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ExampleUsage.cs

int userId = StackOverflowApiHelper.GetUserId("satrom");

Assert.AreEqual(380135, userId);

StackOverflowApiHelper.cs

public static class StackOverflowApiHelper
{

   private const string DefaultUserId = "22656"; //Jon Skeet

   public static string GetUserId(string username)
   {
      var apiEndpoint = string.Format("http://stack2xml.quickmediasolutions.com/stackoverflow/users?filter={0}", username);
      var webRequest = WebRequest.Create(apiEndpoint) as HttpWebRequest;
      var webResponse = webRequest.GetResponse() as HttpWebResponse;

      if (webResponse != null)
      {
         string userDataStream;
         using (var userStream = new StreamReader(webResponse.GetResponseStream()))
         {
            userDataStream = userStream.ReadToEnd();
         }

         return GetUserIdFromXmlResult(userDataStream);
      }

      return DefaultUserId;
   }

   private static string GetUserIdFromXmlResult(string userString)
   {
      XElement userXml = XElement.Parse(userString);
      var users = userXml.Descendants("users").ToList();

      if (users[0] != null)
         return users[0].Element("user").Element("user_id").Value;

      return DefaultUserId;
   }
}