Add Mediator design pattern implementation

Implemented the Mediator pattern with `ChatRoom` and `Participant`
classes to facilitate communication between objects. Updated
`Program.cs` to include a demo of the pattern.

Updated `README.md` to document the Mediator pattern, including its
purpose, usage example, and implementation/test paths.

Added `MediatorTests.cs` to verify the functionality of the pattern.

Author: froghramar

DesignPatterns/Mediator/ChatRoom.cs

@@ -0,0 +1,21 @@
+namespace DesignPatterns.Mediator;
+
+public class ChatRoom
+{
+    private readonly List<Participant> _participants = new();
+
+    public void Register(Participant p)
+    {
+        if (p is null) throw new ArgumentNullException(nameof(p));
+        p.Room = this;
+        _participants.Add(p);
+    }
+
+    public void Broadcast(Participant from, string message)
+    {
+        foreach (var p in _participants)
+        {
+            if (p != from) p.Receive(from.Name, message);
+        }
+    }
+}

DesignPatterns/Mediator/Participant.cs

@@ -0,0 +1,27 @@
+namespace DesignPatterns.Mediator;
+
+public class Participant
+{
+    public string Name { get; }
+    internal ChatRoom? Room { get; set; }
+
+    private readonly List<string> _messages = new();
+
+    public Participant(string name)
+    {
+        Name = name ?? string.Empty;
+    }
+
+    public IReadOnlyList<string> Messages => _messages;
+
+    public void Send(string message)
+    {
+        if (Room is null) throw new InvalidOperationException("Participant is not registered with a chat room.");
+        Room.Broadcast(this, message);
+    }
+
+    internal void Receive(string from, string message)
+    {
+        _messages.Add($"{from}: {message}");
+    }
+}

DesignPatterns/Program.cs

@@ -12,6 +12,7 @@
 using DesignPatterns.Proxy;
 using DesignPatterns.Chain;
 using DesignPatterns.Iterator;
+using DesignPatterns.Mediator;
 
 Console.WriteLine("Factory pattern demo:");
 
@@ -146,6 +147,19 @@
 cmdRemote.PressUndo();
 Console.WriteLine($"After undo, light is on: {light.IsOn}");
 
+// Mediator demo
+Console.WriteLine();
+Console.WriteLine("Mediator pattern demo:");
+var room = new ChatRoom();
+var alice = new Participant("Alice");
+var bob = new Participant("Bob");
+room.Register(alice);
+room.Register(bob);
+
+alice.Send("Hello Bob");
+Console.WriteLine("Bob's messages:");
+foreach (var m in bob.Messages) Console.WriteLine(m);
+
 // Decorator demo
 Console.WriteLine();
 Console.WriteLine("Decorator pattern demo:");

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, Command, and Iterator patterns.
+- `DesignPatterns` - Console demo showing Factory, Singleton, Builder, Prototype, Abstract Factory, Adapter, Bridge, Composite, Decorator, Facade, Flyweight, Proxy, Chain of Responsibility, Command, Iterator, and Mediator patterns.
   - Factory implementation: `DesignPatterns/Factories`
   - Singleton implementation: `DesignPatterns/Singleton/Logger.cs`
   - Builder implementation: `DesignPatterns/Builder/HouseBuilder.cs` and `DesignPatterns/Builder/House.cs`
@@ -19,6 +19,7 @@ Projects
   - Chain of Responsibility implementation: `DesignPatterns/Chain/*`
   - Command implementation: `DesignPatterns/Command/*`
   - Iterator implementation: `DesignPatterns/Iterator/*`
+  - Mediator implementation: `DesignPatterns/Mediator/*`
 - `Tests` - xUnit tests covering the examples.
 
 Included patterns and brief docs
@@ -202,6 +203,20 @@ Included patterns and brief docs
     while (it.HasNext()) Console.WriteLine(it.Next());
     ```
 
+- Mediator Pattern
+  - Purpose: Define an object that encapsulates how a set of objects interact. Mediator promotes loose coupling by keeping objects from referring to each other explicitly.
+  - Example: `DesignPatterns/Mediator/*` contains `ChatRoom` (mediator) and `Participant` (colleagues) that register with the room and broadcast messages.
+  - Usage snippet:
+
+    ```csharp
+    var room = new ChatRoom();
+    var alice = new Participant("Alice");
+    var bob = new Participant("Bob");
+    room.Register(alice);
+    room.Register(bob);
+    alice.Send("Hello Bob");
+    ```
+
 Tests
 - Tests are written with xUnit in the `Tests` project.
   - Factory tests: `Tests/ShapeFactoryTests.cs`
@@ -219,6 +234,7 @@ Tests
   - Chain tests: `Tests/ChainTests.cs`
   - Command tests: `Tests/CommandTests.cs`
   - Iterator tests: `Tests/IteratorTests.cs`
+  - Mediator tests: `Tests/MediatorTests.cs`
 
 Common commands
 - Build solution: `dotnet build`

Tests/MediatorTests.cs

@@ -0,0 +1,22 @@
+using DesignPatterns.Mediator;
+
+namespace Tests;
+
+public class MediatorTests
+{
+    [Fact]
+    public void Participants_CanSendMessagesViaChatRoom()
+    {
+        var room = new ChatRoom();
+        var alice = new Participant("Alice");
+        var bob = new Participant("Bob");
+
+        room.Register(alice);
+        room.Register(bob);
+
+        alice.Send("Hello Bob");
+
+        Assert.Contains("Alice: Hello Bob", bob.Messages);
+        Assert.Empty(alice.Messages); // sender shouldn't receive own message
+    }
+}