Setting lpnFit and alpDx to null will get you the functionality of GetTextExtentPoint.

static extern bool GetTextExtentPoint32(IntPtr hdc, string lpString,

static extern bool GetTextExtentPoint32(IntPtr hdc, string lpString, int cbString, out SIZE lpSize);

    GDI.GetTextExtentPoint32(hdc, text, text.Length,out sz);

static extern bool GetTextExtentPoint32(IntPtr hdc, string lpString,

static extern bool GetTextExtentPoint32(IntPtr hdc, string lpString,

    GDI.GetTextExtentPoint32(hdc, text, text.Length,out sz);

   static extern bool GetTextExtentPoint(IntPtr hdc, string lpString,

       //Do the GetHdc(), GetTextExtentPoint, etc, here. Remember that you have

             GetTextExtentPoint(HDC, line, line.Length, ref MeasureSize);