Linq+lambda表达式+delegate委托详解及代码示例
一、Linq
简介：提供一种统一且对称的方式，让程序员在广义的数据上得到和操作“数据”。简单点说，就是在C#后台像sql查询一样操作集合和数组，从而筛选出我们想要的结果。
c#定义的各种简化查询
代码示例：
int[] scores1 = new int[] { 97, 92, 81, 60, 541, 1, 88, 81, 79 };
int[] scores2 = new int[] { 97, 92, 81, 60, 541, 1, 88, 81, 79 };
var scoreQuery = from score1 in scores1
from score2 in scores2
where score1 == score2 && score1 > 80
orderby score1 descending
select score1 + score2;
LinqToXml
代码示例：
using System.Xml.Linq;
static void BuildXmlDocWithLINQToXml()
{
XDocument doc =new XDocument(
new XDeclaration("1.0","utf-8","yes"),
new XComment("Current Inventory of cars!"),
new XProcessingInstruction("xml-stylesheet","href='MyStyle.css' title='compact' type='text/css'"),
new XElement("Inventory",
new XElement("Car",new XAttribute("ID",1000),
new XElement("PetName","Jimbo"),
new XElement("Color","Red"),
new XElement("Make","Ford")
),
new XElement("Car",new XAttribute("ID","I2"),
new XElement("PetName","Pink"),
new XElement("Color","Yugo"),
new XElement("Make","Melvin")
)
)
);
doc.Save("InventoryWithLinq.xml");
doc.Descendants("PetName").Remove();
Console.WriteLine(doc);
}
static void MakeXElementFromArray()
{
var people = new[] {
new { FirstName="ss",Age=45} ,
new { FirstName="撒大V",Age=43} ,
new { FirstName="阿双方各",Age=65} ,
new { FirstName="帷幄",Age=32}
};
var arrays = from c in people
select
new XElement("Person", new XAttribute("Age", c.Age),
new XElement("FirstName", c.FirstName));
XElement doc=new XElement("People",arrays);
Console.WriteLine(doc);
string myElement = @"
Yellow
Yugo
";
XElement newdoc = XElement.Parse(myElement);
Console.WriteLine(doc);
XElement doc2=XElement.Load("InventoryWithLinq.xml");
//新增元素
doc.Descendants("Person").First().Add(newdoc);
//linq查询
var makeInfo = from Car in doc2.Descendants("Car")
where (string)Car.Element("Color") == "Red"
select Car.Element("Color").Value;
}
linq order by charindex 排序 按给定字符串顺序排序
//list=list.OrderBy(ee => SqlFunctions.CharIndex(“书记,主任,支部委员,村委委员,系统工作人员”, ee.ZhiWu)).ToList(); //linq to sql中报错:只能在linq to entities中调用此方法.
int i = 0;
list = list.OrderBy(ee => { i=Array.IndexOf(new string[] { “书记”, “主任”, “支部委员”, “村委委员”, “系统工作人员” }, ee.ZhiWu);if ( i!= -1) { return i; } else { return int.MaxValue; } }).ToList(); //字符串在字符数组中出现的位置,不在数组中的排在最后
扩展: 如果字符串是 村委委员1 那么就不在字符串数组中了会排到后面去(如上图),下面函数解决模糊匹配问题
int PatIndex(string[] array, string value)
{
int index = -1;
for (int i = 0; i < array.Length; i++)
{
//if (value.Contains(array[i]))
if (value.StartsWith(array[i]))//以搜索字符串开头的
{
index = i;
break;
}
}
return index;
}
list = list.OrderBy(ee => { i=PatIndex(new string[] { "书记", "主任", "支部委员", "村委委员", "系统工作人员" }, ee.ZhiWu);if ( i!= -1) { return i; } else { return int.MaxValue; } }).ToList();
补充 sql:case when then
select * from table order by case when cunorju like ‘%街道%’ then 0 when cunorju like ‘%村委%’ then 1 when cunorju like ‘%居委%’ then 2 else 3 end
sql order by charindex(…):
Select * from table order by case when CharIndex(cunorju ,N’街道村委居委’)>0 then CharIndex(cunorju ,N’街道村委居委’) else 2000000000 end
/符合的放最前面,不符合的排在2000000000位置/
二、lambda表达式
简化版的匿名类
代码示例：
//Func == public delegate string Func(string func);
public static void LambdaFun(string str,Func func)
{
Console.WriteLine(func(str));
}
List Citys = new List()
{
"BeiJing",
"ShangHai",
"Tianjin",
"GuangDong"
};
//结合linq使用表达式
var result = Citys.First(c => c.Length > 7);
Console.WriteLine(result);
//结合委托使用表达式
LambdaFun("BeiJing 2013", s =>
{
if (s.Contains("2013"))
{
s = s.Replace("2013", "2018");
}
return s;
});
--------------利用lambda表达式计算算术树
ParameterExpression a = Expression.Parameter(typeof(int), "i");
ParameterExpression b = Expression.Parameter(typeof(int), "j");
BinaryExpression r1 = Expression.Multiply(a, b);
ParameterExpression c = Expression.Parameter(typeof(int), "w");
ParameterExpression d = Expression.Parameter(typeof(int), "x");
BinaryExpression r2 = Expression.Multiply(c, d);
BinaryExpression result = Expression.Add(r1, r2);
Expression> lambda = Expression.Lambda>(result, a, b, c, d);
Console.WriteLine(lambda + "");
Func f = lambda.Compile();
Console.WriteLine(f(1, 2, 1, 1) + "");
Console.ReadKey();
三、delegate委托：
将方法作为参数传递（类似于C中的指针）
代码示例：
public delegate void GreetingDelegate(string name);
class DelegateTest
{
private static void EnglishGreeting(string name)
{
Console.WriteLine("Morning, " + name);
}
private static void ChineseGreeting(string name)
{
Console.WriteLine("早上好, " + name);
}
private static void EnglishGreetingNight(string name)
{
Console.WriteLine("Good Night, " + name);
}
//注意此方法，它接受一个GreetingDelegate类型的方法作为参数
private static void GreetPeople(string name, GreetingDelegate MakeGreeting)
{
MakeGreeting(name);
}
static void Main(string[] args)
{
GreetPeople("Jimmy Zhang", EnglishGreeting);
GreetPeople("张子阳", ChineseGreeting);
GreetingDelegate del = new GreetingDelegate(EnglishGreeting);
del += ChineseGreeting;
GreetPeople("张子阳", del);
del -= EnglishGreeting;
del += EnglishGreetingNight;
GreetPeople("Jimmy Zhang", del);
GreetingDelegate del;
del = ChineseGreeting;
del += EnglishGreetingNight;
del("hha");
Console.ReadKey();
}
}