C#读写文件的自定义函数，写文件不全的问题的解决方法

/// <summary>
/// winform 读文件 by www.yongfa365.com
/// </summary>
/// <param name="filename">默认读取的文件名</param>
/// <returns></returns>
public string ReadFile(string filename)
{
////////////////////////读文件操作////////////////////////
string html = "";
OpenFileDialog dlg = new OpenFileDialog();
dlg.Title = "打开一个文本文件";
dlg.FileName = filename;
dlg.Filter = "所有文件(*.*)|*.*|文本文件 (*.txt)|*.txt;*.ini;*.sql";
if (dlg.ShowDialog() == DialogResult.OK)
{
filename = dlg.FileName;
StreamReader din = new StreamReader(filename, System.Text.Encoding.Default);
while (din.Peek() > -1)
{
html = html + din.ReadToEnd();
}
din.Close();
}
return html;
}