确保在主UI线程中创建子控件

Question

I'm modifying existing WinForms project. The project has UserControl. This UserControl has DataSet variable which is set from another part of the program in different thread.

What I want to do is to dynamically add another controls to this control depending on the DataSet.

So, after DataSet is loaded, I'm calling RefreshChildControl function and trying to add my new ChildUserControls to flowLayoutPanel. And that's where the problems begin:). I get the "Cross-thread operation not valid: Control 'ChildUserControl' accessed from a thread other than the thread it was created on" exception. I tried to use if(this.InvokeRequired) and Invoke this method, but it does not help. InvokeRequired on MyUserControl is false.

So, is there any good way of performing such task? Or am I missing something important?

EDIT:

I tried to skip InvokeRequired test and just call this.FindForm().Invoke on this method. I've got "Invoke or BeginInvoke cannot be called on a control until the window handle has been created." exception. And, by the way, when I open another form with this control everything worked fine.

Answer 1

First. The simplest solution is to perform Invoke everytime. Nothing bad will happen. Second, use SynchronizationContext.

using System.Threading;
public class YourForm
{
    SynchronizationContext sync;
    public YourForm()
    {
        sync = SynchronizationContext.Current;
        // Any time you need to update controls, call it like this:
        sync.Send(UpdateControls);
    }

    public void UpdateControls()
    {
        // Access your controls.
    }
}

SynchronizationContext will manage all threading issues for you. It checks, whether you call from the same or from the other thread. If from same it will just immediately execute your code. Otherwise it will do Invoke through form's message loop.

Answer 2

If your user control is not immediately visible after you construct it, the handle will not be created on the thread that you think it is created on. It's not the C# object whose thread parent is important, it is the Windows Handle object whose parent is important.

To force a control to be immediately created on the thread that you thought you created it on, then read out the control.Handle which will force the control to actually be made and assigned a handle.

 MyUserControl uc = new MyUserControl(); // the handle is not created here
 uc.Visible = false;
 IntPtr dummy = uc.Handle; //  The control is immediately given a real handle

You can also try to fiddle around with uc.CreateControl, but this won't create the handle if the control is not visible.

Now you can have another thread update your user control even if the user control is not visible.

 uc.BeginInvoke((Action)(() => uc.Text = "ha ha"));

If you leave out the dummy = uc.Handle line, you will get an exception that you can't call BeginInvoke on a control that does not have a handle.

http://msdn.microsoft.com/en-us/library/system.windows.forms.control.createcontrol(v=vs.90).aspx