Creating Line Numbers for RichTextBox in C#

I am using AddLineNumbers() function to call anywhere in the program.

I am also using the getWidth() function that returns the specific width according to number of lines for auto resizing the line numbers width.

Here's code of AddLineNumbers() function:

public void AddLineNumbers()
{
    // create & set Point pt to (0,0)
    Point pt = new Point(0, 0);
    // get First Index & First Line from richTextBox1
    int First_Index = richTextBox1.GetCharIndexFromPosition(pt);
    int First_Line = richTextBox1.GetLineFromCharIndex(First_Index);
    // set X & Y coordinates of Point pt to ClientRectangle Width & Height respectively
    pt.X = ClientRectangle.Width;
    pt.Y = ClientRectangle.Height;
    // get Last Index & Last Line from richTextBox1
    int Last_Index = richTextBox1.GetCharIndexFromPosition(pt);
    int Last_Line = richTextBox1.GetLineFromCharIndex(Last_Index);
    // set Center alignment to LineNumberTextBox
    LineNumberTextBox.SelectionAlignment = HorizontalAlignment.Center;
    // set LineNumberTextBox text to null & width to getWidth() function value
    LineNumberTextBox.Text = "";
    LineNumberTextBox.Width = getWidth();
    // now add each line number to LineNumberTextBox upto last line
    for (int i = First_Line; i <= Last_Line + 2; i++)
    {
        LineNumberTextBox.Text += i + 1 + "\n";
    }
}

Procedure

Step 1: Start Visual Studio & create new Windows Forms Application in C#.

Step 2: Drag & Drop RichTextBox onto Form1 & Set following properties to it.

Name	LineNumberTextBox
BackColor	White
BorderStyle	None
Cursor	PanNE
Dock	Left
ForeColor	Black
ReadOnly	true
ScrollBars	None

You can set BackColor & ForeColor to LineNumberTextBox as you wish.

Here you can also set Enabled property to false, but you can not see ForeColor & BackColor.

For ForeColor & BackColor we are set as above properties to LineNumberTextBox.

Step 3: Drag & Drop RichTextBox onto Form1 & Set Dock property to Fill. T he default name could be richTextBox1.

Step 4: Add above AddLineNumbers() function to your Form1 code or see Complete Code next.

Step 5: Add following events & code to Form1.

• Load:
  

  private void Form1_Load(object sender, EventArgs e)
  {
      LineNumberTextBox.Font = richTextBox1.Font;
      richTextBox1.Select();
      AddLineNumbers();
  }

• Resize
  

  private void Form1_Resize(object sender, EventArgs e)
  {
      AddLineNumbers();
  }

Step 6: Add following events & code to richTextBox1.

1. SelectionChanged
   

   private void richTextBox1_SelectionChanged(object sender, EventArgs e)
   {
       Point pt = richTextBox1.GetPositionFromCharIndex(richTextBox1.SelectionStart);
       if (pt.X == 1)
       {
           AddLineNumbers();
       }
   }

2. VScroll
   

   private void richTextBox1_VScroll(object sender, EventArgs e)
   {
       LineNumberTextBox.Text = "";
       AddLineNumbers();
       LineNumberTextBox.Invalidate();
   }

3. TextChanged
   

   private void richTextBox1_TextChanged(object sender, EventArgs e)
   {
       if (richTextBox1.Text == "")
       {
           AddLineNumbers();
       }
   }

4. FontChanged
   

   private void richTextBox1_FontChanged(object sender, EventArgs e)
   {
       LineNumberTextBox.Font = richTextBox1.Font;
       richTextBox1.Select();
       AddLineNumbers();
   }

Step 7: Add following events & code to LineNumberTextBox.

• MouseDown
  

  private void LineNumberTextBox_MouseDown(object sender, MouseEventArgs e)
  {
      richTextBox1.Select();
      LineNumberTextBox.DeselectAll();
  }

Step 8: Debug your Form & check it out.

Complete Code

using System;
using System.Drawing;
using System.Windows.Forms;

namespace LineNumbers_CSharp
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        public int getWidth()
        {
            int w = 25;
            // get total lines of richTextBox1
            int line = richTextBox1.Lines.Length;

            if (line <= 99)
            {
                w = 20 + (int)richTextBox1.Font.Size;
            }
            else if (line <= 999)
            {
                w = 30 + (int)richTextBox1.Font.Size;
            }
            else
            {
                w = 50 + (int)richTextBox1.Font.Size;
            }

            return w;
        }

        public void AddLineNumbers()
        {
            // create & set Point pt to (0,0)
            Point pt = new Point(0, 0);
            // get First Index & First Line from richTextBox1
            int First_Index = richTextBox1.GetCharIndexFromPosition(pt);
            int First_Line = richTextBox1.GetLineFromCharIndex(First_Index);
            // set X & Y coordinates of Point pt to ClientRectangle Width & Height respectively
            pt.X = ClientRectangle.Width;
            pt.Y = ClientRectangle.Height;
            // get Last Index & Last Line from richTextBox1
            int Last_Index = richTextBox1.GetCharIndexFromPosition(pt);
            int Last_Line = richTextBox1.GetLineFromCharIndex(Last_Index);
            // set Center alignment to LineNumberTextBox
            LineNumberTextBox.SelectionAlignment = HorizontalAlignment.Center;
            // set LineNumberTextBox text to null & width to getWidth() function value
            LineNumberTextBox.Text = "";
            LineNumberTextBox.Width = getWidth();
            // now add each line number to LineNumberTextBox upto last line
            for (int i = First_Line; i <= Last_Line + 2; i++)
            {
                LineNumberTextBox.Text += i + 1 + "\n";
            }
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            LineNumberTextBox.Font = richTextBox1.Font;
            richTextBox1.Select();
            AddLineNumbers();
        }

        private void richTextBox1_SelectionChanged(object sender, EventArgs e)
        {
            Point pt = richTextBox1.GetPositionFromCharIndex(richTextBox1.SelectionStart);
            if (pt.X == 1)
            {
                AddLineNumbers();
            }
        }

        private void richTextBox1_VScroll(object sender, EventArgs e)
        {
            LineNumberTextBox.Text = "";
            AddLineNumbers();
            LineNumberTextBox.Invalidate();
        }

        private void richTextBox1_TextChanged(object sender, EventArgs e)
        {
            if (richTextBox1.Text == "")
            {
                AddLineNumbers();
            }
        }

        private void richTextBox1_FontChanged(object sender, EventArgs e)
        {
            LineNumberTextBox.Font = richTextBox1.Font;
            richTextBox1.Select();
            AddLineNumbers();
        }

        private void LineNumberTextBox_MouseDown(object sender, MouseEventArgs e)
        {
            richTextBox1.Select();
            LineNumberTextBox.DeselectAll();
        }

        private void Form1_Resize(object sender, EventArgs e)
        {
            AddLineNumbers();
        }

    }
}