<InsertItemTemplate>
        <asp:TextBox ID="tbAFML" runat="server" Text='<%# Bind("AFML") %>'/>
        <asp:CustomValidator runat="server" ControlToValidate="tbAFML" 
                                            OnServerValidate="AFML_ServerValidate" 
                                            ValidationGroup="vgFlightDetails"
                                            Text="*" ErrorMessage="AFML already exists" />
        <asp:RequiredFieldValidator runat="server" ControlToValidate="tbAFML" 
                                            Text="*" ErrorMessage="AFML is required field" 
                                            ValidationGroup="vgFlightDetails"/>
    </InsertItemTemplate>

protected void AFML_ServerValidate(object sender, ServerValidateEventArgs e)
        {
            e.IsValid = KB.AFMLExists(e.Value) == null ? true : false;
        }

<asp:TemplateField HeaderText="AFML">
<ItemTemplate>
                                <asp:Label ID="Label8" runat="server" Text='<%# Eval("AFML") %>'/>
                            
</ItemTemplate>
    <EditItemTemplate>
        <asp:TextBox ID="tbAFML" runat="server" Text='<%# Bind("AFML") %>'  CausesValidation="true" ValidationGroup="vgFlightDetails" />
        
        <asp:RequiredFieldValidator runat="server" ControlToValidate="tbAFML" Text="*" ErrorMessage="AFML is required field" 
                                                    ValidationGroup="vgFlightDetails"/>
         <asp:CustomValidator runat="server" ID="cv2" ControlToValidate="tbAFML"  
                                            OnServerValidate="AFML_ServerValidate" 
                                            ValidationGroup="vgFlightDetails"
                                            Text="*" ErrorMessage="AFML already exists" />
    </EditItemTemplate>
    <InsertItemTemplate>
        <asp:TextBox ID="tbAFML" runat="server" Text='<%# Bind("AFML") %>' CausesValidation="true" ValidationGroup="vgFlightDetails"/>
        <asp:RequiredFieldValidator runat="server" ControlToValidate="tbAFML" Text="*" ErrorMessage="AFML is required field" 
                                            ValidationGroup="vgFlightDetails"/>
        <asp:CustomValidator runat="server" ID="cv2" ControlToValidate="tbAFML"  
                                             OnServerValidate="AFML_ServerValidate" 
                                            ValidationGroup="vgFlightDetails"
                                            Text="*" ErrorMessage="AFML already exists" />
    </InsertItemTemplate>
    
</asp:TemplateField>
 <asp:TemplateField ShowHeader="False">
                          
                            <ItemTemplate>
                                <asp:LinkButton ID="LinkButton1" runat="server" CausesValidation="False" 
                                    CommandName="Edit" Text="Edit" />
                            </ItemTemplate>
                            <EditItemTemplate>
                                <asp:LinkButton ID="LinkButton1" runat="server" CausesValidation="True" 
                                    CommandName="Update" Text="Update" ValidationGroup="vgFlightDetails"/>
                                &nbsp;<asp:LinkButton ID="LinkButton2" runat="server" CausesValidation="False" 
                                    CommandName="Cancel" Text="Cancel"/>
                            </EditItemTemplate>
                            <InsertItemTemplate>
                                 <asp:LinkButton ID="LinkButton1" runat="server" CausesValidation="True" 
                                    CommandName="Insert" Text="Insert" ValidationGroup="vgFlightDetails"/>
                            </InsertItemTemplate>
                         </asp:TemplateField>
<asp:ValidationSummary ID="ValidationSummary1"  runat="server" ValidationGroup="vgFlightDetails" />

    protected void AFML_ServerValidate(object sender, ServerValidateEventArgs e)
        {
            e.IsValid = KB.AFMLExists(e.Value) == null ? true : false;
        }


public object AFMLExists(object AFML)
        {
            try
            {
                using (SqlConnection connection = new SqlConnection(connectionString))
                {
                    connection.Open();
                    SqlCommand command = connection.CreateCommand();
                    command.CommandType = CommandType.Text;
                    command.CommandText = @"select 1 from flight where afml=@afml";
                    command.Parameters.AddWithValue("@afml", AFML);
                    return command.ExecuteScalar();
                }
            }
            catch (SqlException e)
            {
                throw new Exception(e.Message);
            }
            
        }