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The Last Configuration Section Handler..

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  

Written by Jeff Atwood

Indoor enthusiast. Co-founder of Stack Exchange and Discourse. Disclaimer: I have no idea what I'm talking about. Find me here: http://twitter.com/codinghorror