cross thread operation is not valid problem

You can not access form controls from another thread directly, there is ways of getting it done. The best way that I use looks like follow:
Create a delegate that you can point to the method to invoke it if needed:
private delegate void TextboxString(string TextToDisplay);
Now create the method that you will call from within the thread that should give the textbox the display string:
private void SetTextboxDisplay(string TextToDisplay)
{
      if(yourTextboxName.InvokeRequired)
      {
         TextboxString textboxString = new TextboxString(SetTextboxDisplay);
            this.Invoke(textboxString, new object[]{TextToDisplay});

      }

     else
      {
            yourTextboxName.Text = TextToDisplay;
      }
}
 
So nou you just call this method within the trhead that must change your textbox text with the parameter of what the text should be
SetTextboxDisplay("The text you want in the textbox");