public class Rectangle
{
   public double Width;
   public double Height;
 
   public double Area()
   {
      return Width*Height;
   }
 
   public double Perimeter()
   {
      return 2*(Width + Height);
   }
}

Rectangle rectangle = new Rectangle();

Rectangle rectangle;

Rectangle rectangle1 = new Rectangle();
Rectangle rectangle2 = rectangle1;

Rectangle rectangle = new Rectangle();
rectangle.Width = 10.5;
rectangle.Height = 10;
double a = rectangle.Area();

// public field (Generally not recommended.)
public double Width;

Rectangle rectangle = new Rectangle();
rectangle.Width = 10.5;

public class Rectangle
{
   public double Width = 5;
   // ...
}

public class Rectangle
{
   // ...
 
   public double Area()
   {
      return Width*Height;
   }
 
   public double Perimeter()
   {
      return 2*(Width + Height);
   }
}

Rectangle rectangle = new Rectangle();
rectangle.Width = 10.5;
rectangle.Height = 10;
double p = rectangle.Perimeter();

public class SimpleMath
{
   public int AddTwoNumbers(int number1, int number2)
   {
      return number1 + number2;
   }
}

SimpleMath obj = new SimpleMath();

Console.WriteLine(obj.AddTwoNumbers(1, 2));

int result = obj.AddTwoNumbers(1, 2);
Console.WriteLine(result);

public class Rectangle
{
    public double Width;
    public double Height;
 
    public double Area()
    {
        return Width*Height;
    }
 
    public double Perimeter()
    {
        return 2*(Width + Height);
    }
}

public class Rectangle
{
    private double _width = 0;
    private double _height = 0;
 
    public double Width
    {
        get { return _width; }
        set { if (value > 0) { _width = value; } }
    }
 
    public double Height
    {
        get { return _height; }
        set { if (value > 0) { _height = value; } }
    }
 
    public double Area()
    {
        return _width * _height;
    }
 
    public double Perimeter()
    {
        return 2*(_width + _height);
    }
}

Rectangle rectangle = new Rectangle();
rectangle.Width = 10;
Console.WriteLine(rectangle.Width);

public class Product
{
    private string _name;
    public string Name
    {
        get { return _name; }
        set { _name = value; }
    }
}

public class Product
{
    public string Name { get; set; }
}

public class Student
{
    public DateTime Birthdate { get; set; }
 
    public double Age { get; private set; }
}

public class Location   
{ 
     public string City { get; set; } 
     public string State { get; set; } 
 
     public Location(string city, string state) 
     { 
           City = city; 
           State = state; 
     } 
}

 var location = new Location("Lake Charles","LA");

 var location = new Tuple<string,string>("Lake Charles","LA");   
// Print out the address
var address = $"{location.Item1}, {location.Item2}";

static Tuple<int, string, string> GetHumanData()
{
   return Tuple.Create(10, "Marcus", "Miller");
}

 var data = GetHumanData();
Console.WriteLine("What is this value {0} or this {1}",  data.Item1, data.Item3);

(int x1, string s1) = (3, "one");
Console.WriteLine($"{x1} {s1}");

 error CS8179: Predefined type 'System.ValueTuple`2' is not defined or imported

 PM> install-package System.ValueTuple

int x2;
string s2;
(x2, s2) = (42, "two");
Console.WriteLine($"{x2} {s2}");

static (int, int) Divide(int x, int y)
{
   int result = x / y;
   int reminder = x % y;
 
   return (result, reminder);
}

 (int result, int reminder) = Divide(11, 3);
Console.WriteLine($"{result} {reminder}");

 (var result1, var reminder1) = Divide(11, 3);
Console.WriteLine($"{result1} {reminder1}");

 var (result1, reminder1) = Divide(11, 3);

static (int, string, string) GetHumanData()
{
   return (10, "Marcus", "Miller");
}

 (int Age, string FirstName, string LastName) results = GetHumanData();
Console.WriteLine(results.Age);
Console.WriteLine(results.FirstName);
Console.WriteLine(results.LastName);

 ValueTuple<int, int> tuple1 = Divide(11, 3);

 (int, int) n = (1,1);
System.Console.WriteLine(n.Item1);

 ValueTuple<int, int> n = new ValueTuple<int, int>(1, 1);
System.Console.WriteLine(n.Item1);

 (int,int,int,int,int,int,int,(int,int))

 var tuple2 = ("Stephanie", 7);
Console.WriteLine($"{tuple2.Item1}, {tuple2.Item2}");

 var tuple3 = (Name: "Matthias", Age: 6);
Console.WriteLine($"{tuple3.Name} {tuple3.Age}");

static (int radius, double area) CalculateAreaOfCircle(int radius)
{
   return (radius, Math.PI * Math.Pow(radius, 2));
}

 var circle = CalculateAreaOfCircle(2);
Console.WriteLine($"A circle of radius, {circle.radius}," +
 $" has an area of {circle.area:N2}.");

class Person
{
    private readonly string _firstName;
    private readonly string _lastName;
 
    public Person(string firstname, string lastname)
    {
        _firstName = firstname;
        _lastName = lastname;
    }
 
    public override String ToString() => $"{_firstName} {_lastName}";
 
    public void Deconstruct(out string firstname, out string lastname)
    {
        firstname = _firstName;
        lastname = _lastName;
    }
}

 var p1 = new Person("Katharina", "Nagel");
(string first, string last) = p1;
Console.WriteLine($"{first} {last}");

public class Rectangle
{
    public Rectangle(int height, int width)
    {
        Height = height;
        Width = width;
    }
 
    public int Width { get; }
    public int Height { get; }
}
 
public static class RectangleExtensions
{
    public static void Deconstruct(this Rectangle rectangle, out int height, out int width)
    {
        height = rectangle.Height;
        width = rectangle.Width;
    }
}

 var r1 = new Rectangle(100, 200);
(int height, int width) = r1;
Console.WriteLine($"height: {height}, width: {width}");

List<Employee> allEmployees = new List<Employee>()
{
  new Employee { ID = 1L, Name = "Fred", Salary = 50000M },
  new Employee { ID = 2L, Name = "Sally", Salary = 60000M },
  new Employee { ID = 3L, Name = "George", Salary = 70000M }
};
var wellPaid =
  from oneEmployee in allEmployees
  where oneEmployee.Salary > 50000M
  select new { EmpName = oneEmployee.Name,
               Income = oneEmployee.Salary };

var wellPaid =
  from oneEmployee in allEmployees
  where oneEmployee.Salary > 50000M
  orderby oneEmployee.Salary descending
  select (EmpName: oneEmployee.Name,
          Income: oneEmployee.Salary);
var highestPaid = wellPaid.First().EmpName;

public Task<(int index, T item)> FindAsync<T>(IEnumerable<T> input, Predicate<T> match)
{
   var dictionary = new Dictionary<(int, int), string>();
   throw new NotSupportedException();
}

public class Triangle
{
    private int _height;
    private int _baseLength;

    public int Height
    {
        get { return _height; }

        set
        {
            if (value < 1 || value > 100)
            {
                // تولید خطا
            }

            _height = value;
        }
    }

    public int BaseLength
    {
        get { return _baseLength; }

        set
        {
            if (value < 1 || value > 100)
            {
                // تولید خطا
            }

            _baseLength = value;
        }
    }

    public double Area
    {
        get { return _height * _baseLength * 0.5; }
    }
}

public Triangle()
{
    _height = _baseLength = 1;
}

Triangle triangle = new Triangle();
 
Console.WriteLine(triangle.Height);
Console.WriteLine(triangle.BaseLength);
Console.WriteLine(triangle.Area);

public Triangle(int height, int baseLength)
{
    Height = height;
    BaseLength = baseLength;
}

Triangle triangle = new Triangle(5, 8);
 
Console.WriteLine(triangle.Height);
Console.WriteLine(triangle.BaseLength);
Console.WriteLine(triangle.Area);

class DotnetTips
{
....
}

class DotnetTips:System.Object
{
...
}

DotnetTips dotnettips=new DotnetTips("i am constructor");

1. اولین کاری که CLR انجام می‌دهد، محاسبه‌ی مقدار حافظه یا تعداد بایت‌های فیلدهای شیء مربوطه است و همچنین اشیایی که از آن‌ها مشتق شده است؛ مثل System.Object که همیشه وجود دارد. هر شیء‌ایی که بر روی حافظه‌ی Heap قرار می‌گیرد، به یک سری اعضای اضافه هم نیاز دارد که Type Object Pointer و Sync Block Index نامیده می‌شوند و CLR از آن‌ها برای مدیریت حافظه استفاده می‌کند. تعداد بایت‌های این اعضای اضافه هم با تعداد بایت‌های شیء مدنظر جمع می‌شوند.
2. طبق تعداد بایت‌های به دست آمده، یک مقدار از حافظه‌ی Heap دریافت می‌شود.
3. دو عضو اضافه که در بند شماره یک به آن اشاره کردیم، آماده سازی می‌شوند.
4. سازنده‌ی شیء صدا زده می‌شود. سازنده بر اساس نوع ورودی شما انتخاب می‌شود و در صورت ارجاع به سازنده‌های والد، ابتدا سازنده‌های والد صدا زده می‌شوند و بعد از اینکه همه‌ی سازنده‌ها صدا زده شدند، سازنده‌ی System.Object صدا زده می‌شود که هیچ کدی ندارد؛ ولی به هر حال عمل Return آن انجام می‌شود.

> create-react-app sample-02
> cd sample-02
> npm start

./src/index.js
  Line 14:15:  'i' is not defined  no-undef

Search for the keywords to learn more about each error.

 const x = 1;
x = 2; // Attempting to override 'x' which is a constant.
 

const person = {
  name: "User 1",
  walk: function() {}, // method
  talk() {} // concise method
};

const person = {
  name: "User 1",
  walk() {},
  talk() {}
};

person.talk();
person.name = "User 3";
person["name"] = "User 2";

const targetMember = "name";
person[targetMember] = "User 2";

const person = {
  name: "User 1",
  walk() {
    console.log(this);
  },
  talk() {}
};

person.walk();

const walk = person.walk;
console.log(walk);

const walk2 = person.walk.bind(person);
console.log(walk2);
walk2();

const square = function(number) {
  return number * number;
};

const square2 = (number) => {
  return number * number;
};

const square2 = number => {
  return number * number;
};

const square3 = number => number * number;
console.log(square3(5));

var activeJobs = jobs.filter(function(job) {
  return job.isActive;
});

var activeJobs2 = jobs.filter(job => job.isActive);

const user = {
  name: "User 1",
  talk() {
    console.log(this);
  }
};
user.talk();

const user = {
  name: "User 1",
  talk() {
    setTimeout(function() {
      console.log(this);
    }, 1000);
  }
};
user.talk();

const user2 = {
  name: "User 2",
  talk() {
    var self = this;
    setTimeout(function() {
      console.log(self);
    }, 1000);
  }
};
user2.talk();

const user3 = {
  name: "User 3",
  talk() {
    setTimeout(() => console.log(this), 1000);
  }
};
user3.talk();

public class Rectangle
{
    public int Length { get; set; }
    public int Height { get; set; }

    public void Grow(int length, int height)
    {
        Length += length;
        Height += height;
    }
}

Rectangle r = new Rectangle();
r.Length = 5;
r.Height = 10;
r.Grow(10, 10);// r.Length is 15, r.Height is 20, same instance of r

// After
public class ImmutableRectangle
{
    int Length { get; }
    int Height { get; }

    public ImmutableRectangle(int length, int height)
    {
        Length = length;
        Height = height;
    }

    public ImmutableRectangle Grow(int length, int height) =>
          new ImmutableRectangle(Length + length, Height + height);
}

ImmutableRectangle r = new ImmutableRectangle(5, 10);
r = r.Grow(10, 10);// r.Length is 15, r.Height is 20, is a new instance of r

public static void Main()
{
    Console.WriteLine(GetSalutation(DateTime.Now.Hour));
}

// imparitive, mutates state to produce a result
/*public static string GetSalutation(int hour)
{
    string salutation; // placeholder value

    if (hour < 12)
        salutation = "Good Morning";
    else
        salutation = "Good Afternoon";

    return salutation; // return mutated variable
}*/

public static string GetSalutation(int hour) => hour < 12 ? "Good Morning" : "Good Afternoon";

public static int Count<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate)
{
    int count = 0;

    foreach (TSource element in source)
    {
        checked
        {
            if (predicate(element))
            {
                count++;
            }
        }
    }

    return count;
}

public static class Extensions
{
    public static Func<T, TReturn2> Compose<T, TReturn1, TReturn2>(this Func<TReturn1, TReturn2> func1, Func<T, TReturn1> func2)
    {
        return x => func1(func2(x));
    }
}

public class Program
{
    public static void Main(string[] args)
    {
        Func<int, int> square = (x) => x * x;
        Func<int, int> negate = x => x * -1;
        Func<int, string> toString = s => s.ToString();
        Func<int, string> squareNegateThenToString = toString.Compose(negate).Compose(square);
        Console.WriteLine(squareNegateThenToString(2));
    }
}

string str = new StringBuilder()
  .Append("Hello ")
  .Append("World ")
  .ToString()
  .TrimEnd()
  .ToUpper();

public static class Extensions
{
    public static StringBuilder AppendWhen(this StringBuilder sb, string value, bool predicate) => predicate ? sb.Append(value) : sb;
}

public class Program
{
    public static void Main(string[] args)
    {
        // Extends the StringBuilder class to accept a predicate
        string htmlButton = new StringBuilder().Append("<button").AppendWhen(" disabled", false).Append(">Click me</button>").ToString();
    }
}

public static void Main()
{
    int[] a = { 1, 2, 3, 4, 5 };

    foreach (int n in GreaterThan(a, 3))
    {
        Console.WriteLine(n);
    }
}


/*public static IEnumerable<int> GreaterThan(int[] arr, int gt)
{
    List<int> temp = new List<int>();
    foreach (int n in arr)
    {
        if (n > gt) temp.Add(n);
    }
    return temp;
}*/

public static IEnumerable<int> GreaterThan(int[] arr, int gt)
{
    foreach (int n in arr)
    {
        if (n > gt) yield return n;
    }
}

List<int> collection = new List<int> { 1, 2, 3, 4, 5 };

// Imparative style of programming is verbose
List<int> results = new List<int>();

foreach(var num in collection)
{
  if (num % 2 != 0) results.Add(num);
}

// Declarative is terse and beautiful
var results = collection.Where(num => num % 2 != 0);

public class User
{
   public string Name { set; get; }
}

public record User
{
   public string Name { set; get; }
}

var user = new User();
user.Name = "User 1";

public record User(string Name);

var user = new User("User 1");
// Error: Init-only property or indexer 'User.Name' can only be assigned
// in an object initializer, or on 'this' or 'base' in an instance constructor
// or an 'init' accessor. [CS9Features]csharp(CS8852)
user.Name = "User 1";

public record User(string Name);

using System;
using System.Collections.Generic;
using System.Runtime.CompilerServices;
using System.Text;
using CS9Features;

public class User : IEquatable<User>
{
 protected virtual Type EqualityContract
 {
  [System.Runtime.CompilerServices.NullableContext(1)]
  [CompilerGenerated]
  get
  {
   return typeof(User);
  }
 }

 public string Name
 {
  get;
  set/*init*/;
 }

 public User(string Name)
 {
  this.Name = Name;
  base..ctor();
 }

 public override string ToString()
 {
  StringBuilder stringBuilder = new StringBuilder();
  stringBuilder.Append("User");
  stringBuilder.Append(" { ");
  if (PrintMembers(stringBuilder))
  {
   stringBuilder.Append(" ");
  }
  stringBuilder.Append("}");
  return stringBuilder.ToString();
 }

 protected virtual bool PrintMembers(StringBuilder builder)
 {
  builder.Append("Name");
  builder.Append(" = ");
  builder.Append((object?)Name);
  return true;
 }

 [System.Runtime.CompilerServices.NullableContext(2)]
 public static bool operator !=(User? r1, User? r2)
 {
  return !(r1 == r2);
 }

 [System.Runtime.CompilerServices.NullableContext(2)]
 public static bool operator ==(User? r1, User? r2)
 {
  return (object)r1 == r2 || (r1?.Equals(r2) ?? false);
 }

 public override int GetHashCode()
 {
  return EqualityComparer<Type>.Default.GetHashCode(EqualityContract) * -1521134295 + EqualityComparer<string>.Default.GetHashCode(Name);
 }

 public override bool Equals(object? obj)
 {
  return Equals(obj as User);
 }

 public virtual bool Equals(User? other)
 {
  return (object)other != null && EqualityContract == other!.EqualityContract && EqualityComparer<string>.Default.Equals(Name, other!.Name);
 }

 public virtual User <Clone>$()
 {
  return new User(this);
 }

 protected User(User original)
 {
  Name = original.Name;
 }

 public void Deconstruct(out string Name)
 {
  Name = this.Name;
 }
}

public virtual User <Clone>$()
{
  return new User(this);
}

public record User(string Name, int Age);

var user1 = new User("User 1", 21);

var user2 = user1 with { Age = 31 };

var user1 = new User("User 1", 21);
var user2 = new User("User 1", 21);

Console.WriteLine("user1.Equals(user2) -> {0}", user1.Equals(user2));
Console.WriteLine("user1 == user2 -> {0}", user1 == user2);

user1.Equals(user2) -> True
user1 == user2 -> True

public virtual bool Equals(User? other)
{
   return (object)other != null &&
 EqualityContract == other!.EqualityContract &&
 EqualityComparer<string>.Default.Equals(Name, other!.Name) && 
EqualityComparer<int>.Default.Equals(Age, other!.Age);
}

    public record User
    {
        public string Name { get; init; }

        public User(string name)
        {
            Name = name;
        }
    }

    public record UserWithAge : User
    {
        public int Age { get; init; }

        public UserWithAge(string name, int age) : base(name)
        {
            Age = age;
        }
    }

var user1 = new User("User 1");
var user2 = new UserWithAge("User 1", 21);

Console.WriteLine("user1.Equals(user2) -> {0}", user1.Equals(user2));
Console.WriteLine("user1 == user2 -> {0}", user1 == user2);

user1.Equals(user2) -> False
user1 == user2 -> False

public abstract record Food(int Calories);
public record Milk(int C, double FatPercentage) : Food(C);

Console.WriteLine(user1.ToString());
Console.WriteLine(user2.ToString());

User { Name = User 1 }
UserWithAge { Name = User 1, Age = 21 }

public record Person(string Name, int Age);

public void Deconstruct(out string Name, out int Age)
{
  Name = this.Name;
  Age = this.Age;
}

var (name, age) = new Person("User 1", 21);

 public record Person([Required] string Name, [Range(0, 150)] int Age);

 public class PersonController
 {
   public IActionResult Index() => View();

   [HttpPost]
   public IActionResult Index(Person person)
   {
    // ...
   }
 }