Question

I already have this code but it gives me the wrong result.

    private void document_PrintPage(object sender, System.Drawing.Printing.PrintPageEventArgs e)
    {
    int charPerLine = e.MarginBounds.Width / (int)e.Graphics.MeasureString("m", txtMain.Font).Width;
    }

The txtMain is a textbox.

Answer 1

This should do the trick. Be careful when dividing by a variable cast to an integer. You are leaving yourself open to a divide-by-zero here in the event that the Width property is less than one, which will be truncated to zero. It may be unlikely that you will have such a small font in your application, but it is still good practice.

private void document_PrintPage(object sender, System.Drawing.Printing.PrintPageEventArgs e)
{
    if( (int)e.Graphics.MeasureString("m", txtMain.Font).Width > 0 )
    {

        int charPerLine = 
            e.MarginBounds.Width / (int)e.Graphics.MeasureString("m", txtMain.Font).Width;
    }
}

The real issue though is why do you even need to know the number of characters per line. Unless you are trying to do some sort of ASCII art, you can use the different overloads of Graphics.DrawString to have GDI+ layout the text for you inside a bounding rectangle without needing to know how many characters fit on a line.

This sample from MSDN shows you how to do this:

// Create string to draw.
String drawString = "Sample Text";

// Create font and brush.
Font drawFont = new Font("Arial", 16);
SolidBrush drawBrush = new SolidBrush(Color.Black);

// Create rectangle for drawing.
float x = 150.0F;
float y = 150.0F;
float width = 200.0F;
float height = 50.0F;
RectangleF drawRect = new RectangleF(x, y, width, height);

// Draw rectangle to screen.
Pen blackPen = new Pen(Color.Black);
e.Graphics.DrawRectangle(blackPen, x, y, width, height);

// Set format of string.
StringFormat drawFormat = new StringFormat();
drawFormat.Alignment = StringAlignment.Center;

// Draw string to screen.
e.Graphics.DrawString(drawString, drawFont, drawBrush, drawRect, drawFormat);

So if you are trying to print a page of text, you can just set the drawRect to the e.MarginBounds and plug a page worth of text in for drawString.

Another thing, if you are trying to print tabular data, you can just partition the page into rectangles – one for each column/row (however you need it), and use e.Graphics.DrawLine overloads to print the table borders.

If you post more details on what you are actually trying to achieve we can help more.