using System;
using System.Collections.Generic;
using using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using namespace 记事本
{
    public partial class Form1 : Form
    {
        private string pathName = null;
        private string SelectText = null;
        private int ChangeText = 0;
        public enum TextChang { NoChang, Chang }
        public Form1()
        {
            InitializeComponent();
        }

        }
        private void 打开ToolStripMenuItem_Click(object sender, EventArgs e)
        {
            openFileDialog1.ShowDialog();
            StreamReader sr = new StreamReader(openFileDialog1.FileName);
            textmain.Text = sr.ReadToEnd();
            sr.Close();
        }
        private void 保存ToolStripMenuItem_Click(object sender, EventArgs e)
        {if(pathName==null)
        {另存为toolStripMenuItem_Click(sender, e);}
        else{textmain.SaveFile(pathName,RichTextBoxStreamType.PlainText);
        }


            //saveFileDialog1.ShowDialog();
            //StreamWriter aw = new StreamWriter(saveFileDialog1.FileName);
            //aw.Write(textmain.Text);
            //aw.Close();只有保存时     
        }
      
        private void 另存为toolStripMenuItem_Click(object sender, EventArgs e)
        {
            SaveFileDialog sfDlg = new SaveFileDialog();
            sfDlg.Filter = "文本文档（*.txt）|*.txt|所有文件(*.*)|*.*";
            if (sfDlg.ShowDialog() == DialogResult.OK)
            {
                pathName = sfDlg.FileName;
                textmain.SaveFile(sfDlg.FileName, RichTextBoxStreamType.PlainText);
                this.Text = Path.GetFileName(pathName);
            }
        }

}

为什么点击打开以后，不选取文件点取消，程序会故障？？

下面的程序块，请在条件判断中使用
if(openFileDialog1.ShowDialog()==DialogResult.OK)
{
    StreamReader sr = new StreamReader(openFileDialog1.FileName);
            textmain.Text = sr.ReadToEnd();
            sr.Close();

}