save box

i hava create a text file
FileStream fs = new FileStream("M:\\a.txt", FileMode.Create);
        StreamWriter sw = new StreamWriter(fs);
        sw.WriteLine("hello");
        sw.WriteLine("hi");
        sw.Close();
        Response.ContentType = "application/txt";
        Response.AddHeader("content-disposition", "attachment; filename=a.txt");
        Response.End()
i want when button click file shuld be save using save file dialog box. created and save box also apper but when i see in download folder a.text is created but with no data but i havw write hello,hi as aboce discuss
when i tried
FileStream fs = new FileStream(&q ...