add namespace using xmlnamespacemanager in C#

I am trying to read the data from XML File. In this elements are prefixed with 'app' and 'gml' text. because of these prefixes I am unable to read the data. For this I am trying to add namespace by using XMLNamespaceManager but not getting.

Edit:

        XmlDocument xmlDoc = new XmlDocument();
        xmlDoc.Load(new StringReader(strResult));
        NameTable nt = new NameTable();
        XmlNamespaceManager prefix = new XmlNamespaceManager(nt);
        string nmspc = xmlDoc.DocumentElement.NamespaceURI;
        prefix.AddNamespace("app:",xmlDoc.DocumentElement.NamespaceURI);

        prefix.PushScope();

Here the strResult contains c# asp.net