How to show add in gridview when text box is empty and edit when there is data in textbox of gridview

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C#

ASP.NET

Gridview

Below is my code with only third column editable in gridview and remaining two columns readonly i am able to save the data when i click Add button in gridview but for that editable coulmn in gridview but what i need is if that editable column in gridview is empty it should show add and after giving the text it should get saved and if that editable column in gridview has some text it should show edit and after giving the text it should get saved How can i do this

<div>
           <asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="False" DataKeyNames="Id" OnPageIndexChanging="GridView1_PageIndexChanging" OnRowEditing="GridView1_RowEditing" OnRowUpdating="GridView1_RowUpdating">
               <Columns>
                   <asp:BoundField DataField="ProductName" HeaderText="ProductName" ReadOnly="true" />
                   <asp:BoundField DataField="Price" HeaderText="Price" ReadOnly="true" />
                   <asp:BoundField DataField="Quantity" HeaderText="Quantity" />
                   <asp:CommandField ShowEditButton="true" CancelText="" DeleteText="" EditText="Add" UpdateText="Save" />

               </Columns>

           </asp:GridView>

       </div>
       <div>
           <asp:Label ID="lblresult" runat="server"></asp:Label>

       </div>

protected void Page_Load(object sender, EventArgs e)
        {
            if (!IsPostBack)
            {
                gvbind();
            }
        }
        protected void gvbind()
        {
            conn.Open();
            SqlCommand cmd = new SqlCommand("Select * from Products", conn);
            SqlDataAdapter da = new SqlDataAdapter(cmd);
            DataSet ds = new DataSet();
            da.Fill(ds);
            conn.Close();
            if (ds.Tables[0].Rows.Count > 0)
            {
                GridView1.DataSource = ds;
                GridView1.DataBind();
            }
            else
            {
                ds.Tables[0].Rows.Add(ds.Tables[0].NewRow());
                GridView1.DataSource = ds;
                GridView1.DataBind();
                int columncount = GridView1.Rows[0].Cells.Count;
                GridView1.Rows[0].Cells.Clear();
                GridView1.Rows[0].Cells.Add(new TableCell());
                GridView1.Rows[0].Cells[0].ColumnSpan = columncount;
                GridView1.Rows[0].Cells[0].Text = "No Records Found";
            }
        }
        protected void GridView1_PageIndexChanging(object sender, GridViewPageEventArgs e)
        {
            GridView1.PageIndex = e.NewPageIndex;
            gvbind();
        }
        protected void GridView1_RowEditing(object sender, GridViewEditEventArgs e)
        {
            GridView1.EditIndex = e.NewEditIndex;
            gvbind();
        }
        protected void GridView1_RowUpdating(object sender, GridViewUpdateEventArgs e)
        {
            int userid = Convert.ToInt32(GridView1.DataKeys[e.RowIndex].Value.ToString());
            GridViewRow row = (GridViewRow)GridView1.Rows[e.RowIndex];
            Label lblID = (Label)row.FindControl("lblID");
            //TextBox textName = (TextBox)row.Cells[0].Controls[0];
            //TextBox textadd = (TextBox)row.Cells[1].Controls[0];
            TextBox textc = (TextBox)row.Cells[2].Controls[0];
            GridView1.EditIndex = -1;
            conn.Open();
            //SqlCommand cmd = new SqlCommand("update Products set ProductName='" + textName.Text + "',Price='" + textadd.Text + "',Quantity='" + textc.Text + "'where id='" + userid + "'", conn);
            SqlCommand cmd = new SqlCommand("update Products set Quantity='" + textc.Text + "'where id='" + userid + "'", conn);
            cmd.ExecuteNonQuery();
            conn.Close();
            gvbind();
        }

What I have tried:

i tried the above code but it is not working