C# 循环时，操作另外一个进程直到操作完成，循环继续执行

using system;
using system.collections.generic;
using system.componentmodel;
using system.data;
using system.drawing;
using system.linq;
using system.text;
using system.threading;
using system.threading.tasks;
using system.windows.forms;

namespace threadtest
{
    public partial class form1 : form
    {
        public form1()
        {
            initializecomponent();
        }

        static eventwaithandle ehandle = new eventwaithandle(false, eventresetmode.manualreset);
        bool flag = false;

        private void button1_click(object sender, eventargs e)
        {
            richtextbox1.clear();
            for (int i = 1; i <= 100; i++) {
                richtextbox1.appendtext(i + "\r\n");
                if (i % 10 == 0) {
                    ehandle = new eventwaithandle(false, eventresetmode.manualreset);
                    thread currentthread = new thread(new threadstart(testprocess));//开辟一个进度条线程
                    currentthread.start();
                    ehandle.waitone();
                    richtextbox1.appendtext("i是10的倍数: i = " + i + "\r\n");
                    if (flag == true)
                    {
                        richtextbox1.appendtext("程序结束" + "\r\n");
                        break;
                    }
                }
            }
        }


        private void testprocess() {
            if (messagebox.show("确定要退出吗？", "message", messageboxbuttons.okcancel, messageboxicon.question) == dialogresult.cancel)
            {
                flag = false;
            }
            else
            {
                flag = true;
            }
            ehandle.set();
        }

        private void richtextbox1_textchanged(object sender, eventargs e)
        {
            richtextbox1.selectionstart = richtextbox1.text.length;
            richtextbox1.scrolltocaret();
        }
    }
}