c#中的多义关键字

Posted 2021-05-08 dotNET跨平台

new

new用三种场景，最常用的实例化，泛型参数时的实例化约束，第三种是显式隐藏继承成员。

using System;using System.Collections.Generic;using System.Data;using System.Linq;using System.Text;using System.Threading.Tasks;using static System.Console;namespace KeyWordsDemo{ class NewDemo : IDemo { public void Run() { //new 1 var table = new DataTable();  //new 2 Show<Parent>(new Child());
 //new 3 Parent parent = new Child(); parent.Print(); } //new 2 static void Show<T>(T t) where T : new() { WriteLine(t.GetType()); }
 #region new 3 public class Parent { public virtual void Print() { WriteLine("Parent Print"); } public virtual void View() { } } public class Child : Parent { //now 3 public new void Print() { WriteLine("Child Print"); } public override void View() { } }        #endregion }}

default

default有两种用法，一是返回类型的默认值，二是在switch中作为非选项值。

using System;using System.Collections.Generic;using System.Linq;using System.Text;using System.Threading.Tasks;using static System.Console;namespace KeyWordsDemo{ class DefaultDemo : IDemo { public void Run() { //default 1 WriteLine(default(int)); WriteLine(default(bool)); WriteLine(default(DateTime)); WriteLine(default(string));
 switch (default(string)) { case "": WriteLine("空字符串"); break; case null: WriteLine("null"); break; //default 2 default: WriteLine("其他"); break; } } }}

out

out有两种用法，一是作为方法输出参数，从方法内部返回数据，二是用在接口和委托的协变中，如案例中的interface IParent<out R>

using System;using System.Collections.Generic;using System.Diagnostics;using System.Linq;using System.Text;using System.Threading.Tasks;using static System.Console;
namespace KeyWordsDemo{ class OutDemo : IDemo { public void Run(){ //out 1 Computer(out int result); WriteLine(result);
 WriteLine("------------"); IParent<object> p1 = new Child<object>(); IParent<object> p2 = new Child<string>(); WriteLine(p1.GetType()); WriteLine(p2.GetType()); p1.Print(); p2.Print(); p1 = p2; p1.Print();
 } //out 1 public void Computer(out int result){ result = 10; }
 //out 2 interface IParent<out R> { R Print(); } class Child<R> : IParent<R> { public R Print(){ var r = default(R); WriteLine($"{typeof(R).Name}"); return r; } } }}