Controls and Threads

Question

I have a button that creates a new thread. That thread contains the following.

foreach (DataRow r in results.Rows)
            {
                var item = new ResultItem(this)
                               {
                                   Image = r[1].ToString(),
                                   Video = r[2].ToString(),
                                   Title = r[3].ToString().Trim(),
                                   Duration = r[4].ToString().Trim()
                               };


                SearchFlow.Controls.Add(item);
            }

I get this error: Controls created on one thread cannot be parented to a control on a different thread. ResItem is custom usercontrol that's part of the project, not a 3rd party control. Is there a way around this? How can I add this control to a flowtable in a new thread?

@davisoa has the right answer. Here are some alternatives for how to use the `InvokeRequired` pattern. http://stackoverflow.com/questions/2367718/c-automating-the-invokerequired-code-pattern — Matt Davis, Nov 06 '10 at 23:21
possible duplicate of ["Cross-thread operation not valid" exception on inner controls](http://stackoverflow.com/questions/974317/cross-thread-operation-not-valid-exception-on-inner-controls) — H H, Nov 07 '10 at 08:52

score 6 · Accepted Answer · answered Nov 06 '10 at 23:00

You must only touch UI controls on the thread that created them. This also means that any control created by a background thread cannot be added to your UI.

The solution is to only create controls using the UI (foreground) thread.

One way to do this is to have a method on the form that creates the controls you need, and have it check to see if it is the foreground thread. If not, it can marshal itself to the foreground. This can be done with any control that implements ISyncronizeInvoke.

private void Foo(object state)
{
    if (this.InvokeRequired)
    {
        this.Invoke(Foo, state);
        return;
    }
    // Do your creation of UI here...
}

Note that this example will block the background thread until the UI has some time to process the method.

Controls and Threads

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