Cross-thread operation not valid: Control 'textBox1' accessed from a thread other than the thread it was created on [duplicate]

The data received in your serialPort1_DataReceived method is coming from another thread context than the UI thread, and that's the reason you see this error.
To remedy this, you will have to use a dispatcher as descibed in the MSDN article:
How to: Make Thread-Safe Calls to Windows Forms Controls

So instead of setting the text property directly in the serialport1_DataReceived method, use this pattern:

delegate void SetTextCallback(string text);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;  }}

So in your case:

private void serialPort1_DataReceived(object sender, System.IO.Ports.SerialDataReceivedEventArgs e){  txt += serialPort1.ReadExisting().ToString();  SetText(txt.ToString());}

I don't know if this is good enough but I made a static ThreadHelperClass class and implemented it as following .Now I can easily set text property of various controls without much coding .

public static class ThreadHelperClass{    delegate void SetTextCallback(Form f, Control ctrl, string text);    /// <summary>    /// Set text property of various controls    /// </summary>    /// <param name="form">The calling form</param>    /// <param name="ctrl"></param>    /// <param name="text"></param>    public static void SetText(Form form, Control ctrl, 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 (ctrl.InvokeRequired)        {            SetTextCallback d = new SetTextCallback(SetText);            form.Invoke(d, new object[] { form, ctrl, text });        }        else        {            ctrl.Text = text;        }    }}

Using the code:

 private void btnTestThread_Click(object sender, EventArgs e) {    Thread demoThread =       new Thread(new ThreadStart(this.ThreadProcSafe));            demoThread.Start(); } // This method is executed on the worker thread and makes  // a thread-safe call on the TextBox control.  private void ThreadProcSafe() {     ThreadHelperClass.SetText(this, textBox1, "This text was set safely.");     ThreadHelperClass.SetText(this, textBox2, "another text was set safely."); }

you can simply do this.

TextBox.CheckForIllegalCrossThreadCalls = false;