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();
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……

来源:网络