protected void gvTrolleys_RowDeleting(object sender, GridViewDeleteEventArgs e)
        {
            object o = gvTrolleys.DataKeys[e.RowIndex].Value;
            dal.DeleteTrolley(e.Keys[0]);
            
        }

dal.DeleteTrolley(gvTrolleys.DataKeys[e.RowIndex].Value);

  public int DeleteTrolley(object id)
        {
            int i = -1;
            try
            {
                using(SqlConnection connection=new SqlConnection(connectionString))
                {
                    connection.Open();
                    SqlCommand command = connection.CreateCommand();
                    command.CommandType = CommandType.Text;
                    command.CommandText = @"update proc_packs set rtype=55 where id=@id";
                    command.ExecuteNonQuery();
                }
            }
            catch (SqlException e)
            {
            }
            return i;
        }

 <asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="false" DataKeyNames="id"
         OnRowDeleting="GridView1_RowDeleting" AutoGenerateDeleteButton="true">
        <Columns>
            <asp:BoundField HeaderText="ID" DataField="id" />
            <asp:BoundField HeaderText="Name" DataField="name" />
            <asp:BoundField HeaderText="Age" DataField="age" />
        </Columns>
    </asp:GridView>

 
protected void GridView1_RowDeleting(object sender, GridViewDeleteEventArgs e)
    {
        int id = (int)GridView1.DataKeys[e.RowIndex].Value;
        string CS = @"Data Source=COMPAQ-PC\SQLEXPRESS;Initial Catalog=DbStudents;Integrated Security=true";
        SqlConnection con = new SqlConnection(CS);
        SqlCommand cmd = new SqlCommand("delete tbl_students where id=" + id, con);
        con.Open();
        cmd.ExecuteNonQuery();
        con.Close();
        BindGridview();
    }

SqlCommand cmd = new SqlCommand("delete tbl_students where id=" + id, con);