LINQ 学习笔记

selectMany   

Demo1:

 DateTime cutoffDate = new DateTime(1997, 1, 1); 

 var orders = 
     from c in customers 
     where c.Region == "WA" 
     from o in c.Orders 
     where o.OrderDate >= cutoffDate 
     select new { c.CustomerID, o.OrderID };

等同于：

customers.Where(c => c.Region == "WA")
    .SelectMany(c => c.Orders)
    .Where(o => o.Orderdate >= cutoffDate)
    .Select(x => new { x.OrderID, x.Customer.CustomerID });

Demo2： Category->Products  one-many type relationshio

写法1：

var redProducts = context.Category.Single(c => c.Name = "red").Products;

建议写法：

var redProducts = context.Category
                         .Where(c => c.Name == "red")
                         .SelectMany(c => c.Products);