用Linq求和嵌套值 [英] Sum nested values with Linq

问题描述

一个简单的问题:我的用户可以拥有很多订单，而这些订单可以拥有很多产品. Linq(lambda)查询看起来像什么，以获取用户的所有Product.Price值的总计?

Simple problem: I have Users that can have many Orders that can have many Products. What does the Linq (lambda) query look like to get a User's grand total of all Product.Price values?

我已经尝试过了:

int total = users.Sum(u => u.Orders.Sum(o => o.Products.Sum(p => p.Price)));

但这给了我

强制转换为值类型'Int32'，因为实例化值为null.结果类型的通用参数或查询必须使用可为空的类型.

The cast to value type 'Int32' failed because the materialized value is null. Either the result type's generic parameter or the query must use a nullable type.

当然，用户可能没有任何订单，并且订单可能没有任何产品.但是Product.Price不是可为空的值.

Of course, a user may not have any orders, and an order may not have any products. But Product.Price is not a nullable value.

所以我尝试了这个，以为它使空的收藏感到窒息:

So I tried this, thinking it was choking on empty collections:

int total = users.Sum(u => u.Orders.Sum(o => o.Products.Sum(p => p.Price) ?? 0) ?? 0) ?? 0;

但这会引发编译错误，表示??的左侧不可为空.

But it's throwing compile errors, saying the left side of the ?? is not nullable.

我在做什么错了?

谢谢.

更新:在我的示例中使用了马克的逻辑后，上述示例的工作版本:

UPDATE: A working version of my examples above after using Marc's logic from his answer:

int total = users.Sum(u => u.Orders.Sum(o => o.Products.Sum(p => (int?)p.Price))) ?? 0;

推荐答案

int total = (from user in users
             from order in user.Orders
             from product in order.Products
             select (int?)product.Price).Sum() ?? 0;

将是我的奉献；有一个令人烦恼的小故障，即SQL中超过0行的SUM是NULL而不是0-上面的方法可以解决此问题.

would be my offering; there is an annoying glitch that the SUM in SQL over 0 rows is NULL, not 0 - the above works around that.

或作为lambdas(来自评论):

or as lambdas (from comments):

int total = users.SelectMany(user => user.Orders)
                 .SelectMany(order => order.Products)
                 .Sum(product => (int?)product.Price) ?? 0;

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