c# - Cross-thread operation not valid: Control 'progressBar1' accessed from a thread other than the thread it was created on

I couldn't find an answer for this:

"Cross-thread operation not valid: Control 'progressBar1' accessed

from a thread other than the thread it was created on."

This is my code:

        private void buttonStart_Click(object sender, EventArgs e)
        {
            ClassCopy cb = new ClassCopy();
            cb.startCopy(textBoxSrc.Text, textBoxDest.Text, true);


            th = new Thread(loading);
            th.Start();

        }

        private loading()
        {
            for (int i = 0; i < 100; i++)
            {
                if (progressBar1.InvokeRequired)

                    progressBar1.Invoke(new Action(loading));
                else
                    progressBar1.Value = i;
            }
        }