sealed class SealedClass  {      public int x;      public int y;  }    // error class SealedTest2:SealedClass  class SealedTest2  {      static void Main()      {          var sc = new SealedClass();          sc.x = 110;          sc.y = 150;          Console.WriteLine($"x = {sc.x}, y = {sc.y}");      }  }  // Output: x = 110, y = 150    //------------------  class X  {      protected virtual void F() { Console.WriteLine("X.F"); }      protected virtual void F2() { Console.WriteLine("X.F2"); }  }    class Y : X  {      sealed protected override void F() { Console.WriteLine("Y.F"); }      protected override void F2() { Console.WriteLine("Y.F2"); }  }    class Z : Y  {      // Attempting to override F causes compiler error CS0239.      // protected override void F() { Console.WriteLine("Z.F"); }        // Overriding F2 is allowed.      protected override void F2() { Console.WriteLine("Z.F2"); }  }

// Assembly1.cs  // Compile with: /target:library    internal class BaseClass  {     public static int intM = 0;  }  // Assembly1_a.cs  // Compile with: /reference:Assembly1.dll    class TestAccess  {     static void Main()     {        var myBase = new BaseClass();   // CS0122  错误     }  }

class A  {      protected int x = 123;  }    class B : A  {      static void Main()      {          var a = new A();          var b = new B();            // Error CS1540, because x can only be accessed by          // classes derived from A.          // a.x = 10;            // OK, because this class derives from A.          b.x = 10;      }  }

public class Shape  {      // A few example members      public int X { get; private set; }      public int Y { get; private set; }      public int Height { get; set; }      public int Width { get; set; }        // Virtual method      public virtual void Draw()      {          Console.WriteLine("Performing base class drawing tasks");      }  }    public class Circle : Shape  {      public override void Draw()      {          // Code to draw a circle...          Console.WriteLine("Drawing a circle");          base.Draw();      }  }  public class Rectangle : Shape  {      public override void Draw()      {          // Code to draw a rectangle...          Console.WriteLine("Drawing a rectangle");          base.Draw();      }  }  public class Triangle : Shape  {      public override void Draw()      {          // Code to draw a triangle...          Console.WriteLine("Drawing a triangle");          base.Draw();      }  }    // Polymorphism at work #1: a Rectangle, Triangle and Circle  // can all be used whereever a Shape is expected. No cast is  // required because an implicit conversion exists from a derived  // class to its base class.  var shapes = new List<Shape>  {      new Rectangle(),      new Triangle(),      new Circle()  };    // Polymorphism at work #2: the virtual method Draw is  // invoked on each of the derived classes, not the base class.  foreach (var shape in shapes)  {      shape.Draw();  }  /* Output:      Drawing a rectangle      Performing base class drawing tasks      Drawing a triangle      Performing base class drawing tasks      Drawing a circle      Performing base class drawing tasks  */

public delegate void StringListEvent(IStringList sender);    public interface IStringList  {      void Add(string s);      int Count { get; }      event StringListEvent Changed;      string this[int index] { get; set; }  }

abstract class A  {      public abstract void F();  }    abstract class B: A  {      public void G() {}  }    class C: B  {      public override void F() {          // actual implementation of F      }  }

// Declare the generic class.  public class GenericList<T>  {      public void Add(T input) { }  }  class TestGenericList  {      private class ExampleClass { }      static void Main()      {          // Declare a list of type int.          GenericList<int> list1 = new GenericList<int>();          list1.Add(1);            // Declare a list of type string.          GenericList<string> list2 = new GenericList<string>();          list2.Add("");            // Declare a list of type ExampleClass.          GenericList<ExampleClass> list3 = new GenericList<ExampleClass>();          list3.Add(new ExampleClass());      }  }

unsafe public static byte[] ToByteArray<T>(this T argument) where T : unmanaged  {      var size = sizeof(T);      var result = new Byte[size];      Byte* p = (byte*)&argument;      for (var i = 0; i < size; i++)          result[i] = *p++;      return result;  }

namespace ExtensionMethods  {      public static class MyExtensions      {          public static int WordCount(this String str)          {              return str.Split(new char[] { ' ', '.', '?' },                               StringSplitOptions.RemoveEmptyEntries).Length;          }      }  }

// percentileQuery is an IEnumerable<IGrouping<int, Country>>  var percentileQuery =      from country in countries      let percentile = (int) country.Population / 10_000_000      group country by percentile into countryGroup      where countryGroup.Key >= 20      orderby countryGroup.Key      select countryGroup;    // grouping is an IGrouping<int, Country>  foreach (var grouping in percentileQuery)  {      Console.WriteLine(grouping.Key);      foreach (var country in grouping)          Console.WriteLine(country.Name + ":" + country.Population);  }

class Person  {      public string FirstName { get; set; }      public string LastName { get; set; }  }    class Pet  {      public string Name { get; set; }      public Person Owner { get; set; }  }    class Cat : Pet  { }    class Dog : Pet  { }    public static void MultipleJoinExample()  {      Person magnus = new Person { FirstName = "Magnus", LastName = "Hedlund" };      Person terry = new Person { FirstName = "Terry", LastName = "Adams" };      Person charlotte = new Person { FirstName = "Charlotte", LastName = "Weiss" };      Person arlene = new Person { FirstName = "Arlene", LastName = "Huff" };      Person rui = new Person { FirstName = "Rui", LastName = "Raposo" };      Person phyllis = new Person { FirstName = "Phyllis", LastName = "Harris" };        Cat barley = new Cat { Name = "Barley", Owner = terry };      Cat boots = new Cat { Name = "Boots", Owner = terry };      Cat whiskers = new Cat { Name = "Whiskers", Owner = charlotte };      Cat bluemoon = new Cat { Name = "Blue Moon", Owner = rui };      Cat daisy = new Cat { Name = "Daisy", Owner = magnus };        Dog fourwheeldrive = new Dog { Name = "Four Wheel Drive", Owner = phyllis };      Dog duke = new Dog { Name = "Duke", Owner = magnus };      Dog denim = new Dog { Name = "Denim", Owner = terry };      Dog wiley = new Dog { Name = "Wiley", Owner = charlotte };      Dog snoopy = new Dog { Name = "Snoopy", Owner = rui };      Dog snickers = new Dog { Name = "Snickers", Owner = arlene };        // Create three lists.      List<Person> people =          new List<Person> { magnus, terry, charlotte, arlene, rui, phyllis };      List<Cat> cats =          new List<Cat> { barley, boots, whiskers, bluemoon, daisy };      List<Dog> dogs =          new List<Dog> { fourwheeldrive, duke, denim, wiley, snoopy, snickers };        // The first join matches Person and Cat.Owner from the list of people and      // cats, based on a common Person. The second join matches dogs whose names start      // with the same letter as the cats that have the same owner.      var query = from person in people                  join cat in cats on person equals cat.Owner                  join dog in dogs on                  new { Owner = person, Letter = cat.Name.Substring(0, 1) }                  equals new { dog.Owner, Letter = dog.Name.Substring(0, 1) }                  select new { CatName = cat.Name, DogName = dog.Name };        foreach (var obj in query)      {          Console.WriteLine(              $"The cat "{obj.CatName}" shares a house, and the first letter of their name, with "{obj.DogName}".");      }  }    // This code produces the following output:  //  // The cat "Daisy" shares a house, and the first letter of their name, with "Duke".  // The cat "Whiskers" shares a house, and the first letter of their name, with "Wiley".

class Test  {      delegate void TestDelegate(string s);      static void M(string s)      {          Console.WriteLine(s);      }        static void Main(string[] args)      {          // Original delegate syntax required          // initialization with a named method.          TestDelegate testDelA = new TestDelegate(M);            // C# 2.0: A delegate can be initialized with          // inline code, called an "anonymous method." This          // method takes a string as an input parameter.          TestDelegate testDelB = delegate(string s) { Console.WriteLine(s); };            // C# 3.0. A delegate can be initialized with          // a lambda expression. The lambda also takes a string          // as an input parameter (x). The type of x is inferred by the compiler.          TestDelegate testDelC = (x) => { Console.WriteLine(x); };            // Invoke the delegates.          testDelA("Hello. My name is M and I write lines.");          testDelB("That's nothing. I'm anonymous and ");          testDelC("I'm a famous author.");            // Keep console window open in debug mode.          Console.WriteLine("Press any key to exit.");          Console.ReadKey();      }  }  /* Output:      Hello. My name is M and I write lines.      That's nothing. I'm anonymous and      I'm a famous author.      Press any key to exit.   */

Action<string> greet = name =>  {      string greeting = $"Hello {name}!";      Console.WriteLine(greeting);  };  greet("World");  // Output:  // Hello World!

int[] numbers = { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 };  var firstSmallNumbers = numbers.TakeWhile((n, index) => n >= index);  Console.WriteLine(string.Join(" ", firstSmallNumbers));  // Output:  // 5 4

public static double StandardDeviation(IEnumerable<double> sequence)  {      var computation = ComputeSumAndSumOfSquares(sequence);        var variance = computation.SumOfSquares - computation.Sum * computation.Sum / computation.Count;      return Math.Sqrt(variance / computation.Count);  }    private static (int Count, double Sum, double SumOfSquares) ComputeSumAndSumOfSquares(IEnumerable<double> sequence)  {      double sum = 0;      double sumOfSquares = 0;      int count = 0;        foreach (var item in sequence)      {          count++;          sum += item;          sumOfSquares += item * item;      }        return (count, sum, sumOfSquares);  }

public static void Main()  {      (string city, int population, double area) = QueryCityData("New York City");        // Do something with the data.  }  public static void Main()  {      var (city, population, area) = QueryCityData("New York City");        // Do something with the data.  }  public static void Main()  {      (string city, var population, var area) = QueryCityData("New York City");        // Do something with the data.  }  public static void Main()  {      string city = "Raleigh";      int population = 458880;      double area = 144.8;        (city, population, area) = QueryCityData("New York City");        // Do something with the data.  }

using System;  using System.Collections.Generic;    public class Example  {      public static void Main()      {          var (_, _, _, pop1, _, pop2) = QueryCityDataForYears("New York City", 1960, 2010);            Console.WriteLine($"Population change, 1960 to 2010: {pop2 - pop1:N0}");      }        private static (string, double, int, int, int, int) QueryCityDataForYears(string name, int year1, int year2)      {          int population1 = 0, population2 = 0;          double area = 0;            if (name == "New York City")          {              area = 468.48;              if (year1 == 1960)              {                  population1 = 7781984;              }              if (year2 == 2010)              {                  population2 = 8175133;              }              return (name, area, year1, population1, year2, population2);          }            return ("", 0, 0, 0, 0, 0);      }  }  // The example displays the following output:  //      Population change, 1960 to 2010: 393,149

using System;    public class Person  {      public string FirstName { get; set; }      public string MiddleName { get; set; }      public string LastName { get; set; }      public string City { get; set; }      public string State { get; set; }        public Person(string fname, string mname, string lname,                    string cityName, string stateName)      {          FirstName = fname;          MiddleName = mname;          LastName = lname;          City = cityName;          State = stateName;      }        // Return the first and last name.      public void Deconstruct(out string fname, out string lname)      {          fname = FirstName;          lname = LastName;      }        public void Deconstruct(out string fname, out string mname, out string lname)      {          fname = FirstName;          mname = MiddleName;          lname = LastName;      }        public void Deconstruct(out string fname, out string lname,                              out string city, out string state)      {          fname = FirstName;          lname = LastName;          city = City;          state = State;      }  }    public class Example  {      public static void Main()      {          var p = new Person("John", "Quincy", "Adams", "Boston", "MA");            // Deconstruct the person object.          var (fName, lName, city, state) = p;          Console.WriteLine($"Hello {fName} {lName} of {city}, {state}!");      }  }  // The example displays the following output:  //    Hello John Adams of Boston, MA!