Fixed the NullReferenceException resulting from invalid or incomplete XRDS documents.

Fixes part of the issue discovered from Google Code Issue 182.
See DNOI 3ab85f11d69a3059cebac203f77a05fe083d7b65.

2 files changed, 9 insertions, 1 deletions

diff --git a/src/DotNetOpenAuth/Xrds/XrdsDocument.cs b/src/DotNetOpenAuth/Xrds/XrdsDocument.cs
index ba8f29b..dd0e19f 100644
--- a/src/DotNetOpenAuth/Xrds/XrdsDocument.cs
+++ b/src/DotNetOpenAuth/Xrds/XrdsDocument.cs
@@ -56,7 +56,9 @@ namespace DotNetOpenAuth.Xrds {
 					}
 				} else {
 					XPathNavigator node = Node.SelectSingleNode("/xrd:XRD", XmlNamespaceResolver);
-					yield return new XrdElement(node, this);
+					if (node != null) {
+						yield return new XrdElement(node, this);
+					}
 				}
 			}
 		}
diff --git a/src/DotNetOpenAuth/Xrds/XrdsNode.cs b/src/DotNetOpenAuth/Xrds/XrdsNode.cs
index 5eda0e6..e27a1b2 100644
--- a/src/DotNetOpenAuth/Xrds/XrdsNode.cs
+++ b/src/DotNetOpenAuth/Xrds/XrdsNode.cs
@@ -7,6 +7,7 @@
 namespace DotNetOpenAuth.Xrds {
 	using System.Xml;
 	using System.Xml.XPath;
+	using DotNetOpenAuth.Messaging;
 
 	/// <summary>
 	/// A node in an XRDS document.
@@ -28,6 +29,9 @@ namespace DotNetOpenAuth.Xrds {
 		/// <param name="node">The node represented by this instance.</param>
 		/// <param name="parentNode">The parent node.</param>
 		protected XrdsNode(XPathNavigator node, XrdsNode parentNode) {
+			ErrorUtilities.VerifyArgumentNotNull(node, "node");
+			ErrorUtilities.VerifyArgumentNotNull(parentNode, "parentNode");
+
 			this.Node = node;
 			this.ParentNode = parentNode;
 			this.XmlNamespaceResolver = this.ParentNode.XmlNamespaceResolver;
@@ -38,6 +42,8 @@ namespace DotNetOpenAuth.Xrds {
 		/// </summary>
 		/// <param name="document">The document's root node, which this instance represents.</param>
 		protected XrdsNode(XPathNavigator document) {
+			ErrorUtilities.VerifyArgumentNotNull(document, "document");
+
 			this.Node = document;
 			this.XmlNamespaceResolver = new XmlNamespaceManager(document.NameTable);
 		}