C#: LINQ to Object 和 LINQ to XML 超好用之一

假設有此一 XML

<?xml version="1.0" encoding="utf-8" ?> <customers>     <customer>         <customerid>ALFKI</customerid>         <city>Berlin</city>         <age>20</age>     </customer>     <customer>         <customerid>BONAP</customerid>         <city>Marseille</city>         <age>21</age>     </customer>     <customer>         <customerid>CONSH</customerid>         <city>London</city>         <age>30</age>     </customer>     <customer>         <customerid>EASTC</customerid>         <city>London</city>         <age>34</age>     </customer>     <customer>         <customerid>FRANS</customerid>         <city>Torino</city>         <age>35</age>     </customer>     <customer>         <customerid>LONEP</customerid>         <city>Portland</city>         <age>40</age>     </customer>     <customer>         <customerid>NORTS</customerid>         <city>London</city>         <age>25</age>     </customer>     <customer>         <customerid>THEBI</customerid>         <city>Portland</city>         <age>36</age>     </customer> </customers>

還有此一物件

public string CustomerID { get; set; } public string City { get; set; } public int Age { get; set; } public static IEnumerable<Customer> CreateCustomers() {    return new List<Customer>    {       new Customer { CustomerID = "ALFKI", City = "Berlin", Age=20   },       new Customer { CustomerID = "BONAP", City = "Marseille" , Age=21},       new Customer { CustomerID = "CONSH", City = "London", Age=30    },       new Customer { CustomerID = "EASTC", City = "London", Age=34    },       new Customer { CustomerID = "FRANS", City = "Torino", Age=35    },       new Customer { CustomerID = "LONEP", City = "Portland", Age=40  },       new Customer { CustomerID = "NORTS", City = "London" , Age=25   },       new Customer { CustomerID = "THEBI", City = "Portland", Age=26  }       };  } }

使用方法範例一：把物件理的值拿來做dropdown list

抓取想要的值，可以拿來做dropdown List 來用

public static List<Customer> GetCities()

{

     var customers = from customer in Customer.CreateCustomers()

                     orderby customer.City

                     select new Customer { City = customer.City };

     return customers.ToList();

}

使用方法範例二：XML來源放到物件裡

我喜歡這方法，我常有一些 需要讀取　web service XML。以前用 XML DOM又慢又沒效率又複雜，又不美觀。。。

public static List<Customer> GetCustomerFromXML(string city)

{

    XDocument xmlDoc = XDocument.Load("CustomerXML.xml");

    var customers = from customer in xmlDoc.Descendants("customer")

                    where customer.Element("city").Value == city

                    select new Customer

                    {

                        CustomerID = customer.Element("customerid").Value,

                        City = customer.Element("city").Value,

                        Age = Convert.ToInt32(customer.Element("age").Value)

                    };

    return customers.ToList();

}

使用方法範例三：從現有List <T>中取出符合條件的<T>

public static List<Customer> GetCustomersFromObject(string city)

{

     var customers = from customer in Customer.CreateCustomers()

                     where customer.City == city

                     select new Customer

                     {

                         CustomerID = customer.CustomerID,

                         City = customer.City,

                         Age = customer.Age

                     };

     return customers.ToList();

 }