Thread Control.Invoke

Question

I have a function

public void ShowAllFly()
{  
        cbFly.Items.Clear();
        cbFly.Items.Add("Uçuş Seçiniz...");

        dsFlyTableAdapters.tblFlyTableAdapter _t=new KTHY.dsFlyTableAdapters.tblFlyTableAdapter();
        dsFly _mds = new dsFly();
        _mds.EnforceConstraints = false;
        dsFly.tblFlyDataTable _m = _mds.tblFly;
        _t.Fill(_m);
        foreach (DataRow _row in _m.Rows)
        {
            cbFly.Items.Add(_row["FlyID"].ToString()+"-"+_row["FlyName"].ToString() + "-" + _row["FlyDirection"].ToString() + "-" + _row["FlyDateTime"].ToString());
        }
        _Thread.Abort();
        timer1.Enabled = false;
        WaitPanel.Visible = false;
}

In Form_Load Function Like this;

{
    _Thread = new System.Threading.Thread(new System.Threading.ThreadStart(ShowAllFly));
    _Thread.Start();
    _Thread.Priority = System.Threading.ThreadPriority.Normal;
}

But When I run it;

in ShowAllFly function

cbFly.Items.Clear(); ----  HERE Gives ERROR  LIKE  Control.Invoke must be used to interact with controls created on a separate thread.

What is the problem?

Answer 1

There are two golden rules of threading in Windows Forms:

• Don't touch any control properties or methods (other than those explicitly listed as being okay) from any thread other than the one which created the control's "handle" (usually there's just one UI thread)
• Don't block the UI thread for any significant length of time, or you'll make the application unresponsive

In order to interact with the UI from a different thread, you need to "marshall" the call to the UI thread, using a delegate and calling Control.Invoke/BeginInvoke. You can test whether or not you need to call Invoke using the InvokeRequired property, but these days I personally tend to just do it anyway - there's not much penalty for invoking when you don't need to.

Lambda expressions in C# 3 (or anonymous methods in C# 2) make this a lot more pleasant as well.

For instance, you could use:

cbFly.Invoke((MethodInvoker)(() => cbFly.Items.Clear()));

All the brackets get in the way a bit, so you might want to add an extension method like this, if you're using C# 3:

public static void Invoke(this Control control, MethodInvoker action)
{
    control.Invoke(action);
}

Then you could do:

cbFly.Invoke(() => cbFly.Items.Clear());

which is a good deal simpler. Usually you can get away with using a MethodInvoker by capturing any variables you need to access within the delegate.

See my threading tutorial or Joe Albahari's for more details.

As a secondary matter, I see you're using Thread.Abort - in fact on your own thread, despite it having other calls after it. Why? Aborting any thread other than your own is an "emergencies only" type call (which should usually be followed by the app being unloaded anyway) and I can't see any reason to abort the current thread when there's still work to be done afterwards...