将状态更新到数据库

Question

我有一个asp页面，通过选择复选框然后按批准按钮批准列表的状态，但是当我选择多个复选框时，它会抛出错误：索引超出范围。我在做什么可能是错的？以下是我的代码。

    <asp:GridView ID="gv2"
        CssClass="table table-striped table-bordered bootstrap-datatable datatable"
        runat="server" AutoGenerateColumns="false"
        EmptyDataText="No records found"
        OnRowDeleting="gv2_RowDeleting"
        DataKeyNames="cancelinvoiceid"
        AllowPaging="True"
        PageSize="10"
        OnPageIndexChanging="gv2_PageIndexChanging">
  <Columns>
  <asp:TemplateField HeaderText="Approval">
    <ItemTemplate>
      <asp:CheckBox ID="CheckBox" runat="server" />
      <asp:HiddenField ID="hdcancelinvoiceid" runat="server" Value='<%# Eval("cancelinvoiceid") %>'></asp:HiddenField>
    </ItemTemplate>
  </asp:TemplateField>

  <asp:BoundField HeaderText="Account No" DataField="account_number"></asp:BoundField>
  <asp:TemplateField HeaderText="Bill Ref">
    <ItemTemplate> <%#Eval("yearref")%>/<%#Eval("monthref")%>-<%#Eval("referencecount")%>
    </ItemTemplate>
  </asp:TemplateField>

  <asp:TemplateField HeaderText="New Reading">
    <ItemTemplate>
      <asp:Label ID="lblApproved1" runat="server" Text='<%# Eval("new_reading") %>'></asp:Label>
    </ItemTemplate>
  </asp:TemplateField>
  <asp:TemplateField HeaderText="Last Reading">
    <ItemTemplate>
      <asp:Label ID="lblApproved2" runat="server" Text='<%# Eval("last_reading") %>'></asp:Label>
    </ItemTemplate>
  </asp:TemplateField>      <asp:TemplateField HeaderText="Requested By">
    <ItemTemplate>
      <asp:Label ID="lblRequestedBy" runat="server" Text='<%# Bind("staff_name") %>'></asp:Label>
    </ItemTemplate>
  </asp:TemplateField>
  <asp:TemplateField HeaderText="Reading Date">
    <ItemTemplate>
      <asp:Label ID="lblFirstApprovedBy" runat="server" Text='<%# Bind("created_on") %>'></asp:Label>
    </ItemTemplate>
  </asp:TemplateField>
  </Columns>
        <PagerStyle CssClass="bottomPaging" HorizontalAlign="Center" />
</asp:GridView>
<asp:Button ID="btnFirstApprove" OnClick="FirstApprove_Click" Text="Approve" runat="server" />
<asp:Button ID="btnFirstDisApprove" CssClass="disapprove" OnClick="FirstDisApprove_Click" Text="Disapprove" runat="server" />

    protected void FirstApprove_Click(object sender, EventArgs e)
{
    bool success = false;

    foreach (GridViewRow gvr in gv2.Rows)
    {
        if (gvr.RowType == DataControlRowType.DataRow)
        {
            CheckBox cb = (CheckBox)(gvr.FindControl("CheckBox"));
            if (cb.Checked == true)
            {
                int cancelinvoiceid = Convert.ToInt32(gv2.DataKeys[gvr.RowIndex].Values["cancelinvoiceid"].ToString());
                force_revisit obj = db.force_revisit.First(x => x.Id == cancelinvoiceid);
                obj.approved_first_by = Convert.ToInt32(Session["CreatedBy"]);
                obj.approved_first_on = Convert.ToDateTime(DateTime.Today);
                obj.approved = true;

                int reading_id = obj.reading_id;

                reading rx = db.readings.First(x => x.Id == reading_id);
                rx.force_revisit = false;
                db.SaveChanges();
                Bindgv2();

                success = true;
            }
        }
    }

    if (success)
    {
        lblMessage.InnerHtml = "Approved successfully.";
        lblMessage.Attributes["class"] = "alert alert-success";
    }
}