c# - LINQ 子选择

Question

我是一名学生，刚接触 LINQ，我们接到了一项处理 LINQ 查询的任务。

我的问题是过去几天我一直在努力寻找执行此步骤的正确方法:打印订单中包含“牛奶”的客户姓名。

Write a LINQ query to select all customers buying milk.
Print the Name of each customer in the query.

为了节省时间，这里是数据的结构，以便您理解它:

        Product milk    = new Product { Name = "Milk",    Price = 13.02m };
        Product butter  = new Product { Name = "Butter",  Price = 8.23m };
        Product bread   = new Product { Name = "Bread",   Price = 17.91m };
        Product cacao   = new Product { Name = "Cacao",   Price = 25.07m };
        Product juice   = new Product { Name = "Juice",   Price = 17.03m };

        Customer c1 = new Customer { Name = "x", City = "g", Orders = new Order[] {
               new Order { Quantity = 2, Product = milk     },
               new Order { Quantity = 1, Product = butter   },
               new Order { Quantity = 1, Product = bread    }
            }
        };

        Customer c2 = new Customer { Name = "y", City = "g", Orders = new Order[] {
                new Order { Quantity = 1, Product = cacao   },
                new Order { Quantity = 1, Product = bread   },
                new Order { Quantity = 2, Product = milk    },
                new Order { Quantity = 2, Product = butter  },
            }
        };

        Customer c3 = new Customer { Name = "z", City = "g",  Orders = new Order[] {
                new Order { Quantity = 3, Product = juice   }
            }
        };

        Customer[] customers = new Customer[] { c1, c2, c3 };

作为我在 LINQ 中使用的语法示例，这里是工作代码的引用:

        var QueryCustomerByCity = from cus in customers.AsEnumerable()
                                     where cus.City == "g"
                                     select cus;

        foreach (Customer c in QueryCustomerByCity)
            Console.WriteLine("Customer {0} lives in {1}", c.Name, c.City);

我真的很努力去理解发生了什么，所以如果你能帮助我，请解释一下你是如何得出这样的结论的:)

非常感谢您的宝贵时间!

Answer 1

您当前的查询:

var QueryCustomerByCity = from cus in customers.AsEnumerable() //for each customer in customers 
                          where cus.City == "g" // where the Customer is from the City "g"
                          select cus; // select the customer

读作“对于每个在 customers 数组中表示为 cus 的客户，其中 customers City 为“g”，然后保留该客户”。因此，您将拥有一系列客户，他们的城市是“g”。

至于你的任务，你追求的是:

var result = from cus in customers // for each customer in customers                  
             where cus.Orders.Any(o => o.Product.Name == "Milk") // where the product name is "Milk"
             select cus; // select the customer

这实际上遍历了 customers 数组中的客户并检查他们的订单，如果有任何产品名称为“Milk”，则保留该特定客户。