[英]LINQ to XML join issue
我在嘗試對兩個單獨的XML文件執行聯接時遇到了真正的麻煩。
我有兩個類描述從XML文件提取的每個對象,它們是
public class XMLCustomers
{
public String CustomerID { get; set; }
public String ContactName { get; set; }
}
和
public class XMLOrders
{
public String OrderID { get; set; }
public String CustomerID { get; set; }
public String OrderDate { get; set; }
public String ShippedDate { get; set; }
public String Freight { get; set; }
public String ShipName { get; set; }
public String ShipCity { get; set; }
public String ShipCountry { get; set; }
}
我的上一課存儲了我在兩組數據上執行的聯接。
public class PostXMLJoinOrder
{
public String OrderID { get; set;}
public String ContactName { get; set;}
public String OrderDate { get; set;}
public String ShippedDate { get; set;}
public String Freight { get; set;}
public String ShipName { get; set;}
public String ShipCity { get; set;}
public String ShipCountry { get; set;}
}
最后,這是我的兩個方法,它們從XML文件中加載信息,第三個方法執行聯接並將信息存儲在IEnumerable中。
public IEnumerable<XMLCustomers> LoadXMLCustomers() {
var custs = from x in XElement.Load(System.Web.HttpContext.Current.Server.MapPath(@"~\App_Data\XCustomers.xml")).Elements()
select new XMLCustomers
{
ContactName = x.Attribute("ContactName").Value,
CustomerID = x.Attribute("CustomerID").Value
};
int size = custs.Count();
return custs;
}
public IEnumerable<XMLOrders> LoadXMLOrders() {
var ords = from x in XElement.Load(System.Web.HttpContext.Current.Server.MapPath(@"~\App_Data\XOrders.xml")).Elements()
select new XMLOrders
{
OrderID = x.Attribute("ContactName").Value,
CustomerID = x.Attribute("CustomerID").Value,
OrderDate = x.Attribute("OrderDate").Value,
ShippedDate = x.Attribute("ShippedDate").Value,
Freight = x.Attribute("Freight").Value,
ShipName = x.Attribute("ShipName").Value,
ShipCity = x.Attribute("ShipCity").Value,
ShipCountry = x.Attribute("ShipCountry").Value
};
int size = ords.Count();
return ords;
}
public IEnumerable<PostXMLJoinOrder> LoadPostXMLJoinOrders() {
var joinQuery = from customer in LoadXMLCustomers()
from orders in LoadXMLOrders()
where customer.CustomerID == orders.CustomerID
select new PostXMLJoinOrder
{
OrderID = orders.OrderID,
ContactName = customer.ContactName,
OrderDate = orders.OrderDate,
ShippedDate = orders.ShippedDate,
Freight = orders.Freight,
ShipName = orders.ShipName,
ShipCity = orders.ShipCity,
ShipCountry = orders.ShipCountry
};
return joinQuery;
}
我已經測試了從LINQ返回的項目數量,該數量仍為0; 我已經仔細檢查了文件中正在加載的所有內容,但似乎無法解決問題。
編輯:其加載問題。 XML文件肯定存儲在正確的App_Data文件夾下。 但是,當個別LoadXMLCustomers()運行時,我在lINQ語句選擇並創建新的Loaded Customer對象的部分得到了NullReferenceException。
我已經確保XML文檔的構建內容正確並且將copyToOutputDirectory設置為較新的版本
這是一個例外,而且var值是null,因此由於某些原因,它肯定不會加載:
解決:我在這里學到的教訓是,您需要密切注意XML和數據。 如果您的某些XML數據具有空值,則需要通過確保select語句可以處理它來進行說明。
上面我有代碼
select new XMLOrders
{
OrderID = x.Attribute("ContactName").Value,
CustomerID = x.Attribute("CustomerID").Value,
OrderDate = x.Attribute("OrderDate").Value,
ShippedDate = x.Attribute("ShippedDate").Value,
Freight = x.Attribute("Freight").Value,
ShipName = x.Attribute("ShipName").Value,
ShipCity = x.Attribute("ShipCity").Value,
ShipCountry = x.Attribute("ShipCountry").Value
};
實際上應該包括對字符串值的強制轉換,因此,如果數據為空,則不會拋出null異常。 其次,我應該獲取元素值而不是屬性值。
select new XMLOrders
{
OrderID = (string)x.Element("OrderID").Value,
CustomerID = (string)x.Element("CustomerID").Value,
OrderDate = (string)x.Element("OrderDate").Value,
ShippedDate = (string)x.Element("ShippedDate").Value,
Freight = (string)x.Element("Freight").Value,
ShipName = (string)x.Element("ShipName").Value,
ShipCity = (string)x.Element("ShipCity").Value,
ShipCountry = (string)x.Element("ShipCountry").Value
};
您是否嘗試過不使用join
而是:
from customer in LoadXMLCustomers()
from order in LoadXMLOrders()
where customer.CustomerID = order.CustomerID
select new PostXMLJoinOrder
...
請注意,在這樣的查詢中,可以為每個客戶再次調用LoadXMLOrders()
。 因此,請先預先保存。
同樣不要忘記實現調用ToArray()
/ ToList()
。
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