private Rectangle GetTextBounds(TextBox textBox, int startPosition, int length)
{
  using (Graphics g = textBox.CreateGraphics())
  {
    g.TextRenderingHint = System.Drawing.Text.TextRenderingHint.AntiAlias;

    CharacterRange[] characterRanges = { new CharacterRange(startPosition, length) };
    StringFormat stringFormat = new StringFormat(StringFormat.GenericTypographic);
    stringFormat.SetMeasurableCharacterRanges(characterRanges);

    Region region = g.MeasureCharacterRanges(textBox.Text, textBox.Font,
                                             textBox.Bounds, stringFormat)[0];
    Rectangle bounds = Rectangle.Round(region.GetBounds(g));

    Point textOffset = textBox.GetPositionFromCharIndex(0);

    return new Rectangle(textBox.Margin.Left + bounds.Left + textOffset.X,
                         textBox.Margin.Top + textBox.Location.Y + textOffset.Y,
                         bounds.Width, bounds.Height);
  }
}