投影
□ 遍历数组索引,Select获取
int[] indexes = {0, 2};
string[] strs = {"a", "b", "c", "d"};
var result = from i in indexes
select strs[i];
foreach (string str in result)
{
Console.Write(str + " ");
}
Console.ReadKey();
结果:a c
□ 使用集合索引投影匿名对象
static void Main(string[] args)
{
int[] numbers = {0, 3, 4, 5, 7};
var result = numbers.Select((n, index) => new {Num = n, IsValueEqualIndex = (n == index)});
foreach (var item in result)
{
Console.WriteLine("{0}:{1}",item.Num,item.IsValueEqualIndex);
}
Console.ReadKey();
}
□ 2个集合的笛卡尔积
int[] numberA = {0,2};
int[] numberB = {1,3};
var result = from a in numberA
from b in numberB
select new {a, b};
foreach (var item in result)
{
Console.WriteLine("{0},{1}",item.a,item.b);
}
Console.ReadKey();
□ 笛卡尔积应用,列出所有客户的所有订单
public class Customer
{
public int Id { get; set; }
public string Name { get; set; }
public List<Order> Orders { get; set; }
}
public class Order
{
public int Id { get; set; }
public decimal Total { get; set; }
}
Order order1 = new Order(){Id = 1,Total = 10.00m};
Order order2 = new Order(){Id = 2,Total = 20.00m};
Order order3 = new Order(){Id = 3,Total = 30.00m};
List<Customer> customers = new List<Customer>()
{
new Customer()
{
Id = 1,
Name = "张三",
Orders = new List<Order>(){order1,order2}
},
new Customer()
{
Id = 2,
Name = "李四",
Orders = new List<Order>() { order3 }
}
};
var result = from c in customers
from o in c.Orders
select new {c.Id, c.Name, o.Total};
foreach (var item in result)
{
Console.WriteLine("客户编号:{0},姓名:{1},订单金额:{2}", item.Id,item.Name,item.Total);
}
Console.ReadKey();
分组
□ group n by 条件 into g, g.Key就是分组标准和条件, g代表分组里所有元素的集合
int[] numbers = { 5, 4, 1, 3, 9, 8, 6};
var result = from n in numbers
group n by n%5 into g
select new {Remainder = g.Key, Numbers = g};
foreach (var item in result)
{
Console.WriteLine("元素除以5的余数为:{0}",item.Remainder);
foreach (var n in item.Numbers)
{
Console.WriteLine(n);
}
}
Console.ReadKey();
连接
□ 根据2个集合的相同部分连接
public class Product
{
public int Id { get; set; }
public string Name { get; set; }
public string Category { get; set; }
}
List<Product> products = new List<Product>()
{
new Product(){Id=1,Name = "体恤衫",Category = "服饰"},
new Product(){Id = 2,Name = "香水",Category = "化妆品"}
};
string[] categoreis = new string[]
{
"服饰",
"化妆品",
"电子"
};
var result = from c in categoreis
join p in products on c equals p.Category
select new {Category = c, p.Name};
foreach (var item in result)
{
Console.WriteLine(item.Name + "属于:" + item.Category);
}
Console.ReadKey();
□ 把其中一个集合的所有元素放到一个别名集合中
List<Product> products = new List<Product>()
{
new Product(){Id=1,Name = "体恤衫",Category = "服饰"},
new Product(){Id = 2,Name = "香水",Category = "化妆品"}
};
string[] categoreis = new string[]
{
"服饰",
"化妆品",
"电子"
};
var result = from c in categoreis
join p in products on c equals p.Category into ps
select new {Category = c, Products = ps};
foreach (var item in result)
{
Console.WriteLine("类别:{0}包括:",item.Category);
foreach (var p in item.Products)
{
Console.WriteLine(" " + p.Name);
}
}
Console.ReadKey();