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 1var table = new DataTable();//new 2Show<Parent>(new Child());//new 3Parent parent = new Child();parent.Print();}//new 2static void Show<T>(T t) where T : new(){WriteLine(t.GetType());}#region new 3public class Parent{public virtual void Print(){WriteLine("Parent Print");}public virtual void View(){}}public class Child : Parent{//now 3public 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 1WriteLine(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 2default: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 1Computer(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 1public void Computer(out int result)
{result = 10;}//out 2interface IParent<out R>{R Print();}class Child<R> : IParent<R>{public R Print()
{var r = default(R);WriteLine($"{typeof(R).Name}");return r;}}}
}