gistfile1.txt

csharp> var x = "<response xmlns='http://example.org/'><video>Hi!</video></response>";           
csharp> var xs = new StringReader(x);                                                            
csharp> var d = new XmlDocument();                                                               
csharp> d.Load(xs);                                                                              
csharp> foreach (XmlNode n in d.DocumentElement.SelectNodes("video")) Console.WriteLine(n.Name); 
# Note: no output
csharp> var mgr = new XmlNamespaceManager (d.NameTable);    
csharp> mgr.AddNamespace("lol", "http://example.org/");
csharp> foreach (XmlNode n in d.DocumentElement.SelectNodes("lol:video", mgr)) Console.WriteLine(n.Name);
video
# Note: you need to pass the XmlNamespaceManager to SelectNodes().