MSBuild Inline Task to Transform a Hierarchy of XML Files using XDT Transforms

base.xml

<?xml version="1.0"?>
<!-- Base XML file; shared stuff -->
<foo xmlns="somens">
  <baz />
</foo>

source.xml

<?xml version="1.0"?>
<!-- inherits is relative to the XML file's directory (e.g. ../base.xml would look one directory up) -->
<foo inherits="base.xml" xmlns="somens" xmlns:xdt="http://schemas.microsoft.com/XML-Document-Transform">
  <bar xdt:Transform="Insert" />
</foo>

Transforms.xml

  <!-- This task takes in a XDT transform file and transforms it, following any inheritance chain.
       There should be at least one base transform for this to work; otherwise just use Microsoft's
       regular TransformXml task. -->
  <!-- EXAMPLE USAGE:
  <TransformXmlHierarchy 
    Source="source.xml" 
    Destination="transformed.xml"
    TaskDirectory="path/to/directory/of/Microsoft.Web.Publishing.Tasks" />
  -->
  <UsingTask
    TaskName="TransformXmlHierarchy"
    TaskFactory="CodeTaskFactory"
    AssemblyFile="$(MSBuildToolsPath)\Microsoft.Build.Tasks.v4.0.dll" >
    <ParameterGroup>
      <Source Required="true" />
      <Destination Required="true" />
      <TaskDirectory Required="true" />
    </ParameterGroup>
    <Task>      
      <Reference Include="System.Xml" />
      <Reference Include="Microsoft.CSharp" />
      <Using Namespace="System"/>
      <Using Namespace="System.Linq"/>
      <Using Namespace="System.IO" />
      <Using Namespace="System.Xml"/>
      <Using Namespace="System.Reflection" />
      <Code Type="Fragment" Language="cs">
<![CDATA[
// TODO: Figure out a way to make the inline task reference this. For the life of me,
//       it wasn't working, so had to load via Reflection.
var taskPath = Path.Combine(TaskDirectory, "Microsoft.Web.Publishing.Tasks.dll");
if (!File.Exists(taskPath))
    throw new Exception("Could not load publishing tasks assembly");

Assembly taskAssembly = Assembly.UnsafeLoadFrom(taskPath);

Func<XmlDocument, string, XmlDocument> transformer = (source, transform) => 
{
  dynamic transformation = taskAssembly.CreateInstance(
    "Microsoft.Web.Publishing.Tasks.XmlTransformation", true, BindingFlags.CreateInstance, 
    null, new object[] { transform }, null, null);
  
  if (transformation == null)
      throw new Exception("Could not create instance of XmlTransformation");

  transformation.Apply(source);

  return source;
};

Func<XmlDocument, string> getParent = (source) => 
{
  if (source == null) return null;

  // Use default namespace of document
  var nsmgr = new XmlNamespaceManager(source.NameTable);
  nsmgr.AddNamespace("x", source.DocumentElement.NamespaceURI);

  // TODO: Probably can safely select first node, to support any kind of XML document
  var attr = source.SelectSingleNode("x:package", nsmgr).Attributes["inherits"];

  return attr == null ? null : attr.Value;
};

var rootDoc = new XmlDocument();
var sources = new List<string>();
var basePath = Path.GetDirectoryName(Source);
var parent = Path.GetFileName(Source);

if (basePath == null) {
    throw new Exception("Could not find base directory of path " + Source);
}

do {
    sources.Add(parent);

    rootDoc.Load(Path.Combine(basePath, parent));

    parent = getParent(rootDoc);

    // TODO: Need to rebase basePath here?
    
    if (parent != null) {
        rootDoc.Load(Path.Combine(basePath, parent));
    }
} while (parent != null);

// Reverse chain
sources.Reverse();

var transformedDoc = sources.Skip(1).Aggregate(rootDoc, 
    (document, transform) => String.IsNullOrEmpty(transform) 
        ? document 
        : transformer(document, Path.Combine(basePath, transform)),
    (document) => document);

Log.LogMessage(MessageImportance.Normal, "Transformed " + Destination);

transformedDoc.Save(Destination);
]]>
      </Code>
    </Task>
  </UsingTask>

Usage.xml

<Import Project="Transforms.xml" />
<Target name="Foo">  
  <TransformXmlHierarchy
      Source="source.xml"
      Destination="transformed\transformed.xml"
      TaskDirectory="$(MSBuildExtensionsPath)\Microsoft\VisualStudio\v10.0\Web\" />
</Target>