textbox is blinking when i update textbox bt a timer.

private void timer1_Tick(object sender, EventArgs e)
{
    if (count < lk.Length)
    {
        if (lk[count] == 'r' && count + 1 < lk.Length && lk[count + 1] == 'n')
        {
            count += 2; 
        }
        else
        {
            lk[count] = '_';
            count++;
        }

        MainText.Text = lk.ToString();

        MainText.SelectionStart = count; 
        MainText.ScrollToCaret();
    }
    else
    {
        timer1.Stop();
        count = 0;
    }
}

i tried to ask chatgpt, but it doesnt work.
im expecting to fix blinkinfg problem