Add Memento Design Pattern implementation

Implemented the Memento pattern with `TextMemento`, `TextEditor`,
and `CareTaker` classes to enable state saving and restoration.
Updated `Program.cs` to include a demo of the Memento pattern.

Enhanced `README.md` to document the Memento pattern, including
its purpose, example files, usage snippet, and test references.

Added `MementoTests.cs` to verify the pattern's functionality,
covering state saving/restoration and empty history handling.

Improved code readability in `Program.cs` by renaming the loop
variable in the `foreach` loop for `bob.Messages`.

Author: froghramar

DesignPatterns/Memento/CareTaker.cs

@@ -0,0 +1,10 @@
+namespace DesignPatterns.Memento;
+
+public class CareTaker
+{
+    private readonly Stack<TextMemento> _history = new();
+
+    public void SaveState(TextMemento m) => _history.Push(m);
+
+    public TextMemento? PopState() => _history.Count > 0 ? _history.Pop() : null;
+}

DesignPatterns/Memento/TextEditor.cs

@@ -0,0 +1,16 @@
+namespace DesignPatterns.Memento;
+
+public class TextEditor
+{
+    public string Text { get; private set; } = string.Empty;
+
+    public TextMemento Save() => new TextMemento(Text);
+
+    public void Restore(TextMemento m)
+    {
+        if (m is null) throw new ArgumentNullException(nameof(m));
+        Text = m.State;
+    }
+
+    public void Type(string input) => Text += input;
+}

DesignPatterns/Memento/TextMemento.cs

@@ -0,0 +1,11 @@
+namespace DesignPatterns.Memento;
+
+public sealed class TextMemento
+{
+    internal string State { get; }
+
+    internal TextMemento(string state)
+    {
+        State = state ?? string.Empty;
+    }
+}

DesignPatterns/Program.cs

@@ -13,6 +13,7 @@
 using DesignPatterns.Chain;
 using DesignPatterns.Iterator;
 using DesignPatterns.Mediator;
+using DesignPatterns.Memento;
 
 Console.WriteLine("Factory pattern demo:");
 
@@ -158,7 +159,23 @@
 
 alice.Send("Hello Bob");
 Console.WriteLine("Bob's messages:");
-foreach (var m in bob.Messages) Console.WriteLine(m);
+foreach (var msg in bob.Messages) Console.WriteLine(msg);
+
+// Memento demo
+Console.WriteLine();
+Console.WriteLine("Memento pattern demo:");
+var editor = new TextEditor();
+var caretaker = new CareTaker();
+
+editor.Type("Hello");
+caretaker.SaveState(editor.Save());
+
+editor.Type(" World");
+Console.WriteLine($"Current text: {editor.Text}");
+
+var m = caretaker.PopState();
+if (m != null) editor.Restore(m);
+Console.WriteLine($"Restored text: {editor.Text}");
 
 // Decorator 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, Command, Iterator, and Mediator patterns.
+- `DesignPatterns` - Console demo showing Factory, Singleton, Builder, Prototype, Abstract Factory, Adapter, Bridge, Composite, Decorator, Facade, Flyweight, Proxy, Chain of Responsibility, Command, Iterator, Mediator, and Memento patterns.
   - Factory implementation: `DesignPatterns/Factories`
   - Singleton implementation: `DesignPatterns/Singleton/Logger.cs`
   - Builder implementation: `DesignPatterns/Builder/HouseBuilder.cs` and `DesignPatterns/Builder/House.cs`
@@ -20,6 +20,7 @@ Projects
   - Command implementation: `DesignPatterns/Command/*`
   - Iterator implementation: `DesignPatterns/Iterator/*`
   - Mediator implementation: `DesignPatterns/Mediator/*`
+  - Memento implementation: `DesignPatterns/Memento/*`
 - `Tests` - xUnit tests covering the examples.
 
 Included patterns and brief docs
@@ -217,6 +218,20 @@ Included patterns and brief docs
     alice.Send("Hello Bob");
     ```
 
+- Memento Pattern
+  - Purpose: Capture and externalize an object's internal state so it can be restored later without violating encapsulation.
+  - Example: `DesignPatterns/Memento/*` contains `TextMemento`, `TextEditor` (originator), and `CareTaker` that stores mementos.
+  - Usage snippet:
+
+    ```csharp
+    var editor = new TextEditor();
+    var caretaker = new CareTaker();
+    editor.Type("Hello");
+    caretaker.SaveState(editor.Save());
+    editor.Type(" World");
+    editor.Restore(caretaker.PopState());
+    ```
+
 Tests
 - Tests are written with xUnit in the `Tests` project.
   - Factory tests: `Tests/ShapeFactoryTests.cs`
@@ -235,6 +250,7 @@ Tests
   - Command tests: `Tests/CommandTests.cs`
   - Iterator tests: `Tests/IteratorTests.cs`
   - Mediator tests: `Tests/MediatorTests.cs`
+  - Memento tests: `Tests/MementoTests.cs`
 
 Common commands
 - Build solution: `dotnet build`

Tests/MementoTests.cs

@@ -0,0 +1,31 @@
+using DesignPatterns.Memento;
+
+namespace Tests;
+
+public class MementoTests
+{
+    [Fact]
+    public void TextEditor_SaveAndRestore_Works()
+    {
+        var editor = new TextEditor();
+        var caretaker = new CareTaker();
+
+        editor.Type("Hello");
+        caretaker.SaveState(editor.Save());
+
+        editor.Type(" World");
+        Assert.Equal("Hello World", editor.Text);
+
+        var m = caretaker.PopState();
+        editor.Restore(m!);
+
+        Assert.Equal("Hello", editor.Text);
+    }
+
+    [Fact]
+    public void CareTaker_ReturnsNullWhenEmpty()
+    {
+        var caretaker = new CareTaker();
+        Assert.Null(caretaker.PopState());
+    }
+}