Setting text in textbox through another class

Hi,

I am spawning a thread from Form1 which has a worker method in anther class(Class1). At the end of the worker method I want to set the text of a Text Box in the Form1 as the result of the computation of the worker method. Writing "TextBox1.Text = string" in worker method of Class1 doesnt work although the string has the right value in it.
Can someone please explain how to do this in simple steps ?

Thanks