Question

I’ve been trying to learn delegates.I just created a button,label and checkbox. If I click checkbox, the time format changes. If i click the button , i print the date accordingly. However when trying to use asynchromous delegate i.e., to use another thread, i am stuck with an error

   public delegate void AsyncDelegate(bool seconds);

public partial class Form1 : Form
{
    AsyncDelegate ad;
    TimeZ t = new TimeZ();
    public Form1()
    {
        InitializeComponent();
    }

 private void btn_async_Click(object sender, EventArgs e)
    {

        ad = new AsyncDelegate(t.GetTime);
        AsyncCallback acb = new AsyncCallback(CB);
        if (chk_sec.Checked)
        {
            ad.BeginInvoke(true, acb, null);
        }
        else
            ad.BeginInvoke(false, acb, null);


    }
    public void CB(IAsyncResult ar)
    {
        t.Tim = ar.ToString();
        ad.EndInvoke(ar);
        lbl_time.Text = t.Tim;
    }

and in another class library i get Timez used above. I add a reference of it in the project

 public class TimeZ
{
  private string tim;
    public string Tim
    {
        get
        {
            return tim;
        }
        set
        {
            tim = value;
        }
    }
    public string GetTime(bool seconds)
    {
        if (seconds)
        {
           return DateTime.Now.ToLongTimeString();
        }
        else
            return DateTime.Now.ToShortTimeString();
    }
}

However i get this error when i run the program:

 Cross-thread operation not valid: Control 'lbl_time' accessed from a thread other than   
    the thread it was created on.

Can u help me out on how to solve this?

Answer 1

You cannot access forms and controls properties and methods from a thread that is not the form thread.

In windows, each window is bound to the thread that created it.

You can do that only with Control.BeginInvoke or the more useful System.Threading.SynchronizationContext class.

See http://msdn.microsoft.com/it-it/library/system.threading.synchronizationcontext(v=vs.95).aspx

See http://msdn.microsoft.com/it-it/library/0b1bf3y3(v=vs.80).aspx

It means, you have to post through synchronization context for example another async delegate in form thread.

public partial class Form1 : Form
{
    AsyncDelegate ad;
    TimeZ t = new TimeZ();

    // Our synchronization context
    SynchronizationContext syncContext;

    public Form1()
    {
        InitializeComponent();

        // Initialize the synchronization context field
        syncContext = SynchronizationContext.Current;
    }

    private void btn_async_Click(object sender, EventArgs e)
    {

        ad = new AsyncDelegate(t.GetTime);
        AsyncCallback acb = new AsyncCallback(CB);
        if (chk_sec.Checked)
        {
            ad.BeginInvoke(true, acb, null);
        }
        else
        {
            ad.BeginInvoke(false, acb, null);
        }
    }

    public void CB(IAsyncResult ar)
    {
        // this will be executed in another thread
        t.Tim = ar.ToString(); // ar.ToString()???? this will not give you the time for sure! why?
        ad.EndInvoke(ar);

        syncContext.Post(delegate(object state)
        {
            // This will be executed again in form thread
            lbl_time.Text = t.Tim;
        }, null);
    }

I don’t know why you need an asynchronous callback to print time however 🙂 really don’t know why, thinking it is just some test code.