if you are loading a string to 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>
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);