C#入门经典 Part 6

CH 23

LINQ查询

var results = 
    from n in names
    where n.StartsWith("S")
    orderby n descending // orderby n.Substring(n.Length - 1)
    select n // select new(n, ...)
    ;

方法语法

var results = names.OrderBy(n => n).Where(n => n.StartsWith("S"));
var results = names.OrderByDescending(n => n.Substring(n.Length - 1)).Where(n => n.StartsWith("S")).select(n => new(n, ...)).Distinct();

聚合运算符

results.Count();
results.Max();
results.Min();
results.Average();
results.Sum(n => (long) n);

Any, All

bool res = customers.Any(c => c.Name == "Tom");
bool res = customers.All(c => c.Name == "Tom");

多级排序

orderby c.Name, c.Age

var res = customers.OrderBy(c => c.Name).ThenBy(c => c.Age)...;

组合查询

var res =
    from c in customers
    group c by c.Region into cg
    select new {TotalSales = cg.Sum(c => c.Sales), Region = cg.Key}
    ;

Take(): 取前n个结果

Skip(): 跳过前n个结果

First(), FirstOrDefault(): 返回第一个匹配给定条件的元素

集合运算符Intersect(), Except(), Union()

Join查询

var res =
    from c in customers
    join o in orders on c.ID equals o.ID
    ;