子对象的父对象和集合

Question

假设我有一个 Customer class 和一个 Invoice class。 Customer 包含一个名为 Invoices 的 List<Invoice> 属性。这可以用单个 Linq 语句填充吗？还是我需要先 select 客户，然后对他们做 foreach 以稍后提取发票？

类似于：

(from customer in customers
 join invoice in invoices on customer.Id = invoice.CustomerId
 select <what>?

我不能 select 那里有客户，或者内部列表未填写。也许 group 代替？

编辑：这行得通。只是想知道是否有不同的方式

var customersWithInvoices =
   (from customer in customers
    let invoiceGroup = (from invoice in invoices where invoice.CustomerId == customer.Id select invoice).ToList()
    select new { customer, invoiceGroup }).ToList();

foreach (var obj in customersWithInvoices) {
   obj.customer.Invoices = obj.invoiceGroup;
   CustomerList.Add(obj.customer);
}

Answer 1

如果您正在从数据库加载并尝试加载集合，您可以使用 .Include() 方法显式加载集合。如果您在与数据库的连接关闭后尝试访问该集合，则会收到错误消息。

您应该使用以下内容：

using(var context = new ServiceContext())
{
  var customers = context.CustomerSet.Include(cust => cust.Invoices);
}

Answer 2

这是不可能的。解决方法是使用 let 子句，如 OP 的编辑区域所示：

var customersWithInvoices =
   (from customer in customers
    let invoiceGroup = (from invoice in invoices where invoice.CustomerId == customer.Id select invoice).ToList()
    select new { customer, invoiceGroup }).ToList();

foreach (var obj in customersWithInvoices) {
   obj.customer.Invoices = obj.invoiceGroup;
   CustomerList.Add(obj.customer);
}

子对象的父对象和集合

Parent and collection of child objects

c#

linq

linq-to-sql