public class PasswordSetter
{
    private P4Connection _p4 = null;
    private string _oldPassword = "";
    private string _newPassword = "";
    PasswordSetter(P4Connection p4)
    {
        _p4 = p4;
    }

    public void SetPassword(string OldPassword, string NewPassword)
    {
        OnPromptEventHandler eh = new OnPromptEventHandler(OnPrompt);
        _p4.OnPrompt += eh;

        _oldPassword = OldPassword;
        _newPassword = NewPassword;

        //run passwd
        P4UnParsedRecordSet r = _p4.RunUnParsed("passwd");

        //Clear the event just in case
        _p4.OnPrompt -= eh;

        //Clear the passwords from memory
        _newPassword = "";
        _oldPassword = "";
    }

    private void OnPrompt(object sender, P4PromptEventArgs e)
    {
        switch (e.Message)
        {
            case ("Enter old password: "):
                e.Response = _oldPassword;
                break;
            case ("Enter new password: "):
                e.Response = _newPassword;
                break;
            case ("Re-enter new password: "):
                e.Response = _newPassword;
                break;
        }
    }
}