Linq 之 Select 和 where 的用法

最近开始学习linq.自己也总结一下，方便以后查阅。

Select 同 Sql 中的 select 类似，即输出我们要的东东，感觉在 linq 中更加强大。

Linq 可以对集合如数组、泛型等操作，这里我们对泛型类型进行举例。建一个类如下：

public class Customer
    {
        public Customer(string firstName, string lastName, string city)
        {
            FirstName = firstName;
            LastName = lastName;
            City = city;
        }
public Customer()
        {}
        public string FirstName { get; set; }
        public string LastName { get; set; }
        public string City { get; set; }
}  

1、 select 出对象，cust 是一个Customer 对象。

static void Main(string[] args)
        {
            List<Customer> customers = new List<Customer>()
            {
                new Customer("Jack", "Chen", "London"),
                new Customer("Sunny","Peng", "Shenzhen"),
                new Customer("Tom","Cat","London")
            };
            //使用IEnumerable<T>作为变量
            var result =from cust in customers
                        where cust.City == "London"
                        select cust;
            foreach (var item in result)
            {
                Console.WriteLine(item.FirstName+":"+item.City);
            }
            Console.Read();
        }  

2、 select 出对象的属性字段

var result =from cust in customerswhere cust.City == "London"
         select cust.City  

多个属性要用new ｛｝如下：

var result =from cust in customers where cust.City == "London"
                        select new{cust.City,cust.FirstName};  

3、  重命名， city 和 name 是随便起的

var result =from cust in customers where cust.City == "London"
                        select new{ city=cust.City, name ="姓名"+ cust.FirstName+""+cust.LastName };
            foreach (var item in result)
            {
                Console.WriteLine(item.name+":"+item.city);
            }  

4、 直接实例化对象

var result =from cust in customers where cust.City == "London"
        select new Customer{ City = cust.City, FirstName = cust.FirstName }; 

5、 selec 中嵌套select

var result =from cust in customers  where cust.City == "London"
            select new{ city = cust.City,
name=from cust1 in customers where cust1.City == "London" select cust1.LastName
};