C#中怎样从指定字符串中查找并替换字符串?

using system;
using system.collections.generic;
using system.componentmodel;
using system.data;
using system.drawing;
using system.linq;
using system.text;
using system.windows.forms;
#region             
#endregion
namespace find
{
    public partial class form1 : form
    {
        public form1()
        {
            initializecomponent();
            string str = "";
            richtextbox1.text = str;
        }
        int start = 0;
        int count = 0;
        /// <summary>
        /// 查找字符串
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>

        private void find_click(object sender, eventargs e)
        {
            string str1;
            str1 = txt_find.text;
            if (start >= richtextbox1.text.length)
            {
                messagebox.show("以查找到尾部");
                start = 0;
            }
            else
            {
                start = richtextbox1.find(str1, start, richtextboxfinds.matchcase);
                if (start == -1)
                {
                    if (count == 0)
                    {
                        messagebox.show("没有该字符！");
                    }
                    else
                    {
                        messagebox.show("以查找到尾部！");
                        start = 0;
                    }
                }
                else
                {
                    start = start + str1.length;
                    richtextbox1.focus();
                }
            }
        }
        /// <summary>
        /// 替换字符串
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        private void replace_click(object sender, eventargs e)
        {
            richtextbox1.text = richtextbox1.text.replace(txt_find.text, txt_replace.text);
        }
        /// <summary>
        /// 输入查找的字符串
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        private void txt_find_textchanged(object sender, eventargs e)
        {
            string str1;
            str1 = txt_find.text;
            start = 0;
            count = 0;
        }

    }
}

实验结果：