如何迭代 LINQ-to-XML 查询返回的对象集合？

Question

我有这个 XML：

<BillingLog>
  <BillingItem>
    <date-and-time>2003-11-04</date-and-time>
    <application-name>Billing Service</application-name>
    <severity>Warning</severity>
    <process-id>123</process-id>
    <description>Timed out on a connection</description>
    <detail>Timed out after three retries.</detail>
  </BillingItem>
  <BillingItem>
    <date-and-time>2010-05-15</date-and-time>
    <application-name>Callback Service</application-name>
    <severity>Error</severity>
    <process-id>456</process-id>
    <description>Unable to process callback</description>
    <detail>Reconciliation timed out after two retries.</detail>
  </BillingItem>
</BillingLog>

我想使用 LINQ-to-XML 将其投影到单个 BillingLog 对象中包含的 BillingItem 对象的集合中。

public class BillingLog
{
    public IEnumerable<BillingItem> items { get; set; }
}

public class BillingItem
{
    public string Date { get; set; }
    public string ApplicationName { get; set; }
    public string Severity { get; set; }
    public int ProcessId { get; set; }
    public string Description { get; set; }
    public string Detail { get; set;}       
}

这是我用来投影 XML 的 LINQ 查询（包含在字符串变量 source 中）。

XDocument xdoc = XDocument.Parse(source);

var log = 
    from i in xdoc.Elements("BillingLog")
    select new BillingLog
    {
        items =
            from j in i.Descendants("BillingItem")
            select new BillingItem
            {
                Date = (string)j.Element("date-and-time"),
                ApplicationName = (string)j.Element("application-name"),
                Severity = (string)j.Element("severity"),
                ProcessId = (int)j.Element("process-id"),
                Description = (string)j.Element("description"),
                Detail = (string)j.Element("detail")
            }
    };

当我尝试使用 foreach 迭代 log 中的对象时。

foreach (BillingItem item in log)
{
Console.WriteLine ("{0} | {1} | {2} | {3} | {4} | {5}", 
                    item.Date, item.ApplicationName, 
                    item.Severity, item.ProcessId.ToString(), 
                    item.Description, item.Detail);
}   

我从 LINQPad 收到以下错误消息。

Cannot convert type 'UserQuery.BillingLog' to 'UserQuery.BillingItem'

提前致谢。

Answer 1

这是因为您的 log 变量包含 BillingLog 对象的集合，而不是 BillingItem。您必须执行以下操作：

foreach( BillingLog l in log )
{
    foreach( BillingItem item in l.items )
    {
        Console.WriteLine( ... );
    }
}

或者，如果您最初的意图是仅选择所有 BillingItem，完全忽略其父 BillingLog，您可以像这样重写查询：

var log = 
        from l in xdoc.Elements("BillingLog")
        from j in l.Descendants("BillingItem")
        select new BillingItem
        {
            Date = (string)j.Element("date-and-time"),
            ApplicationName = (string)j.Element("application-name"),
            Severity = (string)j.Element("severity"),
            ProcessId = (int)j.Element("process-id"),
            Description = (string)j.Element("description"),
            Detail = (string)j.Element("detail")
        }

这将为您提供从所有 BillingLog 下选择的所有 BillingItems 的简单集合，而 BillingLog 本身将被完全丢弃。