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Posted 3 Nov 2005

Numbering lines of RichTextBox in .NET 2.0

, 3 Nov 2005
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The standard RichTextBox does not allow numbering of lines. This user control does.


Numbering of lines in text editors is a well known feature. But the standard RichTextBox in .NET 2.0 does not support this feature. It is also hard to find a suitable solution on the Internet. Especially, a solution that does not directly use Win32 functions.

The RichTextBox is not a standard control in Windows Forms. It does not use the OnPaint method and other similar functions as it should and it also hides some of the important properties required for proper customization. One way to overcome this is to use win32 API functions and override the WndProc functions. I don't like this way of doing it but I am forced to use it again and again. I consider it as a fault of .NET developers.

Fortunately, for numbering lines of RichTextBox, there is a satisfactory solution that does not use pure Win32 API functions.


We implement the RichTextBox with numbered lines as a UserControl. We will not override anything in the RichTextBox, we will only use its events. Our UserControl named NumberedTextBoxUC consists of SplitContainer, Label (numberLabel) and RichTextBox. Label is used for displaying the line number and RichTextBox for the text content, both are contained in SplitterContainer.

The content of numberLabel is updated in the RichTextBox's event handlers. These events are:

  • OnTextChanged
  • OnVScroll
  • OnSizeChanged
  • OnFontChanged


There are several problems with this implementation. The first one is scrolling. Unlike the VS source code editor or TextBox control, RichTextBox uses smooth scrolling, thus scrolling with the scrollbar scrolls the text in pixels, not in lines. You will notice that the first line is displayed in half. This is not always a wanted behaviour and I would appreciate the possibility to turn it off. Another problem is the redrawing speed of large Labels, you cannot afford to print too many lines to Label in each OnTextChanged event handler. Another strange problem is the RichTextBox in .NET 2.0 uses strange line indentation, which is impossible to turn off or set to zero. The same font in Label and RichTextBox results in different line positions when the controls are top aligned.


I am displaying only the numbers of visible lines, thus the unnecessary hidden line numbers are not printed. The update function is called updateNumberLabel(). It uses the RichTextBox functions GetCharIndexFromPosition and GetLineFromCharIndex to determine the first and last visible line numbers.

private void updateNumberLabel()
    //we get index of first visible char and 
    //number of first visible line
    Point pos = new Point(0, 0);
    int firstIndex = richTextBox1.GetCharIndexFromPosition(pos);
    int firstLine = richTextBox1.GetLineFromCharIndex(firstIndex);

    //now we get index of last visible char 
    //and number of last visible line
    pos.X = ClientRectangle.Width;
    pos.Y = ClientRectangle.Height;
    int lastIndex = richTextBox1.GetCharIndexFromPosition(pos);
    int lastLine = richTextBox1.GetLineFromCharIndex(lastIndex);

    //this is point position of last visible char, we'll 
    //use its Y value for calculating numberLabel size
    pos = richTextBox1.GetPositionFromCharIndex(lastIndex);

    //finally, renumber label
    numberLabel.Text = "";
    for (int i = firstLine; i <= lastLine + 1; i++)
        numberLabel.Text += i + 1 + "\n";


For different line indentations I have found a constant, which works best for font size 8. The size of the Label font is bigger for this constant. I hope, in future versions of .NET this issue will be fixed and both the fonts will be exactly the same, as it was in .NET 1.1.

public NumberedTextBoxUC()

    numberLabel.Font = new Font(richTextBox1.Font.FontFamily, 
                              richTextBox1.Font.Size + 1.019f);

Smooth scrolling is another issue which causes a lot of troubles. I use the small numberLabel location update in each OnVScroll event handler. The Label is moved about as many pixels different from multiples of the RichTextBox font height, thus the modulo text position of the font height. The reverse solution, to update the text position to be line aligned, is in my opinion impossible without using Win32 functions. Updating the text position with RichTextBox functions in this way results in text shivering.

private void richTextBox1_VScroll(object sender, EventArgs e)
    //move location of numberLabel for amount 
    //of pixels caused by scrollbar
    int d = richTextBox1.GetPositionFromCharIndex(0).Y % 
                              (richTextBox1.Font.Height + 1);
    numberLabel.Location = new Point(0, d);



I hope this user control helps developers handling RichTextBox. I appreciate your advice and improvements to this control.


This article has no explicit license attached to it but may contain usage terms in the article text or the download files themselves. If in doubt please contact the author via the discussion board below.

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About the Author

Petr Minarik
Web Developer
Czech Republic Czech Republic
Petr Minarik is currently studying at Czech Technical University Prague. He is interested in C# programming, Direct3D and graphics generally and he develops his own realtime 3D scene editor. He likes fun, beer and good people.

You can visit him at his homepage.

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