Como gravar um método de eixo LINQ to XML (LINQ to XML)
Um método de eixo XML recupera uma coleção de elementos XML de um documento XML ou ancestral. Você pode escrever seus próprios métodos do eixo para recuperar coleções de uma árvore XML. Uma das melhores maneiras de fazer isso é gravar um método de extensão que retorna uma coleção de elementos ou atributos. Você pode escrever seu método de extensão para subconjuntos específicos de retorno de elementos ou atributos, com base nos requisitos do seu aplicativo. Este artigo fornece um exemplo para C# e Visual Basic.
Exemplo
O exemplo a seguir usa dois métodos de extensão. O primeiro método de extensão, GetXPath, opera em XObject, e retorna uma expressão XPath que quando avaliada retorna o nó ou do atributo. O segundo método de extensão, Find, opera em XElement. Retorna uma coleção de objetos XAttribute e objetos de XElement que contêm texto especificado.
O exemplo usa o documento XML Arquivo XML de exemplo: várias ordens de compra.
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 doesn't 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);
}
}
}
Imports System.Runtime.CompilerServices
Imports System.Text
Module Module1
Sub Main()
Dim purchaseOrders = XElement.Load("..\..\PurchaseOrders.xml")
Dim subset = From xobj In purchaseOrders.Find("1999")
For Each obj In subset
Console.WriteLine(obj.GetXPath())
If obj.GetType() = GetType(XElement) Then
Console.WriteLine(CType(obj, XElement).Value)
ElseIf obj.GetType() = GetType(XAttribute) Then
Console.WriteLine(CType(obj, XAttribute).Value)
End If
Next
End Sub
End Module
Public Module MyExtensions
Private Function GetQName(ByVal xe As XElement) As String
Dim prefix = xe.GetPrefixOfNamespace(xe.Name.Namespace)
If xe.Name.Namespace = XNamespace.None OrElse prefix Is Nothing Then
Return xe.Name.LocalName
Else
Return prefix & ":" & xe.Name.LocalName
End If
End Function
Private Function GetQName(ByVal xa As XAttribute) As String
Dim prefix = xa.Parent.GetPrefixOfNamespace(xa.Name.Namespace)
If xa.Name.Namespace = XNamespace.None OrElse prefix Is Nothing Then
Return xa.Name.LocalName
Else
Return prefix & ":" & xa.Name.LocalName
End If
End Function
Private Function NameWithPredicate(ByVal el As XElement) As String
If el.Parent IsNot Nothing AndAlso
el.Parent.Elements(el.Name).Count() <> 1 Then
Return GetQName(el) & "[" &
(el.ElementsBeforeSelf(el.Name).Count() + 1) & "]"
Else
Return GetQName(el)
End If
End Function
<Extension()>
Public Function StrCat(Of T)(ByVal source As IEnumerable(Of T),
ByVal separator As String) As String
Return source.Aggregate(New StringBuilder,
Function(sb, i) sb.
Append(i.ToString()).
Append(separator),
Function(s) s.ToString())
End Function
<Extension()>
Public Function GetXPath(ByVal xobj As XObject) As String
If xobj.Parent Is Nothing Then
Dim doc = TryCast(xobj, XDocument)
If doc IsNot Nothing Then Return "."
Dim el = TryCast(xobj, XElement)
If el IsNot Nothing Then Return "/" + NameWithPredicate(el)
' The XPath data model doesn't include white space text nodes
' that are children of a document, so this method returns null.
Dim xt = TryCast(xobj, XText)
If xt IsNot Nothing Then Return Nothing
Dim com = TryCast(xobj, XComment)
If com IsNot Nothing Then
Return "/" &
If(com.Document.Nodes().OfType(Of XComment)().Count() <> 1,
"comment()[" & (com.NodesBeforeSelf().OfType(Of XComment)().Count() + 1) & "]",
"comment()")
End If
Dim pi = TryCast(xobj, XProcessingInstruction)
If pi IsNot Nothing Then
Return "/" &
If(pi.Document.Nodes().OfType(Of XProcessingInstruction)().Count() <> 1,
"processing-instruction()[" &
(pi.NodesBeforeSelf().OfType(Of XProcessingInstruction)().Count() + 1) & "]",
"processing-instruction()")
End If
Return Nothing
Else
Dim el = TryCast(xobj, XElement)
If el IsNot Nothing Then
Return "/" &
el.Ancestors().
InDocumentOrder().
Select(Function(e) NameWithPredicate(e)).StrCat("/") & NameWithPredicate(el)
End If
Dim at = TryCast(xobj, XAttribute)
If at IsNot Nothing Then
Return "/" &
at.Parent.
AncestorsAndSelf().
InDocumentOrder().
Select(Function(e) NameWithPredicate(e)).StrCat("/") & "@" & GetQName(at)
End If
Dim com = TryCast(xobj, XComment)
If com IsNot Nothing Then
Return "/" &
com.Parent.
AncestorsAndSelf().
InDocumentOrder().
Select(Function(e) NameWithPredicate(e)).StrCat("/") &
If(com.Parent.Nodes().OfType(Of XComment)().Count() <> 1,
"comment()[" & (com.NodesBeforeSelf().OfType(Of XComment)().Count() + 1) & "]",
"comment()")
End If
Dim cd = TryCast(xobj, XCData)
If cd IsNot Nothing Then
Return "/" &
cd.Parent.
AncestorsAndSelf().
InDocumentOrder().
Select(Function(e) NameWithPredicate(e)).StrCat("/") &
If(cd.Parent.Nodes().OfType(Of XText)().Count() <> 1,
"text()[" & (cd.NodesBeforeSelf().OfType(Of XText)().Count() + 1) & "]",
"text()")
End If
Dim tx = TryCast(xobj, XText)
If tx IsNot Nothing Then
Return "/" &
tx.Parent.
AncestorsAndSelf().
InDocumentOrder().
Select(Function(e) NameWithPredicate(e)).StrCat("/") &
If(tx.Parent.Nodes().OfType(Of XText)().Count() <> 1,
"text()[" & (tx.NodesBeforeSelf().OfType(Of XText)().Count() + 1) & "]",
"text()")
End If
Dim pi As XProcessingInstruction = TryCast(xobj, XProcessingInstruction)
If pi IsNot Nothing Then
Return "/" &
pi.Parent.
AncestorsAndSelf().
InDocumentOrder().
Select(Function(e) NameWithPredicate(e)).StrCat("/") &
If(pi.Parent.Nodes().OfType(Of XProcessingInstruction)().Count() <> 1,
"processing-instruction()[" &
(pi.NodesBeforeSelf().OfType(Of XProcessingInstruction)().Count() + 1) & "]",
"processing-instruction()")
End If
Return Nothing
End If
End Function
<Extension()>
Public Function Find(ByVal source As XElement, ByVal value As String) As IEnumerable(Of XObject)
Dim results = From att In source.Attributes()
Where att.Value.Contains(value)
Let a As XObject = att
Select a
If source.Elements().Any Then
For Each result In From child In source.Elements() Select Find(child, value)
results = If(results Is Nothing, result, results.Union(result))
Next
Else
If source.Value.Contains(value) Then
results = If(results Is Nothing,
New List(Of XObject) From {source},
results.Union(New List(Of XObject) From {source}))
End If
End If
Return results
End Function
End Module
Esse exemplo gera a saída a seguir:
/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
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