[DllImport("user32.dll", CharSet = CharSet.Auto)]
static extern IntPtr SendMessage(IntPtr hWnd, uint Msg,
    Int32 wParam, int[] lParam);
private const uint EM_SETTABSTOPS = 0xCB;

// Set tab stops inside a TextBox.
private void SetTextBoxTabs(TextBox txt, int[] tabs)
{
    SendMessage(txt.Handle, EM_SETTABSTOPS, tabs.Length, tabs);
}