Question

I’m trying to iterate through the selected values on a ListBox to see if I have a match, and then set a bool if I do. I’ve got this code that works when not threading:

for (int i = 0; i < FileTypesExcludedListBox.SelectedItems.Count; i++)
    if (currentFiles[currentFileLoc].EndsWith(FileTypesExcludedListBox.SelectedItems[i].ToString()))
        doNotCompare = true;

Now I’ve changed the app so this is happening inside a thread that didn’t create the control. I’ve used anonymous delegates to update text labels before, but I need the return value for this and I’m not sure how to do that…

Answer 1

1) You can Invoke calls to control on UI thread. Of course, you can pass array of objects as parameter. But it’s really easy to put some parameter on wrong place. I think, it’s better to use anonymous delegate instead.
Same thing for returning values from control. You just need to specify type of returned value in delegate signature.

public string ExcludedFileType
{
    get 
    {
        if (InvokeRequired)
            return (string)Invoke((Func<string>)delegate { return ExcludedFileType; });

        return (string)listBox1.SelectedItem; 
    }

    set
    {
        if (InvokeRequired)
        {
            Invoke((MethodInvoker)delegate { ExcludedFileType = value; });
            return;
        }

        listBox1.SelectedItem = value;
    }
}

Usage from worker thread:

ExcludedFileType = "jpg";
MessageBox.Show(ExcludedFileType);

2) You can provide parameters when starting thread.

// start async operation    
ThreadPool.QueueUserWorkItem(DoSomething, ExcludedFileTypes);

So, you will not need access to control from worker thread:

private void DoSomething(object state)
{
    IList<string> excluedeFileTypes = (IList<string>)state;

    foreach(string fileType in excluedeFileTypes)                
        if (currentFiles[currentFileLoc].EndsWith(fileType))
            doNotCompare = true;
}

private IList<string> ExcludedFileTypes
{
    get
    {
        List<string> filteTypes = new List<string>();
        foreach (var item in listBox1.SelectedItems)
            filteTypes.Add(item.ToString());
        return filteTypes;
    }
}

3) You can use AOP e.g. PostSharp to implement thread dispatching automatically.