HOW TO：撰寫 LINQ to XML 座標軸方法
更新： November 2007
您可以撰寫自己的座標軸方法來擷取 XML 樹狀結構中的集合。其中一個最好的方式是，撰寫可傳回項目或屬性集合的擴充方法。您可以根據您應用程式的需求，撰寫您的擴充方法來傳回項目或屬性的特定子集。
範例
下列範例使用兩種擴充方法。第一個擴充方法 GetXPath 會在 XObject 上運算，並在接受評估時，傳回將傳回節點或屬性的 XPath 運算式。第二個擴充方法 Find 會在 XElement 上運算。該擴充方法會傳回包含某些指定之文字的 XAttribute 物件集合和 XElement 物件集合。
此範例使用下列 XML 文件：XML 範例檔：多個採購訂單 (LINQ to XML).
.gif "注意事項") 注意事項： |
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下列範例使用 C# 的 yield return 建構函式。在 Visual Basic 2008 中沒有同等的功能，因此，只有在 C# 中提供這個範例。 |
public static class MyExtensions
{
private static string GetQName(XElement xe)
{
string prefix = xe.GetPrefixOfNamespace(xe.Name.Namespace);
if (xe.Name.Namespace == XNamespace.None || prefix == null)
return xe.Name.LocalName.ToString();
else
return prefix + ":" + xe.Name.LocalName.ToString();
}
private static string GetQName(XAttribute xa)
{
string prefix =
xa.Parent.GetPrefixOfNamespace(xa.Name.Namespace);
if (xa.Name.Namespace == XNamespace.None || prefix == null)
return xa.Name.ToString();
else
return prefix + ":" + xa.Name.LocalName;
}
private static string NameWithPredicate(XElement el)
{
if (el.Parent != null && el.Parent.Elements(el.Name).Count() != 1)
return GetQName(el) + "[" +
(el.ElementsBeforeSelf(el.Name).Count() + 1) + "]";
else
return GetQName(el);
}
public static string StrCat<T>(this IEnumerable<T> source,
string separator)
{
return source.Aggregate(new StringBuilder(),
(sb, i) => sb
.Append(i.ToString())
.Append(separator),
s => s.ToString());
}
public static string GetXPath(this XObject xobj)
{
if (xobj.Parent == null)
{
XDocument doc = xobj as XDocument;
if (doc != null)
return ".";
XElement el = xobj as XElement;
if (el != null)
return "/" + NameWithPredicate(el);
// the XPath data model does not include white space text nodes
// that are children of a document, so this method returns null.
XText xt = xobj as XText;
if (xt != null)
return null;
XComment com = xobj as XComment;
if (com != null)
return
"/" +
(
com
.Document
.Nodes()
.OfType<XComment>()
.Count() != 1 ?
"comment()[" +
(com
.NodesBeforeSelf()
.OfType<XComment>()
.Count() + 1) +
"]" :
"comment()"
);
XProcessingInstruction pi = xobj as XProcessingInstruction;
if (pi != null)
return
"/" +
(
pi.Document.Nodes()
.OfType<XProcessingInstruction>()
.Count() != 1 ?
"processing-instruction()[" +
(pi
.NodesBeforeSelf()
.OfType<XProcessingInstruction>()
.Count() + 1) +
"]" :
"processing-instruction()"
);
return null;
}
else
{
XElement el = xobj as XElement;
if (el != null)
{
return
"/" +
el
.Ancestors()
.InDocumentOrder()
.Select(e => NameWithPredicate(e))
.StrCat("/") +
NameWithPredicate(el);
}
XAttribute at = xobj as XAttribute;
if (at != null)
return
"/" +
at
.Parent
.AncestorsAndSelf()
.InDocumentOrder()
.Select(e => NameWithPredicate(e))
.StrCat("/") +
"@" + GetQName(at);
XComment com = xobj as XComment;
if (com != null)
return
"/" +
com
.Parent
.AncestorsAndSelf()
.InDocumentOrder()
.Select(e => NameWithPredicate(e))
.StrCat("/") +
(
com
.Parent
.Nodes()
.OfType<XComment>()
.Count() != 1 ?
"comment()[" +
(com
.NodesBeforeSelf()
.OfType<XComment>()
.Count() + 1) + "]" :
"comment()"
);
XCData cd = xobj as XCData;
if (cd != null)
return
"/" +
cd
.Parent
.AncestorsAndSelf()
.InDocumentOrder()
.Select(e => NameWithPredicate(e))
.StrCat("/") +
(
cd
.Parent
.Nodes()
.OfType<XText>()
.Count() != 1 ?
"text()[" +
(cd
.NodesBeforeSelf()
.OfType<XText>()
.Count() + 1) + "]" :
"text()"
);
XText tx = xobj as XText;
if (tx != null)
return
"/" +
tx
.Parent
.AncestorsAndSelf()
.InDocumentOrder()
.Select(e => NameWithPredicate(e))
.StrCat("/") +
(
tx
.Parent
.Nodes()
.OfType<XText>()
.Count() != 1 ?
"text()[" +
(tx
.NodesBeforeSelf()
.OfType<XText>()
.Count() + 1) + "]" :
"text()"
);
XProcessingInstruction pi = xobj as XProcessingInstruction;
if (pi != null)
return
"/" +
pi
.Parent
.AncestorsAndSelf()
.InDocumentOrder()
.Select(e => NameWithPredicate(e))
.StrCat("/") +
(
pi
.Parent
.Nodes()
.OfType<XProcessingInstruction>()
.Count() != 1 ?
"processing-instruction()[" +
(pi
.NodesBeforeSelf()
.OfType<XProcessingInstruction>()
.Count() + 1) + "]" :
"processing-instruction()"
);
return null;
}
}
public static IEnumerable<XObject> Find(this XElement source, string value)
{
if (source.Attributes().Any())
{
foreach (XAttribute att in source.Attributes())
{
string contents = (string)att;
if (contents.Contains(value))
yield return att;
}
}
if (source.Elements().Any())
{
foreach (XElement child in source.Elements())
foreach (XObject s in child.Find(value))
yield return s;
}
else
{
string contents = (string)source;
if (contents.Contains(value))
yield return source;
}
}
}
class Program
{
static void Main(string[] args)
{
XElement purchaseOrders = XElement.Load("PurchaseOrders.xml");
IEnumerable<XObject> subset =
from xobj in purchaseOrders.Find("1999")
select xobj;
foreach (XObject obj in subset)
{
Console.WriteLine(obj.GetXPath());
if (obj.GetType() == typeof(XElement))
Console.WriteLine(((XElement)obj).Value);
else if (obj.GetType() == typeof(XAttribute))
Console.WriteLine(((XAttribute)obj).Value);
}
}
}
此程式碼會產生下列輸出：
/PurchaseOrders/PurchaseOrder[1]/@OrderDate
1999-10-20
/PurchaseOrders/PurchaseOrder[1]/Items/Item[2]/ShipDate
1999-05-21
/PurchaseOrders/PurchaseOrder[2]/@OrderDate
1999-10-22
/PurchaseOrders/PurchaseOrder[3]/@OrderDate
1999-10-22