401 Error (Unauthorized) When Accessing SharePoint Web Services

We've created application helper classes that accesses all SharePoint lists via the integrated web services (/_vti_bin/*.asmx).   One of the miserable errors we sometimes get when moving through the deployment stages (Development, QA then Production) is that, even with administrative privileges, we still get a 401 Unauthorized error.

Our code goes something like this:

protected WebServiceSPLists.Lists _lists;
const string CClassName = "DataContextSharePointList";

private string GetListNameByTitle(Connection connection, string listTitle)
{
 const string CProcName = "GetListNameByTitle";
 string response = "";
 XmlNode listsNode;
 XmlNode node;

 try
 {
  this.InitConnection(connection);
  _lists.Credentials = ((Connection)connection).Credentials;
  _lists.Url = ((Connection)connection).SiteUrl + "/_vti_bin/lists.asmx";
  Trace.Write(CClassName, CProcName, "GetListCollection");
  listsNode = _lists.GetListCollection();    // <-- 401 error occurs here
  Trace.Write(CClassName, CProcName, "GetListCollection OK");
  
  // Code omitted //
 }
 catch (Exception ex) 
 {
  Trace.Write(CClassName, CProcName, "Error: " + ex.Message);
 }
}

The way we fix this is by either specifying the host names or disabling the loopback check and EACH of the application servers.  See this post for more details.

Important and Useful Links:

• You receive error 401.1 when you browse a Web site that uses Integrated Authentication and is hosted on IIS 5.1 or a later version by Microsoft Support
• How to resolve (401) Unauthorized error for SharePoint web service call from Infopath Webbased form? by Harry Chen