I want send email just once per day. There is one button and any user can click that button. So email must not be send 2 times. When first user click button then email is send and if some other user click button than email will not be send again.

This is how I implemented and it’s working (more/less) There are some cases (once month) when send 2 email.(Don’t know why)

I am sure that there is more elegant way to resolve this problem. Please share with me your ideas.

    protected void Send(string email, String allRecipients, String subject, String body)
    {
        //Be sure that email is send just once:
        //Lock an object, and if its already locked just continue,
        if (Monitor.TryEnter(syncLock))
        {
            try
            {
                //Send email only if is not send already.
                //Check in DB if is send.
                //IsSend function return true if email is send already, else false.
                if (!IsSent())
                {
                    SendEmails(email, allRecipients, subject, body);
                }
            }
            finally
            {
                Monitor.Exit(syncLock);
            }
        }
    }

Function that send email

    public static void SendEmails(string email, String allRecipients, String subject, String body)
    {
        //Flag if email is sent or not:
        Boolean isEmailSend = false;

        SmtpClient client = new SmtpClient();
        //Message:
        MailMessage message = new MailMessage(email, allRecipients, subject, body);
        try
        {
            isEmailSend = true;
            client.Send(message);

        }
        catch (SmtpException e)
        {
            isEmailSend = false;
            throw new SmtpException(e.ToString());
        }
        catch (TimeoutException e)
        {
            //Problem: Not sure if is email send or not:
            isEmailSend = false;
            throw new TimeoutException("TIME OUT:" + e.ToString());
        }
        catch (Exception e)
        {
            //Problem: Not sure if is email send or not:
            isEmailSend = false;
            throw new Exception(e.ToString());
        }
        finally
        {
            if (isEmailSend)
            {
                //Update DB.
                //Set that email is send.
                //This update in DB that email for current day is send already.
                SetAsSend();
            }
        }
    }