我编写了一个简单的记事本，只能打开一段字符。请教高手为什么？谢谢！

我是用richTextBox。

private void MenuItemOpen_Click(object sender, EventArgs e)
        {
            if(needtosave==true)
            {
                DialogResult result=MessageBox.Show("文本内容已经改变，是否需要保存?","保存文件",MessageBoxButtons.YesNoCancel,MessageBoxIcon.Question);
                if(result==DialogResult.Yes)
                {
                    MenuItemSave_Click(sender,e);
                    needtosave=false;
                    richTextBoxText.Clear();
                }
                else if (result == DialogResult.Cancel)
                {
                    return;
                }
                    else
                {
                    string file=GetOpenFile();
                    if(file==null)
                    {
                        return;
                    }
                    else
                    {
                        currenfilename=file;
                        OpenFile();
                        needtosave=false;
                    }
                }
            }
                    
            else
                {
                    richTextBoxText.Clear();
                    string file = GetOpenFile();
                    if (file == null)
                    {
                        return;
                    }
                    else
                    {
                        currenfilename = file;
                        OpenFile();
                        needtosave = false;
                    }
                }
                  
            
        }
        private string GetOpenFile()
        {
            dlg = new OpenFileDialog();
            dlg.Title = "打开文本文件";
            dlg.ValidateNames = true;
            dlg.CheckFileExists = true;
            dlg.CheckPathExists = true;
            dlg.ShowHelp = true;
            dlg.Multiselect = true;
            dlg.Filter = "文本文件(*.txt)|*.txt|所有文件(*.*)|*.*";
           // string dir = Environment.GetFolderPath(Environment.SpecialFolder.Templates);
           // dlg.InitialDirectory = dir;
            if (dlg.ShowDialog() == DialogResult.OK)
            {
                return dlg.FileName;
            }
            else
            {
                return  null;
            }
           
         
        }
        private void OpenFile()
        {
            try
            {
                FileStream fs = new FileStream(currenfilename, FileMode.Open, FileAccess.Read);
                if (fs.CanRead)
                {
                    FileInfo f = new FileInfo(currenfilename);
                    StreamReader sr = new StreamReader(fs, Encoding.Default);
                    string strline = sr.ReadLine();
                    if (strline != null)
                    {
                        //richTextBox1.Text += strline + "\n";
                        richTextBoxText.Text = strline + "\n";
                        strline = null;
                    }
                    sr.Close();
                    this.Text = f.Name;
                }

            }
            catch (Exception e)
            {
                MessageBox.Show(e.Message);
            }
        }

今天又发现：当我编辑了一些汉字的时候，保存后再打开它时，就变成了乱码！请高手帮忙！感激不禁！谢谢~~~~`

richTextBox1.Text = File.ReadAllText(this.openFileDialog1.FileName, Encoding.GetEncoding("gb2312"));
乖老鼠，还是不行啊~~~~~~~~~

TextBoxEdit.Text = File.ReadAllText(this.dlgOpenFile.FileName,Encoding.Default);//Default是获取系统的当前ANSI代码页的编码。
这样就不会出现乱码，也不会只打开一段字符。谢谢乖老鼠的提示！！！感激不禁！