memento design pattern

Program.cs

@@ -18,6 +18,7 @@ using design_patterns.behavioral.chainofresponsability;
 using design_patterns.behavioral.command;
 using design_patterns.behavioral.iterator;
 using design_patterns.behavioral.mediator;
+using design_patterns.behavioral.memento;
 
 namespace design_patterns
 {
@@ -49,7 +50,8 @@ namespace design_patterns
                 // await ChainOfResponsabilitySample.Run();
                 // await CommandSample.Run();
                 // await IteratorSample.Run();
-                await MediatorSample.Run();
+                // await MediatorSample.Run();
+                await MementoSample.Run();
             }
             catch (System.Exception ex)
             {

behavioral/memento/MementoSample.cs

@@ -0,0 +1,136 @@
+using System.Globalization;
+using System.Text;
+using System;
+using System.Collections.Generic;
+using System.Threading.Tasks;
+
+namespace design_patterns.behavioral.memento
+{
+    /// <summary>
+    /// Memento is a behavioral design pattern that lets you 
+    /// save and restore the previous state of an object 
+    /// without revealing the details of its implementation.
+    /// 
+    /// Use it when:
+    /// - you want to produce snapshots of the object’s state 
+    ///     to be able to restore a previous state of the object.
+    /// - direct access to the object’s fields/getters/setters 
+    ///     violates its encapsulation.
+    /// </summary>
+    public class MementoSample
+    {
+        public static async Task Run()
+        {
+            Console.WriteLine("Behavioral - Memento");
+
+            var board = new DrawingBoard();
+            System.Console.WriteLine(board);
+            // init state
+            var memento0 = board.Save();
+
+            board.DrawLine(new Line(0,0, 10, 20, 1, "red"));
+            board.DrawLine(new Line(10,5, 15, 25, 2, "green"));
+
+            System.Console.WriteLine(board);
+            var memento1 = board.Save();
+
+            board.DrawLine(new Line(20,20, 5, 20, 1, "blue"));
+
+            System.Console.WriteLine(board);
+            var memento2 = board.Save();
+
+            board.DrawLine(new Line(7, 7, 7, 7, 3, "new red"));
+            System.Console.WriteLine(board);
+            var memento3 = board.Save();
+
+#if false
+            // only memento
+            System.Console.WriteLine("Restore memento1");
+            board.Restore(memento1);
+            System.Console.WriteLine(board);
+#else
+            // memento with undo()
+            System.Console.WriteLine("Undo one step");
+            board.Undo();
+            System.Console.WriteLine(board);
+
+            System.Console.WriteLine("Undo one more step");
+            board.Undo();
+            System.Console.WriteLine(board);
+
+            System.Console.WriteLine("Undo one more more step");
+            board.Undo();
+            System.Console.WriteLine(board);
+            System.Console.WriteLine("Undo one more more more step");
+            board.Undo();
+            System.Console.WriteLine(board);
+#endif
+        }
+    }
+
+    /// <summary>
+    /// keeps the snapshot of the system/component
+    /// </summary>
+    public class Memento {
+        
+        public List<Line> Lines { get; set; } = new List<Line>();
+
+        public Memento(DrawingBoard board)
+        {
+            // copy inner Lines and NOT the list reference
+            this.Lines.AddRange(board.Lines);
+        }
+    }
+
+    public class DrawingBoard {
+        public List<Line> Lines { get; set; } = new List<Line>();
+        private Stack<Memento> _history = new Stack<Memento>();
+
+        public void DrawLine(Line line)
+        {
+            Lines.Add(line);
+        }
+
+
+        public override string ToString()
+        {
+            StringBuilder sb = new StringBuilder();
+            sb.AppendLine($"Board contains: {this.Lines.Count} lines ->");
+            foreach (var line in Lines)
+            {
+                sb.AppendLine($"Line: from {line.fromX}-{line.fromY} to {line.toX}-{line.toY} width {line.width} color {line.color}");
+            }
+            return sb.ToString();
+        }
+
+        public Memento Save()
+        {
+            var memento = new Memento(this);
+            _history.Push(memento);
+            return memento;
+        }
+
+        public void Restore(Memento memento)
+        {
+            if (memento != null &&  
+                _history.Contains(memento)) {
+                while(!_history.Peek().Equals(memento)) {
+                    _history.Pop();
+                }
+                this.Lines = memento.Lines;
+            }
+        }
+
+        public void Undo() {
+            if(_history.Count>1){
+                _history.Pop();
+                var memento = _history.Peek();
+                Restore(memento);
+            }
+        }
+    }
+
+    public record Line(int fromX, int fromY, 
+        int toX, int toY, 
+        int width, string color);
+}