C#读写文件的自定义函数，写文件不全的问题的解决方法

private string ReadFile(string filename)
// by www.yongfa365.com
{
    ////////////////////////读文件操作////////////////////////
    string html = "";

    //获取文件名
    OpenFileDialog dl = new OpenFileDialog();
    dl.Title = "打开一个文本文件";
    dl.FileName = filename;
    dl.Filter = "所有文件(*.*)|*.*|文本文件 (*.txt)|*.txt;*.ini;*.sql";
    if (dl.ShowDialog() == DialogResult.OK)
        filename = dl.FileName;

    //读内容
    //StreamReader din = new StreamReader(filename, System.Text.Encoding.GetEncoding("gb2312"));
    StreamReader din = new StreamReader(filename, System.Text.Encoding.Default);
    while (din.Peek() > -1)
    {
        html = html + din.ReadToEnd();
    }
    din.Close();

    //返回值
    return html;
}

private void WriteFile(string filename, string content)
// by www.yongfa365.com
{
    ////////////////////////写文件操作////////////////////////
    SaveFileDialog dl = new SaveFileDialog();
    dl.Title = "请选择保存路径";
    dl.Filter = "所有文件(*.*)|*.*|文本文件 (*.txt)|*.txt";
    dl.FileName = filename;
    if (dl.ShowDialog() == DialogResult.OK)
        filename = dl.FileName;

    //StreamWriter dout = new StreamWriter(filename, false, System.Text.Encoding.GetEncoding("utf-8"));
    StreamWriter dout = new StreamWriter(filename);
    dout.Write(content);
    //必须写下边这句，关闭当前流，不然输入的文本文件内容不全，只有半截。
    dout.Close();
}