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……