Muito bem. Caso possa interessar a alguém, vou deixar aqui como resposta a forma como foi resolvido o problema descrito na pergunta.
Começa pelo fato de que como a ideia era de apenas assinar a mensagem SOAP não foi preciso implementar todos os métodos da classe SoapExtension.
Ficou assim:
Imports System
Imports System.Collections.Generic
Imports System.Text
Imports System.Web.Services.Protocols
Imports System.IO
Imports System.Xml
Imports System.Runtime.InteropServices
Public Class TraceExtension
Inherits SoapExtension
Private oldStream As Stream
Private newStream As Stream
Private Shared RequestXml As XmlDocument
Private Shared ResponseXml As XmlDocument
<ComVisible(True)> _
Public Shared ReadOnly Property XmlRequest() As XmlDocument
Get
Return RequestXml
End Get
End Property
Public Shared ReadOnly Property XmlResponse() As XmlDocument
Get
Return ResponseXml
End Get
End Property
Public Overrides Function ChainStream(ByVal stream As Stream) As Stream
oldStream = stream
newStream = New MemoryStream()
Return newStream
End Function
Public Function getXml() As String
Return XmlRequest.InnerXml
End Function
Public Overrides Sub ProcessMessage(ByVal message As SoapMessage)
Select Case message.Stage
Case SoapMessageStage.BeforeSerialize
Case SoapMessageStage.AfterSerialize
''*
''* Assinatura do XML
''*
''Obtem o xml do objeto já com o envelope SOAP
RequestXml = GetSoapEnvelope(newStream)
'Realiza Assinatura (-->AQUI CHAMA A CLASSE QUE IMPLEMENTA A ASSINATURA COM CERTIFICADO X509Certificate2)
RequestXml = XmlSigner.SignXml(RequestXml, "MINHA ID ASSINADA", "MINHA TAG ASSINADA")
''Passa o xml assinado
Dim memoryStream As MemoryStream = New MemoryStream()
Dim xmlWriter As XmlWriter = xmlWriter.Create(memoryStream)
RequestXml.WriteTo(xmlWriter)
xmlWriter.Flush()
''Volta o memoryStream para o início
memoryStream.Position = 0
CopyStream(memoryStream, oldStream)
memoryStream.Close()
Exit Select
Case SoapMessageStage.BeforeDeserialize
CopyStream(oldStream, newStream)
ResponseXml = GetSoapEnvelope(newStream)
Exit Select
Case SoapMessageStage.AfterDeserialize
Exit Select
End Select
End Sub
Private Function GetSoapEnvelope(ByVal stream As Stream) As XmlDocument
Dim xml As XmlDocument = New XmlDocument()
stream.Position = 0
Dim reader As StreamReader = New StreamReader(stream)
xml.LoadXml(reader.ReadToEnd())
stream.Position = 0
Return xml
End Function
Private Sub CopyStream(ByVal from As Stream, ByVal tto As Stream)
Dim reader As StreamReader = New StreamReader(from)
Dim writer As StreamWriter = New StreamWriter(tto)
writer.WriteLine(reader.ReadToEnd())
writer.Flush()
End Sub
#Region " NOP"
Public Overrides Function GetInitializer(ByVal methodInfo As LogicalMethodInfo, ByVal attribute As SoapExtensionAttribute) As Object
Return Nothing
End Function
Public Overrides Function GetInitializer(ByVal WebServiceType As Type)
Return Nothing
End Function
Public Overrides Sub Initialize(ByVal initializer As Object)
End Sub
#End Region
End Class
Para que a classe de interceptação (TraceExtension) funcionasse corretamente, a solução que encontramos foi registra-la antes de executar a chamada para o método do WebService.
Resumindo bastante a chamada do método de registro ficou assim:
Private Sub btnExecutar_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnExecutar.Click
Try
'REGISTRO DA CLASSE DE INTERCEPTACAO
RegisterSoapExtension(GetType(TraceExtension).BaseType, 1, PriorityGroup.Low)
...
RespostaXML = MeuWS.MeuMetodo(EnvioXML)
...
Catch ex As Exception
Throw New Exception(ex.Message)
End Try
End Sub
Também um pouco resumido, o método de registro ficou assim:
<ComVisible(True)> _
<ReflectionPermission(SecurityAction.Demand)> _
Public Sub RegisterSoapExtension(ByVal type As Type, ByVal priority As Integer, ByVal group As PriorityGroup)
Dim wss As WebServicesSection = WebServicesSection.Current
Dim readOnlyField As FieldInfo = GetType(System.Configuration.ConfigurationElementCollection).GetField("bReadOnly", BindingFlags.NonPublic OrElse BindingFlags.Instance)
readOnlyField.SetValue(wss.SoapExtensionTypes, False)
Dim oElement As SoapExtensionTypeElement = New SoapExtensionTypeElement()
oElement.Type = GetType(TraceExtension)
oElement.Priority = priority
oElement.Group = group
wss.SoapExtensionTypes.Add(oElement)
Dim resetModifiedMethod As MethodInfo = GetType(System.Configuration.ConfigurationElement).GetMethod("ResetModified", BindingFlags.NonPublic OrElse BindingFlags.Instance)
resetModifiedMethod.Invoke(wss.SoapExtensionTypes, Nothing)
Dim setReadOnlyMethod As MethodInfo = GetType(System.Configuration.ConfigurationElement).GetMethod("SetReadOnly", BindingFlags.NonPublic OrElse BindingFlags.Instance)
setReadOnlyMethod.Invoke(wss.SoapExtensionTypes, Nothing)
End Sub
E foi assim que funcionou.
