我在尝试对两个单独的 XML 文件执行的连接时遇到了真正的麻烦。
我有两个类来描述我从 XML 文件中提取的每个对象，它们是
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; }
}
我的最后一堂课存储了我对两组数据执行的连接。
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，这是我从 XML 文件加载信息的两种方法，第三种方法执行连接并将信息存储在 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;
}
我已经测试了从我的 LINQ 返回的项目数量，它仍然是 0；我已经仔细检查了从文件中加载的所有内容，但我似乎无法理解它在哪里出错了。
编辑：这是一个加载问题。XML 文件肯定存储在正确的 App_Data 文件夹下。但是当单独的 LoadXMLCustomers() 运行时，我在 lINQ 语句选择并创建新的 Loaded Customer 对象的部分得到 NullReferenceException。
我已经确保 XML 文档的构建是内容，并且 copyToOutputDirectory 设置为如果较新
这是异常，并且 var 值为 null，因此由于某种原因它肯定不会加载：
已解决：我在这里学到的教训是，您需要密切关注您的 XML 和数据。如果您的某些 XML 数据有空值，那么您需要确保 select 语句可以处理它。
上面我有代码
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
};
这确实应该包括对字符串值的强制转换，因此如果有空数据，则 "" 不会引发空异常。其次，我应该得到元素值而不是属性值。
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
};