c#中如何保存文本?!

C#为PPC手机编程,在Textbox中打开文本并作出修改后,如何保存修改后的文本?!
下面的代码实现不了保存功能,是不是不对?
string theFile;
saveFileDialog1.Filter = "Text File(*.txt)|*.txt";
saveFileDialog1.ShowDialog();
theFile = saveFileDialog1.FileName;

FileStream fs = new FileStream(theFile,FileMode.Create);
StreamWriter writer = new StreamWriter(fs);

fs.Close();
writer.Close();

谢谢各位!

写

///////////////////////////////aa

string time = System.DateTime.Now.ToShortTimeString();
string text = TextBox1.Text;
string sss = System.DateTime.Now.ToShortDateString()+".txt";
//指定路径
sss = "d:\\"+sss;
//如果文件a.txt存在就打开，不存在就新建 .append 是追加写
FileStream fst = new FileStream(sss,FileMode.Append);
//写数据到a.txt格式
StreamWriter swt = new StreamWriter(fst,System.Text.Encoding.GetEncoding("utf-8"));
//写入
swt.WriteLine(time+'\r'+"写"+text);
swt.Close();
fst.Close();

读

////////////////////////////////

string ss = System.DateTime.Now.ToShortDateString()+".txt";
ss ="d:\\"+ss;
//FileStream fs = new FileStream(sss,
try
{
StreamReader srt = new StreamReader(ss,Convert.ToBoolean(FileMode.Open));
//存在
TextBox2.Text = srt.ReadToEnd();