C# LINQ

LINQ(Language-Integrated Query)，是.NET Framework 3.5 開始新增的語言功能特色是具備資料查詢的能力，語法就像是 SQL 的語法

查詢作業需要三個部分

取得資料來源
建立查詢
執行查詢

範例說明

class Person {
    public string Name { get; set; }
    public string Gender { get; set; }
}

public static void Main () {
    // 1. 資料來源
    List<Person> person = new List<Person> {
        new Person { Name = "Shawn", Gender = "Male" },
        new Person { Name = "Erin", Gender = "Female" },
        new Person { Name = "David", Gender = "Male" },
        new Person { Name = "Susan", Gender = "Female" },
        new Person { Name = "Joe", Gender = "Male" },
        new Person { Name = "Eric", Gender = "Male" }
    };

    // 2. 建立查詢
    var result = from p in person
            	where p.Gender == "Female"
            	select p;

    // 3. 執行查詢
    foreach (var r in result) {
        Console.WriteLine ("Name is {0}, Gender is {1}", r.Name, r.Gender);
    }
}

output
Name is Erin, Gender is Female
Name is Susan, Gender is Female

from … in ：指定資料來源
where：條件式判斷，套用篩選
select ：取得查詢物件

另提取資料語法中有一個select new，可以依照所設定的屬性產生類別物件，這個語法包含兩個功能：物件初始值及匿名型別

class Person {
    public string Name { get; set; }
    public string Gender { get; set; }
}

public static void Main () {
    List<Person> person = new List<Person> {
        new Person { Name = "Shawn", Gender = "Male" },
        new Person { Name = "Erin", Gender = "Female" },
        new Person { Name = "David", Gender = "Male" },
        new Person { Name = "Susan", Gender = "Female" },
        new Person { Name = "Joe", Gender = "Male" },
        new Person { Name = "Eric", Gender = "Male" }
    };

    var result = from p in person
            	where p.Gender == "Female"
            	select new {
			name = p.Name,
			gender = p.Gender,
			age = 18
		};

    foreach (var r in result) {
        Console.WriteLine("Name {0}, Gender {1}, Age {2}", r.name, r.gender, r.age);
    }
}

output
Name David, Gender Male, Age 20
Name Eric, Gender Male, Age 20
Name Joe, Gender Male, Age 20
Name Shawn, Gender Male, Age 20

LINQ 依照使用對象可以區分為

參考資料：

Coding Diary

查詢作業需要三個部分

Comments