The Last Configuration Section Handler..

December 21, 2004

I stumbled across the Craig Andera post The Last Configuration Section Handler I'll Ever Need a few months ago, but I didn't really understand the implications until I started writing a bunch of configuration section handlers. His approach is very clever; instead of writing a bunch of tedious code to read settings from a .config file, you deserialize an instance of the class using the .config file XML as the input!

Here's the VB.NET version of the necessary ConfigurationSectionHandler:

Imports System.Xml
Imports System.Xml.Xpath
Imports System.Xml.Serialization
Imports System.Configuration

Public Class XmlSerializerSectionHandler
    Implements IConfigurationSectionHandler

    Public Function Create(ByVal parent As Object, ByVal configContext As Object, _
        ByVal section As System.Xml.XmlNode) As Object _
        Implements System.Configuration.IConfigurationSectionHandler.Create

        Dim xpn As XPathNavigator = section.CreateNavigator
        Dim TypeName As String = xpn.Evaluate("string(@type)").ToString
        Dim t as Type = Type.GetType(TypeName)
        Dim xs as XmlSerializer = New XmlSerializer(t)

        Return xs.Deserialize(New XmlNodeReader(section))
    End Function

End Class

And here's an example of what your *.config file would look like:

<configuration>
  <configSections>
    <section name="MyStuff" 
      type="MyClass.XmlSerializerSectionHandler, MyClass" />
  </configSections>

  <MyStuff type="MyClass.MyStuff">
    <Foo>234</Foo>
    <Bar>A bunch of information</Bar>
  </MyStuff> 
</configuration>

Note the type= attrib on the MyStuff element. With the type information in that attribute, the <MyStuff> config section can be deserialized to an instance of the MyStuff object:

Class MyStuff
    Public foo As Integer
    Public bar As String
End Class

.. in a single call!

Dim ms As MyStuff 
ms = CType(ConfigurationSettings.GetConfig("MyStuff"), MyClass.MyStuff)

Before going this route, make sure your class serializes to the same XML format exactly-- otherwise you'll get a bunch of non-intuitive deserialization error messages. Here's a quick way to serialize a class to the console and view the correct XML that is expected for deserialization:

Dim o as New MyStuff
o.foo = 3
o.bar = "stuff"

Dim sb As New Text.StringBuilder
Dim sw As New IO.StringWriter(sb)
Dim xs As XmlSerializer = New XmlSerializer(o.GetType)
Dim xsn As New XmlSerializerNamespaces
xsn.Add("", "")

Dim xtw As New Xml.XmlTextWriter(sw)
xtw.Formatting = Xml.Formatting.Indented
xtw.WriteRaw("")

xs.Serialize(xtw, o, xsn)
Dim s As String = sb.ToString
s = Regex.Replace(s, "(<" & o.GetType.Name & ")(>)", "$1 type=""" & o.GetType.FullName & """$2")
Console.WriteLine(s)

Note that some of the contortions in the above code are necessary to get a "clean" set of XML output, free of namespaces, encoding, and the like. This code was borrowed from Mark Allanson's blog.

It really could be The Last Configuration Section You'll Ever Need.

However, troubleshooting XML that won't deserialize can be.. difficult. Here's an improved, more robust XmlSerializerSectionHandler that provides much better feedback when things go wrong.

''' <summary>
''' Configuration section handler that deserializes configuration settings to an object.
''' </summary>
''' <remarks>The root node must have a type attribute defining the type to deserialize to.</remarks>
Public Class XmlSerializerSectionHandler
    Implements IConfigurationSectionHandler

    Public Function Create(ByVal parent As Object, ByVal configContext As Object, ByVal section As System.Xml.XmlNode) As Object _
        Implements System.Configuration.IConfigurationSectionHandler.Create

        '-- get the name of the type from the type= attribute on the root node
        Dim xpn As XPathNavigator = section.CreateNavigator
        Dim TypeName As String = xpn.Evaluate("string(@type)").ToString
        If TypeName = "" Then
            Throw New ConfigurationException( _
                "The type attribute is not present on the root node of " & _
                "the <" & section.Name & "> configuration section ", _
                section)
        End If

        '-- make sure this string evaluates to a valid type
        Dim t As Type = Type.GetType(TypeName)
        If t Is Nothing Then
            Throw New ConfigurationException( _
                "The type attribute '" & TypeName & "' specified in the root node of the " & _
                "the <" & section.Name & "> configuration section " & _
                "is not a valid type.", section)
        End If
        Dim xs As XmlSerializer = New XmlSerializer(t)

        '-- attempt to deserialize an object of this type from the provided XML section
        Dim xnr As New XmlNodeReader(section)
        Try
            Return xs.Deserialize(xnr)
        Catch ex As Exception
            Dim s As String = ex.Message
            Dim innerException As Exception = ex.InnerException
            Do While Not innerException Is Nothing
                s &= " " & innerException.Message
                innerException = innerException.InnerException
            Loop
            Throw New ConfigurationException( _
                "Unable to deserialize an object of type '" & TypeName & "' from " & _
                "the <" & section.Name & "> configuration section: " & s, _
                ex, section)
        End Try
    End Function

End Class