RequiredFieldValidator error in my asp.net

Question

I have two pages .At first page I have a button1 and when I click that post some sessions to next pages and redirect to second page . At second page I have some RequiredFieldValidators but when I click button1 show me an error like this !!

Server Error in '/' Application.

The ControlToValidate property of 'FirstnameRequiredFieldValidator' cannot be blank.

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. 

Exception Details: System.Web.HttpException: The ControlToValidate property of 'FirstnameRequiredFieldValidator' cannot be blank.

Source Error: 

An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.

Stack Trace: 


[HttpException (0x80004005): The ControlToValidate property of 'FirstnameRequiredFieldValidator' cannot be blank.]
   System.Web.UI.WebControls.BaseValidator.ControlPropertiesValid() +9831920
   System.Web.UI.WebControls.BaseValidator.OnPreRender(EventArgs e) +36
   System.Web.UI.Control.PreRenderRecursiveInternal() +83
   System.Web.UI.Control.PreRenderRecursiveInternal() +155
   System.Web.UI.Control.PreRenderRecursiveInternal() +155
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +974

My second page code:

  <table style="width:100%;">

            <tr>
                <td class="style1">
                    &nbsp;</td>
                <td class="style2">
                    &nbsp;</td>
                <td>
                    &nbsp;</td>
            </tr>
            <tr>
                <td class="auto-style1">
                    <asp:Label ID="Label1" runat="server" Text="نام" Font-Size="Larger"></asp:Label>
                </td>
                <td class="auto-style2">
                    <asp:TextBox ID="firstnametxt" runat="server" Height="22px" MaxLength="20" 
                        Width="158px"></asp:TextBox>
                </td>
              <td>
                  <asp:RequiredFieldValidator ID="FirstnameRequiredFieldValidator" runat="server" ForeColor="Red" ErrorMessage="نام خود را وارد کنید"></asp:RequiredFieldValidator>
              </td>


            </tr>
            <tr>
                <td class="style1">
                    <asp:Label ID="Label2" runat="server" Text="نام خانوادگی" Font-Size="Larger"></asp:Label>
                </td>
                <td class="style2">
                    <asp:TextBox ID="lastnametxt" runat="server" Height="22px" MaxLength="10" 
                        Width="158px"></asp:TextBox>
                </td>
                 <td>
                  <asp:RequiredFieldValidator ID="LastbaneRequiredFieldValidator" runat="server" ForeColor="Red" ErrorMessage="نام خانوادگی خود را وارد کنید."></asp:RequiredFieldValidator>
              </td>

            </tr>

            <tr>
                <td class="style1">
                    <asp:Label ID="Label4" runat="server" Text="شماره تلفن" Font-Size="Larger"></asp:Label>
                </td>
                <td class="style2">
                    <asp:TextBox ID="phonetxt" runat="server" Height="22px" MaxLength="10" 
                        Width="158px"></asp:TextBox>
                </td>
                 <td>
                  <asp:RequiredFieldValidator ID="PhoneRequiredFieldValidator" runat="server" ForeColor="Red" ErrorMessage="شماره تلفن خود را وارد کنید."></asp:RequiredFieldValidator>
                <asp:RegularExpressionValidator ID="Phonevalidator" 
                    runat="server" ForeColor="Red" ControlToValidate="phonetxt"
                        ErrorMessage="شماره وارد شده نامعتبر است ." 
                        ValidationExpression="\d+">
                    </asp:RegularExpressionValidator>
              </td>


            </tr>

            <tr>
                <td class="style1">
                    &nbsp;</td>
                <td class="style2">
                    &nbsp;</td>
                <td>
                    &nbsp;</td>
            </tr>
            <tr>
                <td class="style1">
                    &nbsp;</td>
                <td class="style2">
                    <asp:Button ID="buy" runat="server" onclick="buybtn_Click" Text="Submit" />
                </td>
                <td>
                    &nbsp;</td>
            </tr>
                           <tr>
                <td class="style1">
                    &nbsp;</td>
                <td class="style2">
                    <td><asp:Label ID="msglabel" runat="server" ></asp:Label></td>

                </td>
                <td>
                    &nbsp;</td>
            </tr>
        </table>  

My button code:

protected void button1_Click(object sender, EventArgs e)
{

    Session["Numofclass"] = Numofclass.SelectedItem.Value;
    Session["totalprice"] = totalprice;
    Session["Numofstud"] = Numofstud.SelectedItem.Value;
    Response.Redirect("account.aspx");

}  

My first page code:

.
.
.
 <section class =" container">

        <!-- start Pages -->


        <div id="pages">
            <div class="top_shadow"></div>
                        <!-- Start Price -->
            <div id="price" class="page">
            <h1>برنامه ریزی کلاس مجازی و قیمت</h1>
            <div class="tabs">

                <div id="tab_1" class="pane" style="width:85%;text-align:center">                               
                <p style="text-align:right;border-bottom:1px solid #aaa">برخی اساتید و معلمانی هستند که می‌خواهند آموزش خود را گسترش دهند اما نیاز به فضا و تجهیزاتی دارند که آنها را از این کار باز می‌دارد. با ارتقا عضویت خود در طرح دبیرانه به همه امکانات لازم برای گسترش بدون مرز آموزش خود با همه امکانات و تجهیزات لازم دست پیدا می‌کنید.</p>
                <form id="price_biz_calculator"><input type="hidden" name="biz"/>
                    <p>تعداد نفرات دانشجویان شما:

                    <asp:DropDownList ID="Numofstud" runat="server" AutoPostBack="True" Height="40px" OnSelectedIndexChanged="Numofstud_SelectedIndexChanged" Width="150px">

                    <asp:ListItem>10</asp:ListItem>
                    <asp:ListItem>25</asp:ListItem> 
                    <asp:ListItem>50</asp:ListItem>
                    <asp:ListItem>100</asp:ListItem>
                    <asp:ListItem>200</asp:ListItem>
                    <asp:ListItem>250</asp:ListItem>
                    </asp:DropDownList>
                    </p>
                    <p>تعداد کلاس&zwnj;های همزمان:&nbsp&nbsp&nbsp&nbsp&nbsp

                    <asp:DropDownList ID="Numofclass" runat="server" AutoPostBack="True" Height="40px" style="margin-left: 0px" Width="150px" OnSelectedIndexChanged="Numofclass_SelectedIndexChanged">

                    <asp:ListItem>1</asp:ListItem>
                    <asp:ListItem>2</asp:ListItem>
                    <asp:ListItem>3</asp:ListItem>
                    <asp:ListItem>4</asp:ListItem>
                    <asp:ListItem>5</asp:ListItem>
                    <asp:ListItem>6</asp:ListItem>
                    <asp:ListItem>7</asp:ListItem>
                    <asp:ListItem>8</asp:ListItem>
                    <asp:ListItem>9</asp:ListItem>
                </asp:DropDownList>

            </p>
                </form>
                    &nbsp
                    &nbsp
                    <p id="price_final_biz" style="color:#6b6b6b;font-size:100px;line-height:90px"></p>
                    <span style="font-size:20px;vertical-align:22%">ماهیانه</span>

                        <asp:ScriptManager EnablePartialRendering="true" ID="ScriptManager1" runat="server" ViewStateMode="Inherit" ValidateRequestMode="Inherit"></asp:ScriptManager>

                    <asp:UpdatePanel runat="server" id="UpdatePanel" updatemode="Conditional">
                  <ContentTemplate>
              <asp:Label ID="lblTotalprice"   runat="server"/>
               </ContentTemplate>

               <Triggers>
                    <asp:AsyncPostBackTrigger controlid="Numofclass" EventName="SelectedIndexChanged" />
                   <asp:AsyncPostBackTrigger controlid="Numofstud" EventName="SelectedIndexChanged" />
                </Triggers>
                    </asp:UpdatePanel>

                    <span style="font-size:20px;vertical-align:22%">هزار تومان</span>
                    &nbsp
                    &nbsp


                    <div>
                    <td>  
                        <asp:Button ID="button1" runat="server" onclick="button1_Click"   
Text="ثبت" Height="34px" Width="81px" /> 

                    </td>
                        </div>
            </div>
            </div>
            <!-- End Price -->
                        <div class="bottom_shadow"></div>
            </div>
         </div>
        <!-- End Pages -->


        <div class="clear"></div>


        <!-- End Pages -->  

            </section>

Answer 1

You have not set ControlToValidate property of any of your RequiredFieldValidator. Set ControlToValidate property value to input control to be validated. For e.g.

   <asp:RequiredFieldValidator ControlToValidate="firstnametxt" ID="FirstnameRequiredFieldValidator" runat="server" ForeColor="Red" ErrorMessage="Show Error Msg here"></asp:RequiredFieldValidator>

Answer 2

Reason is you haven't told validator that which control it has to validate. Add ControlToValidate in your mark up.

I guess you want to validate lastnametxt, hence,

Replace validaot mark up from

<asp:RequiredFieldValidator ID="FirstnameRequiredFieldValidator" runat="server" ForeColor="Red" ErrorMessage="نام خود را وارد کنید"></asp:RequiredFieldValidator>

To

<asp:RequiredFieldValidator ID="FirstnameRequiredFieldValidator" runat="server" ForeColor="Red" ControlToValidate="lastnametxt"  ErrorMessage="نام خود را وارد کنید"></asp:RequiredFieldValidator>