How to fix my Background solution to make thread-safe calls?

Question

I tried to set Text property of TextBox from another thread. I got this exception below;

"Cross-thread operation not valid: Control 'recTpcTxt' accessed from a thread other than the thread it was created on."

Then, I used BackgroundWorker to solve this issue. However, I faced with the same exception message.

EDIT[1]: Actually, I take a guide myself this link ; https://msdn.microsoft.com/en-us/library/ms171728(v=vs.110).aspx. I can solve my problem by using invokeproperty. However, I cannot solve my problem with backgroundworker.

Is there something wrong in my solution? How do I fix my solution to set some property of UI variable?

EDIT[2]: More code to clarify the issue;

MqttManager.cs;

public partial class MqttManager : Form
    {
        MqttHandler mqttHandler = new MqttHandler();
        public static MqttManager managerInst;

        public MqttManager()
        {
            InitializeComponent();
            managerInst = this;
            ...
        }

        ...

        private BackgroundWorker backgroundWorker;

        public void NotifyUIForRecMsg(string topic, string message)
        {
            object[] objArr = { topic, message };
            this.backgroundWorker.RunWorkerAsync(objArr);
        }

        private void backgroundWorker_DoWork(object sender, DoWorkEventArgs e)
        {
            System.Threading.Thread.Sleep(5000);
            e.Result = e.Argument;
        }

        private void backgroundWorker_RunWorkerCompleted(
            object sender,
            RunWorkerCompletedEventArgs e)
        {
            object[] res = (object[])e.Result;
            this.recTpcTxt.Text = (String)res[0];
        }
    }

MqttManager.Design.cs;

partial class MqttManager
    {
        /// <summary>
        /// Required designer variable.
        /// </summary>
        private System.ComponentModel.IContainer components = null;

        /// <summary>
        /// Clean up any resources being used.
        /// </summary>
        /// <param name="disposing">true if managed resources should be disposed; otherwise, false.</param>
        protected override void Dispose(bool disposing)
        {
            if (disposing && (components != null))
            {
                components.Dispose();
            }
            base.Dispose(disposing);
        }

        #region Windows Form Designer generated code

        /// <summary>
        /// Required method for Designer support - do not modify
        /// the contents of this method with the code editor.
        /// </summary>
        private void InitializeComponent()
        {
            ...

            this.backgroundWorker = new System.ComponentModel.BackgroundWorker();
            this.backgroundWorker.DoWork += new System.ComponentModel.DoWorkEventHandler(this.backgroundWorker_DoWork);
            this.backgroundWorker.RunWorkerCompleted += new System.ComponentModel.RunWorkerCompletedEventHandler(this.backgroundWorker_RunWorkerCompleted);
        }
        #endregion

        ...
     }

MqttHandler.cs;

class MqttHandler
    {
        MqttClient client;

        ...

        /// <summary>
        /// Publish received event handler.
        /// </summary>
        private void client_MqttMsgPublishReceived(Object sender, MqttMsgPublishEventArgs e)
        {
            MqttManager.managerInst.NotifyUIForRecMsg(e.Topic, Encoding.UTF8.GetString(e.Message));
        }
     }

Answer 1

check this: https://msdn.microsoft.com/en-us/library/ms171728(v=vs.110).aspx

Basically, to set a control propertiy you have to be in the same UI thread.

This simple solution move the call to textbox1.Text = someText in the UI thread

private void SetText(string text)
{
    // InvokeRequired required compares the thread ID of the
    // calling thread to the thread ID of the creating thread.
    // If these threads are different, it returns true.
    if (this.textBox1.InvokeRequired)
    {   
        SetTextCallback d = new SetTextCallback(SetText);
        this.Invoke(d, new object[] { text });
    }
    else
    {
        this.textBox1.Text = text;
    }
}

also, you can use textBox1.BeginInvoke instead of Invoke: it will run in UI thread, without locking the caller thread waiting for SetText delegate to be completed

[Edit] to do it in your backgroundWorker:

private void backgroundWorker_DoWork(object sender, DoWorkEventArgs e)
{
    object[] arg = (object[])e.Argument;
    SetTextToTextBox(recTpcTxt, (string)arg[0]);
    SetTextToTextBox(recMsgTxt, (string)arg[1]);
}

private void SetTextToTextBox(TextBox toSet, string text)
{
    // InvokeRequired required compares the thread ID of the
    // calling thread to the thread ID of the creating thread.
    // If these threads are different, it returns true.
    if (toSet.InvokeRequired)
    {   
        SetTextCallback d = new SetTextCallback(SetText);
        toSet.Invoke(d, new object[] { text });
    }
    else
    {
        toSet.Text = text;
    }
}

[Edit 2] To properly use backgroundworker

Register for events DoWork and RunWorkerCompleted

this.backgroundWorker1.DoWork += new System.ComponentModel.DoWorkEventHandler(this.backgroundWorker1_DoWork);
this.backgroundWorker1.RunWorkerCompleted += new System.ComponentModel.RunWorkerCompletedEventHandler(this.backgroundWorker1_RunWorkerCompleted);

Before exiting backgroundWorker1_DoWork, set result property of eventArgs, and read them in backgroundWorker1_RunWorkerCompleted

private void backgroundWorker1_DoWork(object sender, DoWorkEventArgs e)
{
    System.Threading.Thread.Sleep(5000);
    e.Result = new string[] { "one", "two" };
}
private void backgroundWorker1_RunWorkerCompleted(
    object sender,
    RunWorkerCompletedEventArgs e)
{
    string[] res = (string[])e.Result;
    this.textBox1.Text = res[0];
}