Form2.cs

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

namespace 记事本窗口
{
    public partial class Form2 : Form
    {
        public Form2()
        {
            InitializeComponent();
        }

        public RichTextBox richtextbox;
        public int start = 0;

        private void button1_Click(object sender, EventArgs e)
        {
            richtextbox.SelectionColor = Color.Blue;   //显示为蓝色
            string str;
            str = textBox1.Text;
            if (checkBox1.Checked)
            {
                if (radioButton2.Checked)
                    checkUp(str);
                else
                    checkDown(str);
            }
            else 
            {
                if (radioButton1.Checked)
                    uncheckDown(str);
                else
                    uncheckUp(str);
            }
        }

        private void button2_Click(object sender, EventArgs e)
        {
            this.Close();
        }

        public void checkDown(string ss)       //区分大小写向下查找
        {
            int c = 0;
            int b = 0;

            try
            {
                c = richtextbox.SelectionStart;
                b = richtextbox.Text.IndexOf(ss, c + ss.Length, StringComparison.CurrentCulture);
                richtextbox.SelectionStart = b;
                richtextbox.SelectionLength = ss.Length;
                richtextbox.SelectionColor = Color.Red;
            }
            catch
            {
                MessageBox.Show("以查找到文档的结尾", "查找结束对话框",
                    MessageBoxButtons.OK);
                this.textBox1.SelectionStart = c;
                this.textBox1.SelectionLength = ss.Length;
            }
        }

        public void checkUp(string ss)         //区分大小写向上查找
        {
            int c = 0;
            int b = 0;

            try
            {
                c = richtextbox.SelectionStart;
                b = richtextbox.Text.LastIndexOf(ss, c - ss.Length, StringComparison.InvariantCulture);
                richtextbox.SelectionStart = b;
                richtextbox.SelectionLength = ss.Length;
                richtextbox.SelectionColor = Color.Red;
            }
            catch
            {
                MessageBox.Show("已查找到文档的结尾", "查找结束对话框",
                    MessageBoxButtons.OK);
                richtextbox.SelectionStart = c;
                richtextbox.SelectionLength = ss.Length;
            }
        }

        public void uncheckDown(string ss)           //不区分大小写向下查找
        {
            int c = 0;
            int b = 0;

            try
            {
                c = richtextbox.SelectionStart;
                b = richtextbox.Text.IndexOf(ss, c + ss.Length,
                    StringComparison.CurrentCultureIgnoreCase);
                richtextbox.SelectionLength = ss.Length;
                richtextbox.SelectionColor = Color.Red;
            }
            catch
            {
                MessageBox.Show("已查找到文档的结尾", "查找结束对话框",
                    MessageBoxButtons.OK);
                richtextbox.SelectionStart = c;
                richtextbox.SelectionLength = ss.Length;
            }
        }

        public void uncheckUp(string ss)      //不区分大小写向上查找
        {
            int c = 0;
            int b = 0;

            try
            {
                c = richtextbox.SelectionStart;
                b = richtextbox.Text.LastIndexOf(ss, c - ss.Length,
                    StringComparison.InvariantCultureIgnoreCase);
                richtextbox.SelectionStart = b;
                richtextbox.SelectionLength = ss.Length;
                richtextbox.SelectionColor = Color.Red;
            }
            catch
            {
                MessageBox.Show("已查找到文档的结尾", "查找结束对话框",
                    MessageBoxButtons.OK);
                richtextbox.SelectionStart = c;
                richtextbox.SelectionLength = ss.Length;
            }
        }
    }
}