Question

I am struggling to retrieve custom attributes from a method. As you can see, the method ProcessXML has a custom attribute. The method itself gets passed into an anonymous delegate and then, in that context, I’m looking to get its custom attributes, but I’m not sure quite how to do it.

I’d be really grateful if someone could show me the proper way to do it. Many thanks

Here’s my code

    Public Sub UpdateXML()

        Dim errors = New Errors

        Dim xml = <testxml>
                      <element value="1"/>
                      <element value="2"/>
                      <element value="3"/>
                  </testxml>

        Dim funcWithErrorTrapping = ProcessXML().WithErrorTrapping(errors)

        Dim res = funcWithErrorTrapping(xml)
        If errors.Count = 0 Then
            MessageBox.Show("Success - " & res)
        Else
            MessageBox.Show("Failure - " & errors.Count)
        End If

    End Sub

    <ThrowException(123456, "He's a very naughty boy")>
    Private Overloads Function ProcessXML() As Func(Of XElement, String)

        Return Function(x)
                   For Each e In x.Descendants("element")
                       e.Attribute("value").Value = "set"
                   Next

                   Throw New Exception

                   Return x.ToString

               End Function

    End Function
End Class

Public Class Errors
    Inherits Dictionary(Of Integer, String)
End Class

<AttributeUsage(AttributeTargets.All, inherited:=False, AllowMultiple:=False)>
Public NotInheritable Class ThrowException
    Inherits Attribute

    Private _number As Integer
    Private _description As String

    Public Sub New(ByVal number As Integer, ByVal description As String)
        _number = number
        _description = description
    End Sub

    Public ReadOnly Property Number As Integer
        Get
            Return _number
        End Get
    End Property

    Public ReadOnly Property Description As String
        Get
            Return _description
        End Get
    End Property

End Class

Public Module Extensions

    <Extension()>
    Public Function WithErrorTrapping(Of T, T1)(ByVal process As Func(Of T, T1), ByVal errors As Errors) As Func(Of T, T1)

        Return Function(a)
                   Try
                       Return process.Invoke(a)
                   Catch ex As Exception
                       Dim [exception] = process.Method.GetAttributes(Of ThrowException)()(1)
                       errors.Add([exception].Number, [exception].Description)
                   End Try
               End Function

    End Function

    <Extension()>
    Public Function GetAttributes(Of T As Attribute)(ByVal attributesProvider As ICustomAttributeProvider)
        Dim attributes = New List(Of T)
        For Each attr As Object In attributesProvider.GetCustomAttributes(GetType(T), False)
            If TypeOf attr Is T Then
                attributes.Add(TryCast(attr, T))
            End If
        Next

        Return attributes

    End Function

End Module

Answer 1

So I realised that AOP (Postsharp) was a much better approach to achieveing what I was trying to do here!