation: `DesignPatterns/Factories`
   - Singleton implementation: `DesignPatterns/Singleton/Logger.cs`
   - Builder implementation: `DesignPatterns/Builder/HouseBuilder.cs` and `DesignPatterns/Builder/House.cs`
@@ -18,6 +18,7 @@ Projects
   - Proxy implementation: `DesignPatterns/Proxy/*`
   - Chain of Responsibility implementation: `DesignPatterns/Chain/*`
   - Command implementation: `DesignPatterns/Command/*`
+  - Iterator implementation: `DesignPatterns/Iterator/*`
 - `Tests` - xUnit tests covering the examples.
 
 Included patterns and brief docs
@@ -188,6 +189,19 @@ Included patterns and brief docs
     remote.PressUndo();
     ```
 
+- Iterator Pattern
+  - Purpose: Provide a way to access the elements of an aggregate object sequentially without exposing its underlying representation.
+  - Example: `DesignPatterns/Iterator/*` contains `Book`, `BookCollection`, `BookIterator`, and interfaces `IBookCollection`, `IIterator`.
+  - Usage snippet:
+
+    ```csharp
+    var books = new BookCollection();
+    books.Add(new Book("Title1", "Author1"));
+    books.Add(new Book("Title2", "Author2"));
+    var it = books.CreateIterator();
+    while (it.HasNext()) Console.WriteLine(it.Next());
+    ```
+
 Tests
 - Tests are written with xUnit in the `Tests` project.
   - Factory tests: `Tests/ShapeFactoryTests.cs`
@@ -204,6 +218,7 @@ Tests
   - Proxy tests: `Tests/ProxyTests.cs`
   - Chain tests: `Tests/ChainTests.cs`
   - Command tests: `Tests/CommandTests.cs`
+  - Iterator tests: `Tests/IteratorTests.cs`
 
 Common commands
 - Build solution: `dotnet build`

Tests/IteratorTests.cs

@@ -0,0 +1,24 @@
+using DesignPatterns.Iterator;
+
+namespace Tests;
+
+public class IteratorTests
+{
+    [Fact]
+    public void BookIterator_IteratesOverCollection()
+    {
+        var collection = new BookCollection();
+        collection.Add(new Book("Title1", "Author1"));
+        collection.Add(new Book("Title2", "Author2"));
+
+        var it = collection.CreateIterator();
+        var titles = new List<string>();
+
+        while (it.HasNext())
+        {
+            titles.Add(it.Next().Title);
+        }
+
+        Assert.Equal(new[] { "Title1", "Title2" }, titles);
+    }
+}