C# 3.0新特性初步研究 Part6:使用查询表达式

王朝c#·作者佚名  2006-11-24
窄屏简体版  字體: |||超大  

查询表达式(Query Expression)

大家都应该对SQL语句不陌生吧,在C# 2.0之前,嵌入到代码中的SQL就是下面这个样子:

1public void Test()

2{

3SqlConnection c = new SqlConnection(…);

4 c.Open();

5 SqlCommand cmd = new SqlCommand(

6 @“SELECT c.Name, c.Phone // queries in quotes

7 FROM Customers c

8 WHERE c.City = @p0”

9 );

10 cmd.Parameters[“@po”] = “London”; // arguments loosely bound

11 DataReader dr = c.Execute(cmd);

12 while (dr.Read()) {

13 string name = r.GetString(0);

14 string phone = r.GetString(1); // results loosely typed

15 DateTime date = r.GetDateTime(2); // compiler can’t help catch mistakes

16 }

17 r.Close();

18}

在C# 3.0中,我们可以将“SQL语句”方便的运用到其他地方,当然这里并不是真正的SQL语句~~

我觉得我会在以后的开发过程中使用很多以下的类似代码:

1class Program

2 {

3 static void Main(string[] args)

4 {

5 var contacts = new List<Contact>();

6

7 contacts.Add(new Contact("Michael", "520-331-2718",

8 "33140 SW Liverpool Lane", "WA"));

9 contacts.Add(new Contact("Jennifer", "503-998-1177",

10 "1245 NW Baypony Dr", "OR"));

11 contacts.Add(new Contact("Sean", "515-127-3340",

12 "55217 SW Estate Dr", "WA"));

13

14 var WAContacts =

15 from c in contacts

16 where c.State == "WA"

17 select new { c.Name, c.Phone };

18

19 Console.WriteLine("Contacts in the state of Washington: ");

20 foreach (var c in WAContacts)

21 {

22 Console.WriteLine("Name: {0}, Phone: {1}", c.Name, c.Phone);

23 }

24 }

25 }

26

27 class Contact

28 {

29 public string Name;

30 public string Phone;

31 public string Address;

32 public string State;

33

34 public Contact(string name, string phone, string address, string state)

35 {

36 this.Name = name;

37 this.Phone = phone;

38 this.Address = address;

39 this.State = state;

40 }

41 }

其中出现的代码:

1var WAContacts =

2 from c in contacts

3 where c.State == "WA"

4 select new { c.Name, c.Phone };

是否与我们熟悉的SQL语句有着极大的相似性呢?Of Course!

到底是SQL梦见了C#,还是C#梦见了SQL……

 
 
 
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