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Introduction

Numbering of lines in text editors is a well known feature. But the standardRichTextBox 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.

TheRichTextBox is not a standard control in Windows Forms. It does not use theOnPaint 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 ofRichTextBox, there is a satisfactory solution that does not use pure Win32 API functions.

Implementation

We implement theRichTextBox with numbered lines as aUserControl. We will not override anything in theRichTextBox, we will only use its events. OurUserControl namedNumberedTextBoxUC consists ofSplitContainer,Label (numberLabel) andRichTextBox.Label is used for displaying the line number andRichTextBox for the text content, both are contained inSplitterContainer.

The content ofnumberLabel is updated in theRichTextBox's event handlers. These events are:

• OnTextChanged
• OnVScroll
• OnSizeChanged
• OnFontChanged

Problems

There are several problems with this implementation. The first one is scrolling. Unlike the VS source code editor orTextBox 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 largeLabels, you cannot afford to print too many lines toLabel in eachOnTextChanged event handler. Another strange problem is theRichTextBox in .NET 2.0 uses strange line indentation, which is impossible to turn off or set to zero. The same font inLabel andRichTextBox results in different line positions when the controls are top aligned.

Solution

I am displaying only the numbers of visible lines, thus the unnecessary hidden line numbers are not printed. The update function is calledupdateNumberLabel(). It uses theRichTextBox functionsGetCharIndexFromPosition andGetLineFromCharIndex to determine the first and last visible line numbers.

privatevoid 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 theLabel 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(){    InitializeComponent();    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 smallnumberLabel location update in eachOnVScroll event handler. TheLabel is moved about as many pixels different from multiples of theRichTextBox 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 withRichTextBox functions in this way results in text shivering.

privatevoid richTextBox1_VScroll(object sender, EventArgs e){//move location of numberLabel for amount//of pixels caused by scrollbarint d = richTextBox1.GetPositionFromCharIndex(0).Y %                               (richTextBox1.Font.Height +1);    numberLabel.Location =new Point(0, d);    updateNumberLabel();}

Conclusion

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

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