如何使用组联接过滤 LINQ to Objects 中的结果？ [已关闭]

Question

我有两个列表：一个代表商店，另一个代表与这些商店关联的订单。这是一个简化版本 数据：

var stores = new[] 
{
    new { Code = 1, Name = "Store 1" },
    new { Code = 2, Name = "Store 2" }    
};

var orders = new[] 
{
    new { Code = 1, StoreCode = 1, TotalValue = 4.12 },
    new { Code = 2, StoreCode = 1, TotalValue = 14.12 },
    new { Code = 3, StoreCode = 1, TotalValue = 24.12 }
};

我正在尝试实现以下输出：

StoreName = Store 1 | TotalValue = 38.24

为了实现此目的，我使用 LINQ to Objects 中的组联接来聚合每个商店的订单。但是，我还需要根据应用于订单的条件来过滤结果。

这是我正在使用的查询：

var result = (from s in stores
              join o in orders on s.Code equals o.StoreCode into grp
              where grp.Any(o => o.TotalValue > 10)
              select new
              {
                  StoreName = s.Name,
                  TotalValue = grp.Sum(x => x.TotalValue)
              }).ToList();

但是，我遇到了错误：当前上下文中不存在名称“o”。

这种场景下如何有效过滤orders表中的group join数据？

Answer 1

试试这个：

var result = (from s in stores
              join o in orders.Where(x => x.TotalValue > 10) on s.Code equals o.StoreCode into grp
              select new
              {
                  StoreName = s.Name,
                  TotalValue = grp.Sum(x => x.TotalValue)
              });

或者这个

var result = (from s in stores
              join o in orders on s.Code equals o.StoreCode into grp
              select new
              {
                  StoreName = s.Name,
                  TotalValue = grp.Where(x => x.TotalValue > 10)
                                  .Sum(x => x.TotalValue)
              });

请注意，通常不需要调用

ToList

，因为这需要立即评估查询。大多数时候，将其保留为

IEnumerable<T>

就足够了。此外，这还将返回一行“Store 2”，带有

TotalValue = 0

。如果您也想省略这些行，您可以使用类似的内容。

var result = (from s in stores
              join o in orders.Where(x => x.TotalValue > 10) on s.Code equals o.StoreCode into grp
              let totalValue = grp.Sum(x => x.TotalValue)
              where totalValue > 0
              select new
              {
                  StoreName = s.Name,
                  TotalValue = totalValue 
              });

或者这个

var result = (from s in stores
              join o in orders on s.Code equals o.StoreCode into grp
              let totalValue = grp.Where(x => x.TotalValue > 10)
                                  .Sum(x => x.TotalValue)
              where totalValue > 0
              select new
              {
                  StoreName = s.Name,
                  TotalValue = totalValue 
              });