SelectSingleNode returns null when tag contains xmlNamespace

I'm loading a string into an XML document that contains the following structure:

<?xml version="1.0" encoding="utf-8"?>
<Project ToolsVersion="4.0" DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">                  
  <ItemGroup>
    <Compile Include="clsWorker.cs" />        
  </ItemGroup>      
</Project>

Then I'm loading all into an XmlDocument:

XmlDocument xmldoc = new XmlDocument();
xmldoc.LoadXml(Xml);

Then the following problem occurs:

XmlNode Node = xmldoc.SelectSingleNode("//Compile"); // returns null

When I remove the xmlns attribute from the root element (Project), it works fine.

How do I get SelectSingleNode to return the relevant element?

You should use an XmlNamespaceManager in your call to SelectSingleNode():

XmlNamespaceManager ns = new XmlNamespaceManager(xmldoc.NameTable);
ns.AddNamespace("msbld", "http://schemas.microsoft.com/developer/msbuild/2003");
XmlNode node = xmldoc.SelectSingleNode("//msbld:Compile", ns);

Taken right from the documentation of SelectSingleNode() on the MSDN:

>Note
>If the XPath expression does not include a prefix, it is assumed that the >namespace URI is the empty namespace. If your XML includes a default >namespace, you must still add a prefix and namespace URI to the >XmlNamespaceManager; otherwise, you will not get a node selected. For >more information, see Select Nodes Using XPath Navigation.

And the immediately following sample code is

XmlNamespaceManager nsmgr = new XmlNamespaceManager(doc.NameTable);
nsmgr.AddNamespace("ab", "http://www.lucernepublishing.com");
XmlNode book = doc.SelectSingleNode("//ab:book", nsmgr);

It's not as if this would be "hidden knowledge". ;-)

This way you don't need to specify namespace:

XmlDocument xmlDoc = new XmlDocument();
xmlDoc.LoadXml("your xml");
XmlNode node = xmlDoc.SelectSingleNode("/*[local-name() = 'Compile']");
XmlNode nodeToImport = xmlDoc2.ImportNode(node, true);
xmlDoc2.AppendChild(nodeToImport);

Since the 'ItemGroup' may have multiple 'Compile' children, and you specifically want the 'Compile' children of 'Project/ItemGroup', the following will return all of the desired 'Compile' children and no others:

XmlDocument projectDoc = new XmlDocument();
projectDoc.Load(projectDocPath);
XmlNamespaceManager ns = new XmlNamespaceManager(projectDoc.NameTable);
ns.AddNamespace("msbld", "http://schemas.microsoft.com/developer/msbuild/2003");
XmlNodeList xnList = projectDoc.SelectNodes(@"/msbld:Project/msbld:ItemGroup/msbld:Compile", ns);

Note that the 'msbld:' namespace specification needs to precede each node level.