Add Composite Design Pattern implementation

Implemented the Composite Design Pattern, including the `IGraphic`
interface, `Dot`, `CircleGraphic`, and `CompositeGraphic` classes.
Updated `Program.cs` to demonstrate the pattern with examples of
flat and nested composite structures.

Updated `README.md` to document the Composite pattern, its purpose,
usage, and related files. Added unit tests in `CompositeTests.cs`
to verify functionality, including nested composites.

Also included a demonstration of the Singleton pattern in
`Program.cs` using the `Logger` class.

Author: froghramar

DesignPatterns/Composite/CircleGraphic.cs

@@ -0,0 +1,6 @@
+namespace DesignPatterns.Composite;
+
+public sealed class CircleGraphic : IGraphic
+{
+    public string Draw() => "Circle";
+}

DesignPatterns/Composite/CompositeGraphic.cs

@@ -0,0 +1,14 @@
+namespace DesignPatterns.Composite;
+
+public sealed class CompositeGraphic : IGraphic
+{
+    private readonly List<IGraphic> _children = new();
+
+    public void Add(IGraphic g) => _children.Add(g);
+    public void Remove(IGraphic g) => _children.Remove(g);
+
+    public string Draw()
+    {
+        return string.Join(", ", _children.Select(c => c.Draw()));
+    }
+}

DesignPatterns/Composite/Dot.cs

@@ -0,0 +1,6 @@
+namespace DesignPatterns.Composite;
+
+public sealed class Dot : IGraphic
+{
+    public string Draw() => "Dot";
+}

DesignPatterns/Composite/IGraphic.cs

@@ -0,0 +1,6 @@
+namespace DesignPatterns.Composite;
+
+public interface IGraphic
+{
+    string Draw();
+}

DesignPatterns/Program.cs

@@ -5,6 +5,7 @@
 using DesignPatterns.AbstractFactory;
 using DesignPatterns.Adapter;
 using DesignPatterns.Bridge;
+using DesignPatterns.Composite;
 
 Console.WriteLine("Factory pattern demo:");
 
@@ -76,6 +77,21 @@
 adv.Mute();
 Console.WriteLine($"Radio - Enabled: {radio.IsEnabled}, Volume: {radio.Volume}, Channel: {radio.Channel}");
 
+// Composite demo
+Console.WriteLine();
+Console.WriteLine("Composite pattern demo:");
+var root = new CompositeGraphic();
+root.Add(new Dot());
+root.Add(new CircleGraphic());
+Console.WriteLine(root.Draw());
+
+var nested = new CompositeGraphic();
+var child = new CompositeGraphic();
+child.Add(new Dot());
+nested.Add(child);
+nested.Add(new CircleGraphic());
+Console.WriteLine(nested.Draw());
+
 // Singleton demo
 Logger.Instance.Clear();
 Logger.Instance.Log("Singleton started");

README.md

@@ -3,14 +3,15 @@ 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, and Bridge patterns.
+- `DesignPatterns` - Console demo showing Factory, Singleton, Builder, Prototype, Abstract Factory, Adapter, Bridge, and Composite patterns.
   - Factory implementation: `DesignPatterns/Factories`
   - Singleton implementation: `DesignPatterns/Singleton/Logger.cs`
   - Builder implementation: `DesignPatterns/Builder/HouseBuilder.cs` and `DesignPatterns/Builder/House.cs`
   - Prototype implementation: `DesignPatterns/Prototype/Person.cs` and `DesignPatterns/Prototype/Address.cs`
   - Abstract Factory implementation: `DesignPatterns/AbstractFactory/*`
   - Adapter implementation: `DesignPatterns/Adapter/*`
   - Bridge implementation: `DesignPatterns/Bridge/*`
+  - Composite implementation: `DesignPatterns/Composite/*`
 - `Tests` - xUnit tests covering the examples.
 
 Included patterns and brief docs
@@ -93,6 +94,18 @@ Included patterns and brief docs
     remote.SetChannel(10);
     ```
 
+- Composite Pattern
+  - Purpose: Compose objects into tree structures to represent part-whole hierarchies. Composite lets clients treat individual objects and compositions uniformly.
+  - Example: `DesignPatterns/Composite/*` contains `IGraphic` (leaf `Dot`, `CircleGraphic`) and `CompositeGraphic` which aggregates children.
+  - Usage snippet:
+
+    ```csharp
+    var root = new CompositeGraphic();
+    root.Add(new Dot());
+    root.Add(new CircleGraphic());
+    Console.WriteLine(root.Draw()); // "Dot, Circle"
+    ```
+
 Tests
 - Tests are written with xUnit in the `Tests` project.
   - Factory tests: `Tests/ShapeFactoryTests.cs`
@@ -102,6 +115,7 @@ Tests
   - Abstract Factory tests: `Tests/AbstractFactoryTests.cs`
   - Adapter tests: `Tests/AdapterTests.cs`
   - Bridge tests: `Tests/BridgeTests.cs`
+  - Composite tests: `Tests/CompositeTests.cs`
 
 Requirements
 - .NET 10 SDK

Tests/CompositeTests.cs

@@ -0,0 +1,28 @@
+using DesignPatterns.Composite;
+
+namespace Tests;
+
+public class CompositeTests
+{
+    [Fact]
+    public void Composite_DrawsAllChildren()
+    {
+        var composite = new CompositeGraphic();
+        composite.Add(new Dot());
+        composite.Add(new CircleGraphic());
+
+        Assert.Equal("Dot, Circle", composite.Draw());
+    }
+
+    [Fact]
+    public void Composite_CanNestComposites()
+    {
+        var root = new CompositeGraphic();
+        var child = new CompositeGraphic();
+        child.Add(new Dot());
+        root.Add(child);
+        root.Add(new CircleGraphic());
+
+        Assert.Equal("Dot, Circle", root.Draw());
+    }
+}