Transmitfile gives error could not find a part of the path 'C:\upload\'

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C#

Gridview

I want to Edit Grid View Rows along with file download option. I can execute Edit and File download separately. But when I execute both, and press Edit button, it gives me error:Could not find a part of the path 'c:\upload\'. This path is right and file download can be executed from this path.

namespace gerp_support
{
    public partial class Admin : System.Web.UI.Page
    {
        string cs = ConfigurationManager.ConnectionStrings["GerpConnStr"].ConnectionString;
        SqlConnection con;
        SqlDataAdapter adapt;
        DataTable dt;  


        protected void Page_Load(object sender, EventArgs e)
        {
            if (!this.IsPostBack)
            {
                this.ShowData();
            }
        }

        private void ShowData()
        {
            dt = new DataTable();
            con = new SqlConnection(cs);
            con.Open();
            adapt = new SqlDataAdapter("SELECT ID,request_no,requestor,request_type, request_area, subject,description,attachment,req_status,admin_comment FROM request where req_status='Open' order by ID", con);



            adapt.Fill(dt);
            if (dt.Rows.Count > 0)
            {
                GridView1.DataSource = dt;
                GridView1.DataBind();
            }
            con.Close();

        }




        protected void GridView1_RowEditing(object sender, System.Web.UI.WebControls.GridViewEditEventArgs e)
        {
            //NewEditIndex property used to determine the index of the row being edited.  
            GridView1.EditIndex = e.NewEditIndex;
            ShowData();
        }



        protected void gvFiles_RowCommand(object sender, GridViewCommandEventArgs e)
        {
            Response.Clear();
            Response.ContentType = "application/octet-stream";

           // Response.AppendHeader("Content-Disposition", "filename=" + e.CommandArgument);

            Response.AppendHeader("Content-Disposition", "attachment; filename=" + e.CommandArgument);

            string p = @"c:\upload\";

             Response.TransmitFile(p + e.CommandArgument);

          //  Response.TransmitFile(@"~\upload\" + e.CommandArgument);

            Response.End();
        }  

        protected void GridView1_RowUpdating(object sender, System.Web.UI.WebControls.GridViewUpdateEventArgs e)
        {
            //Finding the controls from Gridview for the row which is going to update  

            Label id = GridView1.Rows[e.RowIndex].FindControl("lbl_id") as Label;
            Label request_no = GridView1.Rows[e.RowIndex].FindControl("lbl_request_no") as Label;
            Label requestor = GridView1.Rows[e.RowIndex].FindControl("lbl_requestor") as Label;
            Label request_type = GridView1.Rows[e.RowIndex].FindControl("lbl_request_type") as Label;
            Label request_area = GridView1.Rows[e.RowIndex].FindControl("lbl_request_area") as Label;
            Label subject = GridView1.Rows[e.RowIndex].FindControl("lbl_subject") as Label;
            Label description = GridView1.Rows[e.RowIndex].FindControl("lbl_description") as Label;




            TextBox req_status = GridView1.Rows[e.RowIndex].FindControl("txt_req_status") as TextBox;
            TextBox admin_comment = GridView1.Rows[e.RowIndex].FindControl("txt_admin_comment") as TextBox;

            con = new SqlConnection(cs);
            con.Open();

            SqlCommand cmd = new SqlCommand("Update request set req_status='" +req_status.Text+ "',admin_comment='" +admin_comment.Text+ "' where ID=" + Convert.ToInt32(id.Text), con);
            cmd.ExecuteNonQuery();
 con.Close();

           GridView1.EditIndex = -1;

            ShowData();
}

What I have tried:

I have tried Edit Grid View Row and File Download separately and it worked. But when I execute both and press Edit button, it gives error:Could not find a part of the path 'c:\upload\'