lambda方法
使用运算符=>，左边是输入参数，右边是表达式或语句块。用在基于方法的 LINQ 查询中，作为诸如 Where 和 Where 等标准查询运算符方法的参数。
下列规则适用于 Lambda 表达式中的变量范围：
捕获的变量将不会被作为垃圾回收，直至引用变量的委托超出范围为止。
在外部方法中看不到 Lambda 表达式内引入的变量。
Lambda 表达式无法从封闭方法中直接捕获 ref 或 out 参数。
Lambda 表达式中的返回语句不会导致封闭方法返回。
Lambda 表达式不能包含其目标位于所包含匿名函数主体外部或内部的 goto 语句、break 语句或 continue 语句。
Lambda表达式的本质是“匿名方法”，即当编译我们的程序代码时，“编译器”会自动将“Lambda表达式”转换为“匿名方法”。
例：用lambda表达式简化委托：
//委托 逛超市
delegate int GuangChaoshi(int a);
static void Main(string[] args)
{
GuangChaoshi gwl = JieZhang;
Console.WriteLine(gwl(10) + ""); //打印20，委托的应用
Console.ReadKey();
}
//结账
public static int JieZhang(int a)
{
return a + 10;
}
利用lambda表达式处理方法：
//委托 逛超市
delegate int GuangChaoshi(int a);
static void Main(string[] args)
{
// GuangChaoshi gwl = JieZhang;
GuangChaoshi gwl = p => p + 10;
Console.WriteLine(gwl(10) + ""); //打印20，表达式的应用
Console.ReadKey();
}
LINQ语言集成查询
语言集成查询 (LINQ) 是一系列直接将查询功能集成到 C# 语言的技术统称。
例：
class LINQQueryExpressions
{
static void Main()
{
// Specify the data source.
int[] scores = new int[] { 97, 92, 81, 60 };
// Define the query expression.
IEnumerable<int> scoreQuery =
from score in scores
where score > 80
select score;
// Execute the query.
foreach (int i in scoreQuery)
{
Console.Write(i + " ");
}
}
}
// Output: 97 92 81
排序
升序
string[] words = { "the", "quick", "brown", "fox", "jumps" };
IEnumerable<string> query = from word in words
orderby word.Length
select word;
//以单词的长度为标准从小到大排序
foreach (string str in query)
Console.WriteLine(str);
/* This code produces the following output:
the
fox
quick
brown
jumps
*/
降序
string[] words = { "the", "quick", "brown", "fox", "jumps" };
IEnumerable<string> query = from word in words
orderby word.Substring(0, 1) descending
select word;
//按字符串的第一个字母对字符串进行降序排序
foreach (string str in query)
Console.WriteLine(str);
/* This code produces the following output:
the
quick
jumps
fox
brown
*/
次要排序
string[] words = { "the", "quick", "brown", "fox", "jumps" };
IEnumerable<string> query = from word in words
orderby word.Length, word.Substring(0, 1)
select word;
//首先按字符串长度，其次按字符串的第一个字母，对字符串进行升序排序
foreach (string str in query)
Console.WriteLine(str);
/* This code produces the following output:
fox
the
brown
jumps
quick
*/
集运算
去重Distinct
string[] planets = { "Mercury", "Venus", "Venus", "Earth", "Mars", "Earth" };
IEnumerable<string> query = from planet in planets.Distinct()
select planet;
foreach (var str in query)
{
Console.WriteLine(str);
}
/* This code produces the following output:
*
* Mercury
* Venus
* Earth
* Mars
*/
排除Except
string[] planets1 = { "Mercury", "Venus", "Earth", "Jupiter" };
string[] planets2 = { "Mercury", "Earth", "Mars", "Jupiter" };
IEnumerable<string> query = from planet in planets1.Except(planets2)
select planet;
foreach (var str in query)
{
Console.WriteLine(str);
}
/* This code produces the following output:
*
* Venus
*/
相交Intersect
string[] planets1 = { "Mercury", "Venus", "Earth", "Jupiter" };
string[] planets2 = { "Mercury", "Earth", "Mars", "Jupiter" };
IEnumerable<string> query = from planet in planets1.Intersect(planets2)
select planet;
foreach (var str in query)
{
Console.WriteLine(str);
}
/* This code produces the following output:
*
* Mercury
* Earth
* Jupiter
*/
联合Union
string[] planets1 = { "Mercury", "Venus", "Earth", "Jupiter" };
string[] planets2 = { "Mercury", "Earth", "Mars", "Jupiter" };
IEnumerable<string> query = from planet in planets1.Union(planets2)
select planet;
foreach (var str in query)
{
Console.WriteLine(str);
}
/* This code produces the following output:
*
* Mercury
* Venus
* Earth
* Jupiter
* Mars
*/
筛选数据where
string[] words = { "the", "quick", "brown", "fox", "jumps" };
IEnumerable<string> query = from word in words
where word.Length == 3
select word;
foreach (string str in query)
Console.WriteLine(str);
/* This code produces the following output:
the
fox
*/
限定符运算
全部
以下示例使用 All 检查所有字符串是否为特定长度
class Market
{
public string Name { get; set; }
public string[] Items { get; set; }
}
public static void Example()
{
List<Market> markets = new List<Market>
{
new Market { Name = "Emily's", Items = new string[] { "kiwi", "cheery", "banana" } },
new Market { Name = "Kim's", Items = new string[] { "melon", "mango", "olive" } },
new Market { Name = "Adam's", Items = new string[] { "kiwi", "apple", "orange" } },
};
// Determine which market have all fruit names length equal to 5
IEnumerable<string> names = from market in markets
where market.Items.All(item => item.Length == 5)
select market.Name;
foreach (string name in names)
{
Console.WriteLine($"{name} market");
}
// This code produces the following output:
//
// Kim's market
}
任意
以下示例使用 Any 检查是否有字符串以“o”开头。
class Market
{
public string Name { get; set; }
public string[] Items { get; set; }
}
public static void Example()
{
List<Market> markets = new List<Market>
{
new Market { Name = "Emily's", Items = new string[] { "kiwi", "cheery", "banana" } },
new Market { Name = "Kim's", Items = new string[] { "melon", "mango", "olive" } },
new Market { Name = "Adam's", Items = new string[] { "kiwi", "apple", "orange" } },
};
// Determine which market have any fruit names start with 'o'
IEnumerable<string> names = from market in markets
where market.Items.Any(item => item.StartsWith("o"))
select market.Name;
foreach (string name in names)
{
Console.WriteLine($"{name} market");
}
// This code produces the following output:
//
// Kim's market
// Adam's market
}
包含
以下示例使用 Contains 检查是否包含特定元素
class Market
{
public string Name { get; set; }
public string[] Items { get; set; }
}
public static void Example()
{
List<Market> markets = new List<Market>
{
new Market { Name = "Emily's", Items = new string[] { "kiwi", "cheery", "banana" } },
new Market { Name = "Kim's", Items = new string[] { "melon", "mango", "olive" } },
new Market { Name = "Adam's", Items = new string[] { "kiwi", "apple", "orange" } },
};
// Determine which market contains fruit names equal 'kiwi'
IEnumerable<string> names = from market in markets
where market.Items.Contains("kiwi")
select market.Name;
foreach (string name in names)
{
Console.WriteLine($"{name} market");
}
// This code produces the following output:
//
// Emily's market
// Adam's market
}