User Authenticattion

[C# Programming]

how to authenticate the user using the active directory and then finally assign certain exccess level to the user....any help..

private bool AuthenticateAD(string userName, string password, string domain, out string message)
{




message = "";


DirectoryEntry entry = new
DirectoryEntry("LDAP://" + domain, userName, password);
try
{
object obj = entry.NativeObject;
DirectorySearcher search = new DirectorySearcher(entry);
search.Filter = "(SAMAccountName=" + userName + ")";
search.PropertiesToLoad.Add("cn");
SearchResult result = search.FindOne();


if (null == result)
{
return false;
}


}
catch (Exception ex)
{
message = ex.Message;
return false;
//throw new Exception("Error authenticating user. " + ex.Message);
}


return true;




}