LINQ举例

//可迭代的数据
string[] names = { };

//从数组中筛选以【S】开头的字符串
var t1 = from n in names
         where n.StartsWith("S")
         select n;

var t2 = names.Where(n => n.StartsWith("S"));

//从数组中筛选以【S】开头的字符串，并降序
var t11 = from n in names         
          where n.StartsWith("S")
          orderby n descending
          select n;
//使用lambda达到同样的功能
var t22 = names.OrderByDescending(n => n).Where(n => n.StartsWith("S"));
var t222 = names.OrderBy(n => n.Substring(n.Length - 1)).Where(n => n.StartsWith("S"));

//类似数据库中的集合查询
n1.Max();
n1.Min();
n1.Average();
n1.Sum(n => (long)n);//强转

//待枚举的类
class Customer
{
     public string ID { get; set; }
     public string City { get; set; }
     public string Region { get; set; }
}

//这里的例子展现了迭代某些类是，利用类的一些属性过滤
List<Customer> customers = new List<Customer>();
var c1 = from c in customers
         where c.Region == "Asia"
         select c;

//强大的lambda表达式
var c11 = customers.Where(c => c.Region=="Asia");
//无法想数据库中语法select a,b,c
//需要使用new语法
var c2 = from c in customers
         where c.Region == "Asia"
         select new { c.Region, c.ID, c.City};
//这里注意到先是Seclect再Where
var c22 = customers
          .Select(c => new { c.Region, c.ID, c.City })
          .Where(c => c.Region == "Asia");

// Any All
// 判断一个集合中存在什么或者每一个都满足什么
var any = numbers.Any(n => n < 1000);
var all = numbers.All(n => n > 100);

//多级排序
var o1 = from c in customers
         orderby c.Region, c.ID descending, c.City
         select new { c.Region, c.ID, c.City 

var o11 = customers
          .OrderBy(c => c.Region)
          .ThenByDescending(c => c.ID)
          .ThenBy(c => c.City)
          .Select(c => new { c.Region, c.ID, c.City });

//组合查询

//取前几条数据或者后几条数据
//Take & Skip
foreach (var item in o11.Take(5)) { }

foreach (var item in o11.Skip(5)) { }

//返回第一条匹配结果 后者：如果没有结果返回null，前者会报错
//First & FirstOrDefault

//Intersect Except Union

//Join
//同样注意到这里使用的new，不能使用并排
var f = from c in customers
        join n in numbers on c.ID equals n.ToString()
        select new { c.ID, c.Region, n};