即使在try/catch块中，也会解除CommunicationException

我目前正在为WP7开发一个需要调用WCF服务应用程序的应用程序。我用一个小的WPF应用程序测试了该服务，一切都很顺利。但现在我从我的WP7应用程序中调用它，我系统地得到了以下异常：

An exception of type 'System.ServiceModel.CommunicationException' occurred in
System.ServiceModel.ni.dll but was not handled in user code
System.ServiceModel.CommunicationException was unhandled by user code
    HResult=-2146233087
    Message=The remote server returned an error: NotFound.
    Source=System.ServiceModel
    InnerException: System.Net.WebException
        HResult=-2146233079
        Message=The remote server returned an error: NotFound.
        Source=System.Windows
        InnerException: System.Net.WebException
             HResult=-2146233079
             Message=The remote server returned an error: NotFound.
             Source=System.Windows
             InnerException: 

尽管我在这样的try/catch块中（在MyProjectPath.Model.User.cs中）进行了服务调用，但异常仍在不断解除：

public Task<User> Load(string logon, string pwHash)
{
    TaskCompletionSource<User> tcs = new TaskCompletionSource<User>();
    client.GetUserByCredsCompleted += ((s, e) =>
        {
            if (e.Error == null) tcs.TrySetResult(e.Result);
            else
            {
                StringBuilder sb = new StringBuilder();
                sb.AppendLine("Error encountered while getting data :");
                sb.AppendLine(e.Error.Message);
                MessageBox.Show(sb.ToString());
            }
        });
    try
    {
        client.GetUserByCredsAsync(logon, pwHash);
    }
    catch (System.ServiceModel.CommunicationException ex)
    {
        MessageBox.Show(ex.Message);
    }
    catch (Exception ex)
    {
        MessageBox.Show(ex.Message);
    }
    return tcs.Task;
}

当执行时，异常发生在这里（在System.ServiceModel.ni.dll中）：

public MyProjectPath.ServiceReference.User EndGetUserByCreds(System.IAsyncResult result) {
    object[] _args = new object[0];
        // Exception gets lifted by the following line :
    MyProjectPath.ServiceReference.User _result = ((MyProjectPath.ServiceReference.User)(base.EndInvoke("GetUserByCreds", _args, result)));
    return _result;
}

有人已经遇到并解决了这个问题吗？我必须承认我在这里很无知。。。