A sample for the required functionality:
public void AddMarker(RadMap Map)
{
    SearchRequest r = new SearchRequest();
    r.Query = "Paris, France";
    r.UserData = new Guid();           
    bingProvider.SearchCompleted += BingProvider_SearchCompleted;
    bingProvider.SearchAsync(r);
}

private void BingProvider_SearchCompleted(object sender, SearchCompletedEventArgs e)
{
    //one cannot access the UserData here
}