父對象和子對象的集合

Question

假設我有一個Customer類和一個Invoice類。 Customer包含一個名為Invoices的List<Invoice>屬性。 可以用單個Linq語句填充嗎？ 還是我需要先選擇客戶，然后對他們進行一次foreach以便以后提取發票？

就像是：

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

我不能在那里選擇客戶，或者內部列表沒有填寫。 也許一group呢？

編輯：這有效。 只是想知道是否有其他方式

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

如果要從數據庫加載並嘗試加載集合，則可以使用.Include()方法顯式加載集合。 如果您在關閉與數據庫的連接后嘗試訪問集合，則將收到錯誤消息。

您應該使用以下內容：

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

Answer 2

這是不可能的。 解決方法是使用let子句，如OP的Edit區域所示：

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);
}