Q:

Updating a listbox from worker thread?

I have found an article on this subject but I can't quite get my head around how to implement the answer proposed.
What I am getting is a cross-thread exception and I realise that the GUI is in one thread and the worker is in another thread. The exception:
System.InvalidOperationException was unhandled by user code
  HResult=-2146233079
  Message=Cross-thread operation not valid: Control 'listBoxCodes' accessed from a thread other than the thread it was created on.
  Source=System.Windows.Forms
  StackTrace:
       at System.Windows.Forms.Control.get_Handle()
       at System.Windows.Forms.Control.SendMessage(Int32 msg, Int32 wparam, String lparam)
       at System.Windows.Forms.ListBox.NativeInsert(Int32 index, Object item)
       at System.Windows.Forms.ListBox.ObjectCollection.AddInternal(Object item)
       at System.Windows.Forms.ListBox.ObjectCollection.Add(Object item)
       at Find_Duplicate_MX_codes.MyThread.backgroundWorker_DoWork(Object sender, DoWorkEventArgs e) in D:\My Programs\Find Duplicate MX codes\Find Duplicate MX codes\MyThread.cs:line 69
       at System.ComponentModel.BackgroundWorker.OnDoWork(DoWorkEventArgs e)
       at System.ComponentModel.BackgroundWorker.WorkerThreadStart(Object argument)
  InnerException: 

Now, I was trying to initially to deal with this by writing my code as follows:
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.IO;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace Find_Duplicate_MX_codes
{
    public struct ThreadSettings
    {
        public Find_Duplicate_MX_codes.Form1 pMainForm { get; set; }
        public ProgressBar progressBar { get; set; }
        public TextBox progressLabel { get; set; }
        public String strFile { get; set; }
        public ListBox lbCodes { get; set; }
        public ListBox lbCodesDuplicate { get; set; }
    }

    class MyThread
    {
        private BackgroundWorker m_backgroundWorker;
        ThreadSettings m_sThreadSettings;

        public MyThread(ThreadSettings sThreadSettings)
        {
            m_sThreadSettings = sThreadSettings;

            m_backgroundWorker = new BackgroundWorker();
            m_backgroundWorker.DoWork += new DoWorkEventHandler(backgroundWorker_DoWork);
            m_backgroundWorker.ProgressChanged += new ProgressChangedEventHandler(backgroundWorker_ProgressChanged);
            m_backgroundWorker.RunWorkerCompleted += new RunWorkerCompletedEventHandler(backgroundWorker_RunWorkerCompleted);
            m_backgroundWorker.WorkerReportsProgress = true;
        }

        public void start()
        {
            if(m_sThreadSettings.pMainForm != null)
                m_sThreadSettings.pMainForm.Enabled = false; // Stop user interacting with the form
            m_backgroundWorker.RunWorkerAsync();
        }

        public void stop()
        {
            m_backgroundWorker.CancelAsync();
        }

        public void backgroundWorker_DoWork(object sender, DoWorkEventArgs e)
        {
            m_backgroundWorker.ReportProgress(0, "Extracting MX codes {0}%)");

            using (var reader = new StreamReader(m_sThreadSettings.strFile))
            {
                Stream baseStream = reader.BaseStream;
                long length = baseStream.Length;

                string line;
                while ((line = reader.ReadLine()) != null)
                {
                    if (line.Length > 8 && line.Substring(0, 4) == "080,")
                    {
                        string strCode = line.Substring(4, 4);

                        if (m_sThreadSettings.lbCodes.FindStringExact(strCode) == -1)
                        {
                            m_sThreadSettings.lbCodes.Items.Add(strCode);
                        }
                        else
                            m_sThreadSettings.lbCodesDuplicate.Items.Add(strCode);
                    }

                    m_backgroundWorker.ReportProgress(Convert.ToInt32(baseStream.Position / length * 100), "Extracting MX codes {0}%)");
                }
            }
        }

        private void backgroundWorker_ProgressChanged(object sender, ProgressChangedEventArgs e)
        {
            if(m_sThreadSettings.progressBar != null)
                m_sThreadSettings.progressBar.Value = e.ProgressPercentage;
            if(m_sThreadSettings.progressLabel != null)
                m_sThreadSettings.progressLabel.Text = String.Format((string)e.UserState, e.ProgressPercentage);
        }

        private void backgroundWorker_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
        {
            int iResult = Convert.ToInt32(e.Result);
            if (m_sThreadSettings.pMainForm != null)
                m_sThreadSettings.pMainForm.Enabled = true; // User can interact with form again
        }
    }
}

If I comment out the line of code that tries to add an item to the list box is works. Progress bar/label are updating. It is when I try to update the listbox that it fails.
In the answer on the other post it suggests:
UserContrl1_LOadDataMethod()
{
    if(textbox1.text=="MyName") //<<======Now it wont give exception**
    {
        //Load data correspondin to "MyName"
        //Populate a globale variable List<string> which will be
        //bound to grid at some later stage
        if(InvokeRequired)
        {
            // after we've done all the processing, 
            this.Invoke(new MethodInvoker(delegate {
                // load the control with the appropriate data
            }));
            return;
        }
    }
}

But I am struggling to understand how to make use of that answer for my lines of code:
m_sThreadSettings.lbCodes.Items.Add(strCode);

How do I change it so that I can get the list boxes populated and:
a) Not get the cross-thread exception
b) Not choke the GUI due to the updating of the list boxes
Thank you!
Update: I have tried this code:
Invoke(new MethodInvoker(delegate { m_sThreadSettings.lbCodes.Items.Add(strCode); }));

But I get warnings that I don't fully understand:

Update 2: I now realise that Invoke is part of the Form object. So I changed my code to:
if (m_sThreadSettings.lbCodes.FindStringExact(strCode) == -1)
    m_sThreadSettings.pMainForm.Invoke(new MethodInvoker(delegate { m_sThreadSettings.lbCodes.Items.Add(strCode); }));
else
    m_sThreadSettings.pMainForm.Invoke(new MethodInvoker(delegate { m_sThreadSettings.lbCodesDuplicate.Items.Add(strCode); }));

That now seems to work. But watch this animation:
https://www.dropbox.com/s/mznoqhqll8m28x7/Results0001.mp4?dl=0
It just doesn't all process quite as I expect. It seems to still be doing all the processing  of the GUI once the reading has finished. Confused.
Update 3: I used Console.Beep() to establish when my progress change handler was being fired. In addition, I confirmed when the progress actually changes. And it does 0 for all apart from towards the end when it does 100. So this BaseSteam.Position might be the fault.

A:

There turned out to be unrelated issues with my code.
ISSUE 1
This is how to update the list box from a worker thread:
public void backgroundWorker_DoWork(object sender, DoWorkEventArgs e)
{
    m_backgroundWorker.ReportProgress(0, "Extracting MX codes {0}%");

    using (var reader = new StreamReader(m_sThreadSettings.strFile))
    {
        Stream baseStream = reader.BaseStream;
        long length = baseStream.Length;

        string line;
        while ((line = reader.ReadLine()) != null)
        {
            if (line.Length > 8 && line.Substring(0, 4) == "080,")
            {
                string strCode = line.Substring(4, 4);

                if (m_sThreadSettings.lbCodes.FindStringExact(strCode) == -1)
                    m_sThreadSettings.pMainForm.Invoke(new MethodInvoker(delegate { m_sThreadSettings.lbCodes.Items.Add(strCode); }));
                else
                    m_sThreadSettings.pMainForm.Invoke(new MethodInvoker(delegate { m_sThreadSettings.lbCodesDuplicate.Items.Add(strCode); }));
            }

            m_backgroundWorker.ReportProgress(Convert.ToInt32((100 / (double)length) * baseStream.Position), "Extracting MX codes {0}%");
        }
    }
}

You have to use the Form's Invoke method.
ISSUE 2
The logic for calculating the progress needed to be adjusted because in this context the result was always 0 due to the way the calculation worked. So I now use:
m_backgroundWorker.ReportProgress(
      Convert.ToInt32((100 / (double)length) * baseStream.Position),
      "Extracting MX codes {0}%");

Voila! My form list b ox content is getting update and the progress bar is being updated when expected ... :)

