密封类

sealed，无法被继承

var

可以定义匿名对象

static void test1()
{
    var t = 1;
    t = "jack";//报错，类型已经确定好了
    var s = new
    {
        id = 1,
        name = "tom"
    };
    Console.WriteLine(s.id + s.name);
}

扩展方法

对现有类型做方法的扩展，密封类也可以实现
不在同一命名空间需要引入，返回类型根据自己需要来决定

系统类型扩展

密封类扩展 

Linq 

static void Main(string[] args)
{
    test1();
}

static void test1()
{
    int[] nums = { 1, 7, 2, 6, 5, 4, 9, 13, 20 };
    List<int> list = new List<int>(nums);
    var res = nums
        .Where(x => x % 2 == 1)
        .Select(x => x * x)
        .OrderByDescending(x => x);
    foreach (var item in res)
    {
        Console.WriteLine(item);
    }
}

class Program
{
    static void Main(string[] args)
    {
        test1();
    }

    static void test1()
    {
        Student s1 = new Student() { id = 1, name = "tom1" };
        Student s2 = new Student() { id = 1, name = "tom2" };
        Student s3 = new Student() { id = 2, name = "tom3" };

        List<Student> list = new List<Student>() { s1, s2, s3 };
        //根据id升序，id相同按照name降序
        var res = list
            .OrderBy(s => s.id)
            .ThenByDescending(s => s.name);
        foreach (var item in res)
        {
            Console.WriteLine(item.ToString());
        }
        //从1开始，按顺序生成10个数
        var num1 = Enumerable.Range(1, 10);
        //生成10个abcd
        var num2 = Enumerable.Repeat("abcd", 10);
        foreach (var item in num1)
        {
            Console.WriteLine(item);
        }
        foreach (var item in num2)
        {
            Console.WriteLine(item);
        }
    }

}