C#学习日志 day7 --------------LINQ与Lamda语句的初步尝试以及XML的生成

LINQ是一种集成在计算机语言里的信息查询语句，是c#3.0中最惹人瞩目的功能。

在C#中，LINQ语句有两种写法。

第一种写法与SQL语句类似：

IEnumerable<Customer> result =  from   customer in customers
      where  customer.FirstName == "Donna“
      select customer;

第二种写法更加接近c#语句:

IEnumerable<Customer> result =
    customers.Where(customer => customer.FirstName == "Donna")
    .Select(customer => customer);

这种写法易于理解，所以我认为这种写法更加好。

在Where和Select后面填入的是Lamda语句，这种语句是Delegate的简化，有利于提升代码的阅读性。

Lamda表达式的形式通常是这样的

people=>people.age>30

第一个people指的是传入的参数， =>是Lamda表达式的特定符号，右边是一个表达式，在查询语句中，此符号后面的表达式返回值通常是布尔类型的。例如上面这条语句放在Where中可以筛选出年龄大于三十的人。

下面是一个简单的LINQ和Lambda表达式的运用

customer类：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApplication3
{
    public class Customer
    {
        public string FirstName { get; set; }
        public string LastName { get; set; }
        public string HomeAddress { get; set; }
        //now override the ToString function of Object class.
        public override string ToString()
        {
            return string.Format("{0} {1}\n Enmail:{2}", FirstName, LastName, HomeAddress);
        }
        public static List<Customer> CreateCustomerList()
        {
            List<Customer> customers = new List<Customer>
            {
                 new Customer { FirstName = "Orlando",LastName = "Gee", HomeAddress = "[email protected]"},
                 new Customer { FirstName = "Keith", LastName = "Harris",HomeAddress = "[email protected]" },
                 new Customer { FirstName = "Donna", LastName = "Carreras",HomeAddress = "[email protected]" },
                 new Customer { FirstName = "Janet", LastName = "Gates",HomeAddress = "[email protected]" },
                 new Customer { FirstName = "Lucy", LastName = "Harrington",HomeAddress = "[email protected]" }
            };
            return customers;
        }
    }
}

在main函数中查询以D开头的记录

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApplication3
{
    class Program
    {
        public static void Main()
        {
            List<Customer> customers = Customer.CreateCustomerList();
            IEnumerable<Customer> result =
                customers.Where(customer => customer.FirstName.StartsWith("D"));
            foreach (Customer customer in result)
            {
                Console.WriteLine(customer.ToString());
            }
        }
    }
}

关于Xml我用上一个数据库简单的创建了一个xml文档

 public static void Main()
        {
            List<Customer> customers = Customer.CreateCustomerList();
            XmlDocument customerXml = new XmlDocument();
            XmlElement rootElem = customerXml.CreateElement("customers");
            customerXml.AppendChild(rootElem);
            foreach (Customer cust in customers) {
                XmlElement customerElm = customerXml.CreateElement("customer");

                XmlElement firstElm = customerXml.CreateElement("firstName");
                firstElm.InnerText = cust.FirstName;
                customerXml.AppendChild(firstElm);

                XmlElement second = customerXml.CreateElement("lastName");
                second.InnerText = cust.LastName;
                customerXml.AppendChild(second);

                XmlElement third = customerXml.CreateElement("emailAddress");
                third.InnerText = cust.Address;
                customerXml.AppendChild(third);

                rootElem.AppendChild(customerElm);
            }
            Console.WriteLine(customerXml.OuterXml);
        }

运行结果是这样的

<customers>
<customer>
<firstName>Orlando</firstName>
<lastName>Gee</lastName>
<emailAddress>[email protected]</emailAddress></customer>
<customer>
<firstName>Keith</firstName>
<lastName>Harris</lastName>
<emailAddress>[email protected]</emailAddress></customer>
<customer>
<firstName>Donna</firstName>
<lastName>Carreras</lastName>
<emailAddress>[email protected]</emailAddress>
</customer>
<customer>
<firstName>Janet</firstName>
<lastName>Gates</lastName>
<emailAddress>[email protected]</emailAddress></customer>
<customer>
<firstName>Lucy</firstName>
<lastName>Harrington</lastName>
<emailAddress>[email protected]</emailAddress></customer></customers>

这里的XmlElement firstElm = customerXml.CreateElement("firstName");语句是定义firstElm标签，这在html中是不行的

而xml的产生需要用到System.Xml.linq;命名空间。