C#中我想把listbox里面的内容写到一个文件，怎么编程

string str = "";
for (int i = 0; i < listBox1.Items.Count; i++)
{
str += listBox1.Items[i].ToString()+"\r\n";
}
FileStream fs = new FileStream("c:\\1.txt", FileMode.OpenOrCreate);
StreamWriter sw = new StreamWriter(fs);
sw.Write(str);
sw.Flush();
sw.Close();
fs.Close();
MessageBox.Show("ok");

昏~先打开一个文本啊.然后listbox一行一行输出