C# 语言变更历史 - YangLong's Blog

.Net Framework 的版本信息

.NET Standard	1.0	1.1	1.2	1.3	1.4	1.5	1.6	2.0
.NET Core	1.0	1.0	1.0	1.0	1.0	1.0	1.0	2.0
.NET Framework ¹	4.5	4.5	4.5.1	4.6	4.6.1	4.6.1	4.6.1	4.6.1
Mono	4.6	4.6	4.6	4.6	4.6	4.6	4.6	5.4
Xamarin.iOS	10.0	10.0	10.0	10.0	10.0	10.0	10.0	10.14
Xamarin.Mac	3.0	3.0	3.0	3.0	3.0	3.0	3.0	3.8
Xamarin.Android	7.0	7.0	7.0	7.0	7.0	7.0	7.0	8.0
Universal Windows Platform	10.0	10.0	10.0	10.0	10.0	10.0.16299	10.0.16299	10.0.16299
Windows	8.0	8.0	8.1
Windows Phone	8.1	8.1	8.1
Windows Phone Silverlight	8.0

VERSION 2.0

Geenerics 泛型

可以在类、方法中对使用的类型进行参数化

// 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());
    }
}

泛型约束

Partial types 部分类

public partial class Employee
{
    public void DoWork()
    {
    }
}

public partial class Employee
{
    public void GoToLunch()
    {
    }
}

Anonymous methods 匿名方法

delegate void Del(int x);
Del d = delegate(int k) { /* ... */ }; 
System.Threading.Thread t1 = new System.Threading.Thread (delegate() { System.Console.Write("Hello, "); } );

Nullable types 可控类型

System.Nullable简写为T ? 可空类型System.Nullable，可空类型仅针对于值类型，不能针对引用类型去创建

Iterators 迭代器

static void Main()  
{  
    DaysOfTheWeek days = new DaysOfTheWeek();  

    foreach (string day in days)  
    {  
        Console.Write(day + " ");  
    }  
    // Output: Sun Mon Tue Wed Thu Fri Sat  
    Console.ReadKey();  
}  

public class DaysOfTheWeek : IEnumerable  
{  
    private string[] days = { "Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat" };  

    public IEnumerator GetEnumerator()  
    {  
        for (int index = 0; index < days.Length; index++)  
        {  
            // Yield each day of the week.  
            yield return days[index];  
        }  
    }  
}  

Covariance and Contravariance 委托的协变和逆变

// Assignment compatibility.   
string str = "test";  
// An object of a more derived type is assigned to an object of a less derived type.   
object obj = str;  

// Covariance.   
IEnumerable<string> strings = new List<string>();  
// An object that is instantiated with a more derived type argument   
// is assigned to an object instantiated with a less derived type argument.   
// Assignment compatibility is preserved.   
IEnumerable<object> objects = strings;  

// Contravariance.             
// Assume that the following method is in the class:   
// static void SetObject(object o) { }   
Action<object> actObject = SetObject;  
// An object that is instantiated with a less derived type argument   
// is assigned to an object instantiated with a more derived type argument.   
// Assignment compatibility is reversed.   
Action<string> actString = actObject;  

Default 默认类型

以使用在类型参数上：default(T); 对于值类型，返回0，引用类型，返回null，对于结构类型，会返回一个成员值全部为0的结构实例

Global 上层空间

解决类名重复问题，从最外面逐渐向内部寻找System类

global::System.Console.WriteLine(number);

Fiexd 固定类型

public fixed char pathName[128]; 

VERSION 3.0

Lamada表达式（=>)

委托 Delegate

Delegate至少0个参数，至多32个参数，可以无返回值，也可以指定返回值类型如：

public delegate int MethodDelegate(int x, int y);
 private static MethodDelegate method;
 static void Main(string[] args)
 {
            method = new MethodDelegate(Add);
            Console.WriteLine(method(10,20));
            Console.ReadKey();
 }
 private static int Add(int x, int y)
 {
            return x + y;
 }

Action 无返回值的委托

一个无返回值、泛型的委托，有16重载，分别是0个参数~15个参数

delegate void Action(); delegate void Action(T obj); delegate void Action<in T,in T1>(T obj,T1 obj1); delegate void Action<in T,in T1,in T2>(T obj,T1 obj1,T2 obj2); 如:

this.button1.Click += new Action<object, EventArgs>((sender, e) => {
                MessageBox.Show("hellow");
            });

如:

Thread t = new Thread(new ThreadStart(new Action(() => { 
			                //线程代码
			            })));
			            t.Start();

Func 有返回值委托

带有返回值的委托。也是有15个重载 delegate TR Func(out TR); delegate TR Func<in T,out TR>(T obj); delegate TR Func<in T,in T1,out TR>(T obj,T1 obj1); delegate TR Func<in T,in T1,in T2,out TR>(T obj,T1 obj1,T2 obj2);

MessageBox.Show(new Func<string>(() => {
                System.Net.WebClient wc = new System.Net.WebClient();
                return wc.DownloadString("http://www.baidu.com");
            })());

Expression 表达式树

Expression<Func<type,returnType» = (param) => lamdaexpresion;

Expression<Func<int, int, int>> expr = (x, y) => x+y;
Expression<Func<int, int, int>> expr = (x, y) => { return x + y; };  //会出现错误，需用API创建复杂的表达式树

类型推断（Var）

var i = 5;
var s = "Hello";

扩展方法

public static class JeffClass
{
    public static int StrToInt32(this string s)
    {
        return Int32.Parse(s);
    }
 }

自动属性（get、set）

会自动生成一个后台的私有变量

public Class Point
{
   public int X { get; set; }
   public int Y { get; set; }
}

查询表达式

from g in
from c in customers
           group c by c.Country
select new { Country = g.Key, CustCount = g.Count() }

VERSION 4.0

斜变和逆变

支持针对泛型接口的协变和逆变

IList<string> strings = new List<string>();
IList<object> objects = strings;

可选参数和命名参数

private void CreateNewStudent(string name, int studentid = 0, int year = 1)
CreateNewStudent(year:2, name:"Hima", studentid: 4);  

VERSION 5.0

异步编程

如：

static async void DownloadStringAsync2(Uri uri)
{
    var webClient = new WebClient();
    var result = await webClient.DownloadStringTaskAsync(uri);
    Console.WriteLine(result);
}

之前的方法：

static void DownloadStringAsync(Uri uri)
{
  var webClient = new WebClient();
  webClient.DownloadStringCompleted += (s, e) =>
      {
          Console.WriteLine(e.Result);
      };
  webClient.DownloadStringAsync(uri);
 }

调用用法信息

public void DoProcessing()
{
    TraceMessage("Something happened.");
}

VERSION 6.0

自动属性初始化(Auto-property initializers)

public class Account
{
    public string Name { get; set; } = "summit";
    public int Age { get; set; } = 22;
    public IList<int> AgeList
    {
        get;
        set;
    } = new List<int> { 10,20,30,40,50 };
}

字符串嵌入值(String interpolation)

Console.WriteLine($"年龄:{account.Age}  生日:{account.BirthDay.ToString("yyyy-MM-dd")}");
Console.WriteLine($"年龄:{account.Age}");

导入静态类(Using Static)

using static System.Math;//导入类
Console.WriteLine($"之前的使用方式: {Math.Pow(4, 2)}");
Console.WriteLine($"导入后可直接使用方法: {Pow(4,2)}");

空值运算符(Null-conditional operators)

var age = account.AgeList?[0].ToString();
Console.WriteLine("{0}", (person.list?.Count ?? 0));

异常过滤器(Exception filters)

static void TestExceptionFilter()
{
    try
    {
        Int32.Parse("s");
    }
    catch (Exception e) when (Log(e))
    {
        Console.WriteLine("catch");
        return;
    }
}

nameof表达式 (nameof expressions)

private static void Add(Account account)
{
     if (account == null)
         throw new ArgumentNullException("account");
}

catch和finally语句块里使用await(Await in catch and finally blocks)

Resource res = null;
try
{
    res = await Resource.OpenAsync(…);       // You could do this.
    …
}
catch(ResourceException e)
{
    await Resource.LogAsync(res, e);         // Now you can do this …
}
finally
{
    if (res != null) await res.CloseAsync(); // … and this.
}

属性使用Lambda表达式(Expression bodies on property-like function members)

public string Name =>string.Format("姓名: {0}", "summit");
public void Print() => Console.WriteLine(Name);

方法成员上使用Lambda表达式

static int LambdaFunc(int x, int y) => x*y;
public void Print() => Console.WriteLine(First + " " + Last);

VERSION 7.0

元祖 Tuple

( string, string, string, string) getEmpInfo()
{
    //read EmpInfo from database or any other source and just return them
    string strFirstName = "abc";
    string strAddress = "Address";
    string strCity= "City";
    string strState= "State";
    return (strFirstName, strAddress, strCity, strState); // tuple literal
                 return (strFName: strFirstName, strAdd: strAddress, strCity: strC, strState: strSt);
}
var empInfo= getEmpInfo();

析构函数

( string strFName,  string strAdd,  string strC, string strSt) = getEmpInfo(); 
 Console.WriteLine($"Address: { strAdd }, Country: { strC }");

OUT输出变量

 void AssignVal(out string strName)
 {
        strName = "I am from OUT";
 }
  AssignVal(out string szArgu); 

不允许为空类型

int objNullVal;     //non-nullable value type
int? objNotNullVal;    //nullable value type
string! objNotNullRef; //non-nullable reference type
string objNullRef;  //nullable reference type

本地方法和函数

private static void Main(string[] args)
{
    int local_var = 100;
    int LocalFunction(int arg1)
    {
        return local_var * arg1;
    }
 
    Console.WriteLine(LocalFunction(100));
}

模式匹配

class Calculate();
class Add(int a, int b, int c) : Calculate;
class Substract(int a, int b) : Calculate;
class Multiply(int a, int b, int c) : Calculate;
Calculate objCal = new Multiply(2, 3, 4);
switch (objCal)
{
    case Add(int a, int b, int c):
        //code goes here
        break;
    case Substract(int a, int b):
        //code goes here
        break;
    case Multiply(int a, int b, int c):
        //code goes here
        break;
    default:
        //default case
        break;
}

通过Ref返回

ref string getFromList(string strVal, string[] Values)
{
 foreach (string val1 in Values)
 {
     if (strVal == val1)
        return ref val1; //return location as ref not actual value
 }
}
string[] values = { "a", "b", "c", "d" };

ref string strSubstitute = ref getFromList("b", values);

strSubstitute = "K"; // replaces 7 with 9 in the array

System.Write(values[1]); // it prints "K"

表达式错误

Throw Exception from Expression

public string getEmpInfo( string EmpName)
{
    string[] empArr = EmpName.Split(",");
    return (empArr.Length > 0) ? empArr[0] : throw new Exception("Emp Info Not exist");
}