Task Status: Pending Activation -DownloadStringTaskAsync -WP8

The status of the task is always "Pending activation." The result of the task = "". I don’t understand why ... Thanks for your help. The user interface calls the GetDocLibs method.

public class ServerFunctions
{
    public static List<BdeskDocLib> GetDocLibs(bool onlyDocLibPerso)
    {
        string xmlContent = GetXml();
        List<BdeskDocLib> result = BdeskDocLib.GetListFromXml(xmlContent,  onlyDocLibPerso);
        return result;
    }

   private static String GetXml()
    {  
        Task<String>task=requesteur.Query(dataRequestParam);
        task.Wait();
        xmlResult = task.Result;
        return xmlResult;
    }
}

public class DataRequest
{
    public Task<String> Query(DataRequestParam dataRequestParam)
    {
       try
       {
        WebClient web = new WebClient();    
        if (!string.IsNullOrEmpty(dataRequestParam.AuthentificationLogin))
        {
            System.Net.NetworkCredential account = new NetworkCredential(dataRequestParam.AuthentificationLogin, dataRequestParam.AuthentificationPassword);
            web.Credentials = account;
        }
        return  web.DownloadStringTaskAsync(dataRequestParam.TargetUri).ConfigureAwait(false); 
     }  
 catch(WebException we)
        {
            MessageBox.Show(we.Message);
            return null;
        }
   } 
}