Question

EDIT: this is a winform application, sorry for the inconvenience

Disclaimer: this is an assignment we got in college, and I’m stuck over this particular section of code.

I have 2 solutions in Visual Studio 2008, one for a Form and one for a DLL which the form can use for functionality. The idea is to send HTML mails from the client, and to use the Delegate to confirm this.

One class of the DLL contains nothing else but a single Delegate:

namespace Console.Grand
{
    public delegate void ObserverDelegate(string info);
}

In a file called Delegate.cs

In the form, I have the following method which I will use for the Delegate:

private void Update(string info)
{
   this.logBox.Text += Environment.NewLine + info;
}

The logBox variable is a TextArea on the form.

On transmitting, the following occurs(BL stands for “Business Layer”):

BL_MailOut bm = new BL_MailOut(s1,ListViewAdresses());
ObserverDelegate deleg = new ObserverDelegate(Update);
bm.SendMail(deleg);

The BL_MailOut constructor looks like this(we’re in the DLL now):

public BL_MailOut(StandardPage page, List<MailAddress> list)
{
    this.s = page;
    this.adresslist = new List<MailAddress>();
    foreach (MailAddress m in list)
    {
        this.adresslist.Add(m);
    }
}

And the method SendMail:

public void SendMail(ObserverDelegate deleg)
{
    IO_MailOut im = new IO_MailOut(s, adresslist, deleg);
    Thread t = new Thread(new ThreadStart(im.Send));
    t.Start();
}

And finally, we arrive at the method Send():

public void Send()
{
    SmtpClient sc;
    MailMessage msg;
    string info;
    foreach (MailAddress adress in this.list)
    {
        try
        {
            sc = new SmtpClient(HOST);
            msg = new MailMessage(SENDER, adress.Address);
            msg.IsBodyHtml = true;
            msg.Subject = "test";
            msg.Body = page.ToString();
            sc.Send(msg);
            info = "(" + DateTime.Now + ") MAIL SENT TO" + Environment.NewLine + adress.Address;
            deleg(info);
        }
    }

I do catch the needed errors, I just left that out here to save room.

When deleg(info); is reached, expected behavior would be that the textBox receives the needed text. However, it does not. The instance of the delegate is preserved and the compiler gives no error. I’ve read the material on the MSDN site on Delegates, but nothing there helped.

Answer 1

Your Update method on the form is performing a cross-thread operation, which is not allowed.

Change your Update method on the form to this

    private void Update(string info)
    {
     ObserverDelegate callBack =  new ObserverDelegate((x) =>
            {
                this.logBox.Text += Environment.NewLine + info;
            });             

        if (this.InvokeRequired)
            this.Invoke(callBack, info);
        else
          callBack(info);
    }