string connection = ConfigurationManager.ConnectionStrings["ADConnection"].ToString();

        DirectorySearcher dssearch = new DirectorySearcher(connection);
        dssearch.Filter = "(sAMAccountName=" + current_User + ")";
        SearchResult sresult = dssearch.FindOne();
        DirectoryEntry dsresult = sresult.GetDirectoryEntry();

string connection = ConfigurationManager.ConnectionStrings["ADConnection"].ToString();

DirectorySearcher dssearch = new DirectorySearcher(connection);
dssearch.Filter = "(&(objectCategory=Person))";
dssearch.Filter = "(sAMAccountName=" + current_User + ")";

SearchResultCollection searchResult = dssearch.FindAll();

foreach (SearchResult srUSers in searchResult)
{
    DirectoryEntry de = srUsers.GetDirectoryEntry();
    dropDownList1.Items.Add(de.Name.ToString());
}