這兩天花了一點時間大概看了一些What is Linq? Linq's design goal. Why Linq? History等不是很想看的東西。
With LINQ, Microsoft's intention was to provide a solution for the problem of object-relational mapping, as well as to simplify the interaction between objects and data sources.
這是我唯幾有畫的段落= =，不知道究竟是不是個key。還是比較想來體驗一下code。
跟hello world一樣，首先要來個hello linq:
1: using System;
2: using System.Linq;
3:
4: static class HelloWorld
5: { 6: static void Main()
7: { 8: //先來個collection
9: string[] words =
10: { "hello", "wonderful", "linq", "beautiful", "world" }; 11:
12: //傳說中的linq語法在這from~ where~ select
13: var shortWords =
14: from word in words
15: where word.Length <= 5
16: select word;
17:
18: //把剛剛處理好的資料印出來，結果就是: hello linq world 各一行
19: foreach (var word in shortWords)
20: Console.WriteLine(word);
21: }
22: }
Linq特別強調data query不再只是一串string，他變成是程式的一部份，所以你不會在執行程式時才發現sql下錯了、字拼錯、參數名字打錯這類的錯誤。Linq背後可存取的資料來源很多種，不管是存取資料庫或只是一個collection，只要學了Linq，就可以用統一的方法對這些資料來源進行存取。
再來一段長一點的，下面要表達的是，Linq也可以把資料弄成XML的樣子，重點是: 輕易地
1: using System;
2: using System.Linq;
3: using System.Xml;
4: using System.Xml.Linq;
5:
6: class Book
7: { 8: public string Publisher;
9: public string Title;
10: public int Year;
11: public Book(string title, string publisher, int year)
12: { 13: Title = title;
14: Publisher = publisher;
15: Year = year;
16: }
17: }
18:
19: static class HelloLinqToXml
20: { 21: static void Main()
22: { 23: //用上面那個book class產生3筆資料
24: Book[] books = new Book[] { 25: new Book("Ajax in Action", "Manning", 2005), 26: new Book("Windows Forms in Action", "Manning", 2006), 27: new Book("RSS and Atom in Action", "Manning", 2006) 28: };
29:
30: //重頭戲: 只能意會不能言傳XD
31: XElement xml = new XElement("books", // layer1 32: from book in books
33: where book.Year == 2006
34: select new XElement("book", //layer2 35: new XAttribute("title", book.Title), // attribute of book(layer2) 36: new XElement("publisher", book.Publisher) //layer3 37: )
38: );
39: Console.WriteLine(xml);
40:
41: //result:
42:
43: //<books> ==> layer1
44: // <book title="Windows Forms in Action"> ==> layer2 with attribute(title)
45: // <publisher>Manning</publisher> ==> layer3
46: // </book>
47: // <book title="RSS and Atom in Action">
48: // <publisher>Manning</publisher>
49: // </book>
50: //</books>
51: }
52: }
接下來應該是最常應用的: 連接資料庫
1: using System;
2: using System.Linq;
3: using System.Data.Linq;
4: using System.Data.Linq.Mapping;
5:
6: static class HelloLinqToSql
7: { 8: //declaring the classes to represent the application data: entities
9: //mapping object and database by adding customer attritube
10: [Table(Name="Contacts")]
11: class Contact
12: { 13: [Column(IsPrimaryKey=true)]
14: public int ContactID { get; set; } 15: [Column(Name="ContactName")]
16: public string Name { get; set; } 17: [Column]
18: public string City { get; set; } 19: }
20:
21: static void Main()
22: { 23: string path = //使用northwind資料庫
24: System.IO.Path.GetFullPath(@"..\..\Data\northwnd.mdf");
25: //translate requests for objects into SQL queries
26: DataContext db = new DataContext(path);
27:
28: var contacts =
29: from contact in db.GetTable<Contact>()
30: where contact.City == "Paris"
31: select contact;
32:
33: foreach (var contact in contacts)
34: Console.WriteLine("Bonjour "+contact.Name); 35: }
36: }
完畢。上面那一段程式，Linq做掉了一些東西:
....如此一般如此一般.... 他其實還是有點複雜的...
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