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。