How to specify Clientside-Download path using C#

protected void Button1_Click(object sender, EventArgs e)
    {
        Response.Redirect("download.aspx?file=~/images/aa.txt");
    }

protected void Page_Load(object sender, EventArgs e)
    {
        string strRequest =(string) Request.QueryString["file"];       
        if (!string.IsNullOrEmpty(strRequest))
        {
            string path = Server.MapPath(strRequest);           
            System.IO.FileInfo file = new System.IO.FileInfo(path);           
            if (file.Exists)
            {
                Response.Clear();
                Response.AddHeader("Content-Disposition", "attachment; filename=" + file.Name);
                Response.AddHeader("Content-Length", file.Length.ToString());
                Response.ContentType = "application/octet-stream";
                Response.WriteFile(file.FullName);
                Response.End();              
            }
            else
            {
                Response.Write("This file does not exist.");
            }           
        }
        else
        {
            Response.Write("Please provide a file to download.");
        }


    }