Question

I want to get a control’s property from a BackgroundWorker in my form:

foreach (ListViewItem i in ListView.CheckedItems) { //error: Cross-thread operation not valid: Control 'ListView' accessed from a thread other than the thread it was created on.
    //do something with i
}

Can anyone suggest the simplest and easiest method to do this?

Answer 1

Let me take another stab at this…

1.) Drag a ListView onto the Form

2.) Drag a BackgroundWorker onto the Form

3.) Create a method do iterate through the ListViewItem collection

private void LoopThroughListItems()
{
    foreach (ListViewItem i in listView1.CheckedItems)
        DoSomething(); 

}

4.) Add code to call LoopThroughListItems() inside the BackgroundWorker’s DoWork Event

private void backgroundWorker1_DoWork(object sender, DoWorkEventArgs e)
{
    LoopThroughListItems();
}

5.) In your Form Load – execute the code on the main thread (it works) then on the backgroundWorkder thread (it fails)

private void Form1_Load(object sender, EventArgs e)
{
    // Try it on the UI Thread - It works
    LoopThroughListItems();

    // Try it on a Background Thread - It fails
    backgroundWorker1.RunWorkerAsync();

}

6.) Modify your code to use IsInvokeRequired/Invoke

private void LoopThroughListItems()
{

    // InvokeRequired == True when executed by non-UI thread
    if (listView1.InvokeRequired)
    {
        // This will re-call LoopThroughListItems - on the UI Thread
        listView1.Invoke(new Action(LoopThroughListItems));
        return;
    }

    foreach (ListViewItem i in listView1.CheckedItems)
        DoSomething(); 
}

7.) Run the app again – now it works on the UI thread and the non-UI thread.

That solve the problem. The checking IsInvokeRequired/Invoking is a common pattern you’ll get used to a lot (which is why it’s included on all Controls). If you are doing it all over the place, you can do something clever and wrap it all up – as described here: Automating the InvokeRequired code pattern