MeasureString in RichTextBox Problem

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C#

WinForms

VSTO

I need to get the last line of a rtb when the vertical scroolbar becomes visible.

WordWrap is true

I allready have a methode which tells me when the scroolbar becomes visible.

I measured the string of the last line and compared it to the width > everything which is bigger is in the last line. My Problem is, that when it reaches about the middle, it says it allready reached the end.

Heres my code:

float sum = 0;
int zeichen = 0;
Graphics g = rtb.CreateGraphics();
g.PageUnit = GraphicsUnit.Pixel;
for (int i = 0; i < rtb.Lines[rtb.Lines.Length - 1].Length && sum < rtb.Width; i++)
{
    zeichen = i;
    rtb.Select(rtb.GetFirstCharIndexFromLine(rtb.Lines.Length - 1) + i, 1);
    sum = sum + g.MeasureString(rtb.SelectedText, rtb.SelectionFont).Width;
}
string s = rtb.Text.Substring(zeichen);
rtb.Text = rtb.Text.Substring(0, zeichen);

SelectionFont = new System.Drawing.Font("Agency FB", 10)

rtb.width = 605

Thanks :)