必填字段验证器不工作行更新gridview

Question

嗨，下面是gridview rowupdating事件的代码。为什么要求现场验证器不起作用？

if (string.IsNullOrEmpty(txtKMStart.Text) && string.IsNullOrEmpty(txtKMEnd.Text))
{
    RequiredFieldValidator rfvS = new RequiredFieldValidator();
    rfvS.ControlToValidate = txtKMStart.ID;
    rfvS.ErrorMessage = "Km Start Required!";

    RequiredFieldValidator rfvE = new RequiredFieldValidator();
    rfvE.ControlToValidate = txtKMEnd.ID;
    rfvE.ErrorMessage = "Km End Required!";
    ValidationSummary vs = new ValidationSummary();
    vs.ShowSummary = false;
    vs.ShowMessageBox = true;      
}

和gridview在

之下

<asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="False" 
                            AutoGenerateEditButton="True"  
                            BackColor="White" BorderColor="#336666" 
                            BorderStyle="Double" BorderWidth="3px" 
                            CellPadding="4"  GridLines="Horizontal" onrowcancelingedit="GridView1_RowCancelingEdit" 
                            onrowdatabound="GridView1_RowDataBound" 
                            onrowediting="GridView1_RowEditing" DataKeyNames="Sno" 
                            onrowupdating="GridView1_RowUpdating" AllowPaging="True" 
                            onpageindexchanging="GridView1_PageIndexChanging" PageSize="15">
</asp:GridView>