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C# 3.0新特性初步研究 Part6:使用查詢表達式

編輯：關於C語言

查詢表達式（Query Expression）
大家都應該對SQL語句不陌生吧，在C# 2.0之前，嵌入到代碼中的SQL就是下面這個樣子：
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，我們可以將“SQL語句”方便的運用到其他地方，當然這裡並不是真正的SQL語句~~
我覺得我會在以後的開發過程中使用很多以下的類似代碼：
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    }
其中出現的代碼：
1var WAContacts =
2                    from c in contacts
3                     where c.State == "WA"
4                     select new { c.Name, c.Phone };
是否與我們熟悉的SQL語句有著極大的相似性呢？Of Course!
到底是SQL夢見了C#，還是C#夢見了SQL……