using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Drawing;
using System.Data;
using System.Text;
using System.Windows.Forms;

namespace NumberedTextBox
{
    public partial class NumberedTextBoxUC : UserControl
    {

        public NumberedTextBoxUC()
        {
            InitializeComponent();
            numberLabel.Font = new Font(richTextBox1.Font.FontFamily, richTextBox1.Font.Size + 1.019f);
        }

        private void updateNumberLabel()
        {
            //得到第一个字符和第一行
            Point pos = new Point(0, 0);
            int firstIndex = richTextBox1.GetCharIndexFromPosition(pos);
            int firstLine = richTextBox1.GetLineFromCharIndex(firstIndex);

            //得到最后一个字符和最后一行
            pos.X = ClientRectangle.Width;
            pos.Y = ClientRectangle.Height;
            int lastIndex = richTextBox1.GetCharIndexFromPosition(pos);
            int lastLine = richTextBox1.GetLineFromCharIndex(lastIndex);

            //这是最后一个字符的位置，我们通过Y值计算字符长度
            pos = richTextBox1.GetPositionFromCharIndex(lastIndex);

           
            //在每一行前面加行号
            numberLabel.Text = "";
            for (int i = firstLine; i <= lastLine + 1; i++)
            {
                numberLabel.Text += i + 1 + "\n";
            }

        }


        private void richTextBox1_TextChanged(object sender, EventArgs e)
        {
            updateNumberLabel();
        }

        private void richTextBox1_VScroll(object sender, EventArgs e)
        {
            int d = richTextBox1.GetPositionFromCharIndex(0).Y % (richTextBox1.Font.Height + 1);
            numberLabel.Location = new Point(0, d);

            updateNumberLabel();
        }

        private void richTextBox1_Resize(object sender, EventArgs e)
        {
            richTextBox1_VScroll(null, null);
        }

        private void richTextBox1_FontChanged(object sender, EventArgs e)
        {
            updateNumberLabel();
            richTextBox1_VScroll(null, null);
        }
    }
}

------解决方案--------------------
原来写过相关的方法，后来用icsharp的editor控件去了，都忘记了

------解决方案--------------------
1楼的代码好强，帮你顶了