Which namespace is necessary to use SelectSingleNode() method (using default namespace and can't use the method)

Question

Hi I have xml file (which is actually msbuild file) that uses different namespace

Answer 1

You need to use the correct namespace, which is "http://schemas.microsoft.com/developer/msbuild/2003".

Try

XmlDocument xml = new XmlDocument();
xml.Load("ref.props");        
XmlNamespaceManager nsmgr = new XmlNamespaceManager(xml.NameTable);
nsmgr.AddNamespace("ms", "http://schemas.microsoft.com/developer/msbuild/2003");

XmlNode platform_node
  = xml.SelectSingleNode("/ms:Project/ms:PropertyGroup[contains(@Condition, '1111')]",
                         nsmgr);

Don't confuse the namespace prefix (which was empty in the XML) with the namespace, which is "http://schemas.microsoft.com/developer/msbuild/2003".