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

I want to send temperature value from a microcontroller using UART to C# interface and Display temperature on Label.Content. Here is my microcontroller code:

while(1) {
   key_scan(); // get value of temp
   if (Usart_Data_Ready())
   {
      while(temperature[i]!=0)
      {
         if(temperature[i]!=' ')
         {
            Usart_Write(temperature[i]);
            Delay_ms(1000);
         }
         i = i + 1;
      }
      i =0;
      Delay_ms(2000);
   }
}

and my C# code is:

private void serialPort1_DataReceived(object sender, System.IO.Ports.SerialDataReceivedEventArgs e)
{
   txt += serialPort1.ReadExisting().ToString();
   textBox1.Text = txt.ToString();
}

but exception arises there "Cross-thread operation not valid: Control 'textBox1' accessed from a thread other than the thread it was created on" Please tell me how to get temperature string from my microcontroller and remove this Error!

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;

Use the following extensions and just pass the action like:

_frmx.PerformSafely(() => _frmx.Show());
_frmx.PerformSafely(() => _frmx.Location = new Point(x,y));

Extension class:

public static class CrossThreadExtensions
{
	public static void PerformSafely(this Control target, Action action)
	{
		if (target.InvokeRequired)
		{
			target.Invoke(action);
		}
		else
		{
			action();
		}
	}

	public static void PerformSafely<T1>(this Control target, Action<T1> action,T1 parameter)
	{
		if (target.InvokeRequired)
		{
			target.Invoke(action, parameter);
		}
		else
		{
			action(parameter);
		}
	}

	public static void PerformSafely<T1,T2>(this Control target, Action<T1,T2> action, T1 p1,T2 p2)
	{
		if (target.InvokeRequired)
		{
			target.Invoke(action, p1,p2);
		}
		else
		{
			action(p1,p2);
		}
	}
}

Along the same lines as previous answers, but a very short addition that Allows to use all Control properties without having cross thread invokation exception.

Helper Method

    /// <summary>
    /// Helper method to determin if invoke required, if so will rerun method on correct thread.
    /// if not do nothing.
    /// </summary>
    /// <param name="c">Control that might require invoking</param>
    /// <param name="a">action to preform on control thread if so.</param>
    /// <returns>true if invoke required</returns>
    public bool ControlInvokeRequired(Control c,Action a)
    {
        if (c.InvokeRequired) c.Invoke(new MethodInvoker(delegate { a(); }));
        else return false;

        return true;
    }

Sample Usage

    // usage on textbox
    public void UpdateTextBox1(String text)
    {
        //Check if invoke requied if so return - as i will be recalled in correct thread
        if (ControlInvokeRequired(textBox1, () => UpdateTextBox1(text))) return;
        textBox1.Text = ellapsed;
    }

    //Or any control
    public void UpdateControl(Color c,String s)
    {
        //Check if invoke requied if so return - as i will be recalled in correct thread
        if (ControlInvokeRequired(myControl, () => UpdateControl(c,s))) return;
        myControl.Text = s;
        myControl.BackColor = c;
    }

Use a shared container to transfer data between threads.