diff --git a/C# Full Course for free/14.OOP.md b/C# Full Course for free/14.OOP.md
index 4ece27c..ae32a71 100644
--- a/C# Full Course for free/14.OOP.md	
+++ b/C# Full Course for free/14.OOP.md	
@@ -1,4 +1,6 @@
-# Classes
+# OOP
+
+# classes
 
 In another file
 ```C#
@@ -90,6 +92,8 @@ class Human
 }
 
 ```
+
+```C#
 // in the main class
 
 Human human1 = new Human("Rick",65)
@@ -287,6 +291,7 @@ Class Car
 ```
 
 ```C#
+// main
 Car[] garage = new Car[3]
 
 Car car1 = new Car("Mustang");
@@ -307,3 +312,380 @@ forach(Car car in garage)
 }
 ```
 
+## Object as arguments
+
+
+```C#
+Class Car 
+{
+    public String model;
+    public String color;
+
+    public Car(String model, String color)
+    {
+        this.model = model;
+        this.color = color;
+    }
+}
+```
+
+```C#
+// main
+
+Car car1 = new car("Mustang", "red");
+Car car 2 = Copy(car1)
+
+ChangeColor(car1, "siver");
+
+Console.WriteLine(car1.color + " " + car1.model)
+
+Console.WriteLine(car2.color + " " + car2.model)
+
+public static void ChangeColor(Car car, String color)
+{
+    car.color = color;
+}
+
+public static Car Copy(Car car)
+{
+    return new Car(car.model, car.color);
+}
+```
+
+## method overiding
+
+provides a new version of a method inherited from a parent class
+inherited method must be: abstract, virtual, or already overriden
+Used with Tostring(), polymporphism
+
+```C#
+Class Animal
+{
+    public virtual void Speak()
+    {
+        Console.WriteLine("The animal goes *brrr*")
+    }
+}
+Class Dog : Animal
+{
+    public override void Speak()
+    {
+        Console.WriteLine("The dog goes *Woof*")
+    }
+
+}
+Class Cat : Animal
+{
+    public override void Speak()
+    {
+        Console.WriteLine("The cat goes *Meow*")
+    }
+}
+```
+
+```C#
+    Dog dog = new Dog();
+    Cat cat = new Cat();
+
+    dog.Speak();
+    cat.Speak();
+```
+
+## ToString
+
+Converts an object ot its string representation so that it is suitable for display
+
+```C#
+class Car
+{
+    String make;
+    String model;
+    int year;
+    String color;
+
+    public Car(String make,String model,int year,String color)
+    {
+        this make = make;
+        this model = model;
+        this year = year;
+        this color = color;
+    }
+    
+    public overide ToString()
+    {
+        String message = "this is a " + make + " " + model;
+        return message;
+    }
+}
+```
+
+```C#
+// Main
+
+Car car = new Car("Chevy", "Corvette", 2022, "blue");
+
+Console.WriteLine(car);
+```
+
+## Polymporphism
+
+Objects can e identified by more than one type
+Ex. A Dog is also: Canine, Animal, Organisms
+
+```C#
+class Vehicle
+{
+    public int speed = 0
+    
+    // have to add virtual for polymporphism
+    public virtual void Go()
+    {
+        Console.WriteLine("this vehicle is moving !");
+    }
+}
+
+Class Car : Vehicle
+{
+    public int wheels = 4;
+    public overide void Go()
+    {
+        Console.WriteLine("this car is moving !");
+    }
+}
+Class Bicycle : Vehicle
+{
+    public int wheels = 2;
+    public overide void Go()
+    {
+        Console.WriteLine("this go is moving !");
+    }
+}
+Class Boat : Vehicle
+{
+    public int wheels = 0;
+    public overide void Go()
+    {
+        Console.WriteLine("this boat is moving !");
+    }
+}
+```
+
+```C#
+// Main
+Car car = new Car();
+Bicycle bicycle = new Bicycle();
+Boat boat = new Boat();
+
+Vehicle[] vehicles = {car, bicycle, boat};
+
+forach (vehicle vehicle in vehicle)
+{
+    vehicle.Go();
+}
+```
+
+## interfaces
+
+defines a "contract" that all the classes inheriting from should follow
+An inerface declares "what a class should have"
+An inheriting class defines "how it should do it"
+Benefit are security + multiple iheritance + "plug-and-play"
+
+
+```C#
+interface IPrey
+{
+    void Flee();
+}
+
+interface IPredator
+{
+    void Hunt();
+
+}
+
+class Rabbit : IPrey
+{
+    pulic void Flee()
+    {
+        Console.WriteLine("The rabbit run away!");
+    
+    }
+
+}
+
+class hawk : IPredator
+{
+    pulic void Hunt()
+    {
+        Console.WriteLine("The hawk is searching for food!");
+    }
+
+}
+
+class fish : IPrey, IPredator
+{
+
+    pulic void Flee()
+    {
+        Console.WriteLine("The fish swims away!");
+    
+    }
+    pulic void Hunt()
+    {
+        Console.WriteLine("The fish is searching for food!");
+    }
+    
+
+}
+
+```
+```C#
+// Main
+
+Rabbit rabbit = new Rabbit();
+Hawk hawk = new Hawk();
+Fish fish = new Fish();
+
+rabit.flee();
+hawk.hunt();
+fish.flee();
+fish.hunt();
+```
+
+## getter and setter
+
+getter & setters add security to fields by encapsulation
+They're accessors found within properties
+
+properties = combine aspects of both fields and methods (share name with a field)
+get accessor = used to return the property value
+set accessor = used to assign a new value
+value keyword = defines the value being assigned by the set (parameter)
+
+
+```C#
+Class Car
+{
+    private int speed;
+
+    public Car(int speed)
+    {
+        Speed = speed;
+    }
+
+    // properties
+    public int Speed
+    {
+        
+        get { return speed}; //read
+        set
+        {
+            if value(value > 500)
+            {
+                speed = 500;
+            }else{
+                speed = value;
+}
+        }
+
+    }
+}
+```
+
+```C#
+// Main
+
+Car car = new Car();
+
+Car Speed = 10000000000; 
+
+Console.WriteLine(car.Speed);
+```
+
+## Auto-Implemted properties
+
+shortcut when no additional logic is required in the property
+you do not have to define a field for a property,
+you only have to write get; and/or set; insid the property;
+
+```C#
+Class Car
+{
+    // shortcut
+    public String Model {get; set;}
+    public Car(String model)
+    {
+        this.Model = model;
+    }
+}
+```
+
+## Enums
+
+Special "class" that contains a set of named integer constans.
+use enums when you have values that you know will not change,
+To get the integer value from an item, you must explicitly convert to an int
+
+name = integer
+
+```C#
+enum Planets
+{
+    Mecury, 
+    Venus, 
+    Earth, 
+    Mars, 
+    Jupiter, 
+    Saturn, 
+    Uranus, 
+    Neptune, 
+    Pluto
+}
+
+enum PlanetsRadius
+{
+    Mecury = 2439, 
+    Venus = 6051, 
+    Earth = 6371, 
+    Mars = 3389, 
+    Jupiter = 69911, 
+    Saturn = 58232, 
+    Uranus = 25362, 
+    Neptune = 24622, 
+    Pluto = 1188
+}
+
+
+Console.WriteLine(Planets.Pluto + " is a planet")
+
+
+String name = PlanetsRadius.Earth.ToString();
+// return : Earth
+
+int radius = (int)PlanetsRadius.Earth;
+// return : 6371
+```
+
+## generics
+
+not specific to a particular data type
+add <T> to : classes, methods, fields, etc.
+or other name ex : <Things>
+allow for code reusability for different data types
+
+```C#
+int[] intArray = { 1, 2, 3};
+double[] doubleArray = { 1.0, 2.0, 3.0};
+String[] stringArray = {"1","2","3"};
+
+// don't work for other array
+public static void() displayElements<T>(T[] array)
+{
+    foreach(T item in array)
+    {
+        Console.Write(item + " ")
+    }
+
+}
+```
diff --git a/C# Full Course for free/15.List.md b/C# Full Course for free/15.List.md
new file mode 100644
index 0000000..76e84b7
--- /dev/null
+++ b/C# Full Course for free/15.List.md	
@@ -0,0 +1,101 @@
+# List
+
+data structure that represents a list of objects that can be
+accessed by index.
+Similar to array, but can dynamically increase/decrease in size
+using System.Collection.Generic
+
+```C#
+// error with array
+String[] food = new string[3];
+
+food[0] = "pizza";
+food[1] = "hamburger";
+food[2] = "hotdog";
+food[3] = "Salad";
+
+foreach(String item in food)
+{
+    Console.WriteLine(item);
+}
+
+```
+
+```C#
+using System.Collection.Generic
+
+List<String> food = new List<String>();
+
+food.Add("fries");
+food.Add("pizza");
+food.Add("hamburger");
+food.Add("hotdog");
+food.Add("Salad");
+food.Add("fries");
+
+foreach(String item in food)
+{
+    Console.WriteLine(item);
+}
+
+// accessing an element
+Console.WriteLine(food[1]);
+
+food.Remove("fries");
+
+food.Insert(0,"sushie");
+
+Console.WriteLine(food.Count());
+
+Console.WriteLine(food.IndexOf("hotdog"));
+Console.WriteLine(food.LastIndexOf("fries"));
+Console.WriteLine(food.Cotains("pizza"));
+food.Sort();
+food.Reverse();
+food.Clear();
+String[] foodArray  = food.ToArray();
+food.Clear();
+```
+
+## List of object
+
+```C#
+List<Player> player = new List<Player>();
+
+Player player1 = new Player ("Chad")
+Player player2 = new Player ("Steve")
+Player player3 = new Player ("Karen")
+
+
+players.Add(player1);
+players.Add(player2);
+players.Add(player3);
+
+// or anonymously
+
+players.Add(new Player ("Karen"));
+players.Add(new Player ("Chad"));
+players.Add(new Player ("Steve"));
+
+foreach(Player player in players)
+{
+    Console.WriteLine(player);
+}
+
+class Player
+{
+    public string username;
+    
+    public Player(String username)
+    {
+        this.username=username;
+    }
+
+    public override string ToString()
+    {
+        return username;
+    
+    }
+}
+
+```
diff --git a/C# Full Course for free/16.multithreading.md b/C# Full Course for free/16.multithreading.md
new file mode 100644
index 0000000..f0c078c
--- /dev/null
+++ b/C# Full Course for free/16.multithreading.md	
@@ -0,0 +1,90 @@
+# multithreading
+
+thread is an execution path of a program
+we can use multiple threads to perform,
+different taks of our program at the same time.
+Current thread running is "main" thread
+using System.Threading;
+
+## Single Thread
+```C#
+// single Thread
+using System.Threading;
+
+Thread mainThread = Thread.CurrenThread;
+mainThread.Name = "Main Thread";
+Console.WriteLine(mainThread.Name);
+```
+
+### Timer Example
+
+```C#
+public static void CountDown()
+{
+    for (int i = 10; i >=0; i--)
+    {
+        Console.WriteLine("Timer #1 : " + i + " seconds");
+        Thread.Sleep(1000)
+    }
+    Console.WriteLine("Timer #1 is complete !")
+}
+
+public static void CountUp()
+{
+    for (int i = 0; i >=0; i++)
+    {
+        Console.WriteLine("Timer #2 : " + i + " seconds");
+        Thread.Sleep(1000)
+    }
+    Console.WriteLine("Timer #2 is complete !")
+}
+```
+
+```C#
+// Main
+
+CountDown();
+CountUp();
+```
+
+## multiple Thread
+
+```C#
+public static void CountDown()
+{
+    for (int i = 10; i >=0; i--)
+    {
+        Console.WriteLine("Timer #1 : " + i + " seconds");
+        Thread.Sleep(1000)
+    }
+    Console.WriteLine("Timer #1 is complete !")
+}
+
+public static void CountUp()
+{
+    for (int i = 0; i >=0; i++)
+    {
+        Console.WriteLine("Timer #2 : " + i + " seconds");
+        Thread.Sleep(1000)
+    }
+    Console.WriteLine("Timer #2 is complete !")
+}
+```
+
+```C#
+// Main
+
+Thread thread1 = new Thread(CountDown);
+Thread thread2 = new Thread(CountUp);
+
+// if the function got a parameter
+// use a lambda
+// Thread thread1 = new Thread(() => CountDown("Thimer #1"));
+
+
+thread1.start();
+thread2.start();
+
+CountDown();
+CountUp();
+```