How to resolve the exception the controls collection cannot be modified because the control contains code blocks (i.e. <% ... %>). While reading from the config key to javascript?

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I am reading the web.config key value to my javascript function.
I am getting below exception
The Controls collection cannot be modified because the control contains code blocks (i.e. <% ... %>).

<script language="javascript" type="text/javascript">
   function test()
   {
      alert(1);
      var key1 = '<%=System.Configuration.ConfigurationManager.AppSettings["AESKey"].ToString() %>';
       alert(key1);

   }
   </script>

What I have tried:

Below is the complete page level source

<%@ Page Language="C#" AutoEventWireup="true" CodeFile="Login.aspx.cs" Inherits="Login" %>

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head id="Head1" runat="server">
    
    <script language="javascript" type="text/javascript">
    function test()
    {
       alert(1);
       var key1 = '<%=System.Configuration.ConfigurationManager.AppSettings["AESKey"].ToString() %>';
        alert(key1);
        
    }
    </script>
    
</head>
<body >
 
    <form id="form1" runat="server">
    <table width="997" border="0" cellspacing="0" cellpadding="0" align="center">
        <tr>
            <td>
                <table width="100%" border="0" cellspacing="0" cellpadding="0">
                    <tr>
                        <td width="40%" class="NeST_LOGIN_IMG1">
                        </td>
                        <td width="3%" class="NeST_LOGIN_IMG2">
                        </td>
                        <td width="28%" class="NeST_LOGIN_IMG3">
                        </td>
                        <td width="29%" align="right" height="135">
                            <table width="50%" border="0" cellspacing="0" cellpadding="0" class="NeST_BANK_LOGO1">
                                <tr>
                                    <td width="11%" rowspan="3"><img src="images/sec/ddsedge_04.jpg" width="25" height="135" /></td>
                                    <td width="79%" style="background-image: url(images/sec/ddsedge_05.jpg)"><img src="images/ddsedge_07.jpg" width="120" /></td>
                                    <td width="10%" rowspan="3"><img src="images/sec/ddsedge_06.jpg" width="24" height="135" /></td>
                                </tr>
                            </table>
                        </td>
                    </tr>
                </table>
            </td>
        </tr>
        <tr>
            <td>
                <table width="100%" border="0" cellspacing="0" cellpadding="0" id="tblLoginPart">
                    <tr>
                        <td width="40%" class="NeST_LOGIN_IMG9">
                        </td>
                        <td width="1%" class="NeST_LOGIN_IMG10">
                        </td>
                        <td width="59%" class="NeST_LOGIN_IMG11" valign="top">
                            <div class="NeST_BANK_LOGO2" > 
                            
                             <table width="50%" border="0" cellspacing="0" cellpadding="0" >
                                    <tr>
                                        <td width="11%" rowspan="3">
                                            <img src="images/sec/ddsedge_04.jpg" width="25" height="135" />
                                        </td>
                                        <td width="79%" style="background-image: url(images/sec/ddsedge_05.jpg); background-repeat:repeat-x ;">
                                           <img src="images/NBKddsedge_07.jpg" width="120px" />
                                        </td>
                                        <td width="10%" rowspan="3">
                                            <img src="images/sec/ddsedge_06.jpg" width="24" height="135" />
                                        </td>
                                    </tr>
                                </table>
                            
                            </div>
                            <table width="100%" border="0" cellspacing="0" cellpadding="0">
                                <tr>
                                    <td width="12%" height="122">
                                         
                                    </td>
                                    <td width="68%">
                                         
                                    </td>
                                </tr>
                                <tr>
                                    <td>
                                         
                                    </td>
                                    <td>
                                        <label class="NeST_LOGIN_LABEL">
                                            Username
                                        </label>
                                        <asp:RequiredFieldValidator ID="Requiredfieldvalidator1"
                                            runat="server" ControlToValidate="txtLoginID" ErrorMessage="[Username cannot be empty]"
                                            CssClass="VALIDATECLASS_LOGIN" EnableClientScript="False"></asp:RequiredFieldValidator>
                                    </td>
                                </tr>
                                <tr>
                                    <td>
                                         
                                    </td>
                                    <td>
                                        <asp:TextBox runat="server" ID="txtLoginID" name="textfield" 
                                            CssClass="NeST_LOGIN_TEXT" AutoCompleteType="Disabled" />
                                    </td>
                                </tr>
                                <tr>
                                    <td>
                                         
                                    </td>
                                    <td>
                                        <label class="NeST_LOGIN_LABEL">
                                            Password
                                        </label>
                                       
                                        <asp:RequiredFieldValidator ID="Requiredfieldvalidator2" runat="server" ControlToValidate="txtPassword"
                                            ErrorMessage="[Password cannot be empty]" CssClass="VALIDATECLASS_LOGIN" EnableClientScript="False"></asp:RequiredFieldValidator>
                                    </td>
                                </tr>
                                <tr>
                                    <td>
                                         
                                    </td>
                                    <td>
                                        <asp:TextBox runat="server" ID="txtPassword" TextMode="Password" name="textfield2"
                                            onkeypress="return RestrictSpace()" oncopy="return false" onpaste="return false"
                                            oncut="return false" CssClass="NeST_LOGIN_TEXT" 
                                            AutoCompleteType="Disabled" />
                                    </td>
                                </tr>
                                <tr>
                                    <td>
                                         
                                    </td>
                                    <td>
                                         
                                    </td>
                                </tr>
                                <tr>
                                    <td>
                                         
                                    </td>
                                    <td>
                                        <label>
                                        </label>
                                        <table width="100%" border="0" cellspacing="0" cellpadding="0">
                                            <tr>
                                                <td width="11%">
                                                     
                                                </td>
                                                <td width="9%">
                                                     
                                                </td>
                                                <td width="80%" align="center">
                                                    <asp:Button ID="btnLogin" runat="server" CssClass="buttonLogin" Text="Login" runat="server"
                                                      OnClientClick="javascript:test();"   OnClick="btnLogin_Click" TabIndex="3" Width="60px"></asp:Button>
                                                  
                                                </td>
                                            </tr>
                                        </table>
                                        <label>
                                        </label>
                                    </td>
                                </tr>
                            </table>
                        </td>
                    </tr>
                </table>
            </td>
        </tr>
        <tr>
            <td  >
                <table width="100%" border="0" cellspacing="0" cellpadding="0">
                    <tr>
                        <td width="41%"  height="62" align="right" class="NeST_LOGIN_LOGO_SMALL" ></td>
                        <td width="59%" class="footertext"> © 2013 NeST IT. All rights reserved</td>
                    </tr>
                </table>
            </td> 
        </tr>
    <tr>
        <td>
             
        </td>
    </tr>
    </table>

    <script>

        var txt = document.getElementById("txtLoginID");
        txt.focus();
    </script>

    </form>
</body>
</html>

Posted 31-Oct-21 22:33pm

ranio

Updated 2-Nov-21 1:23am

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OriginalGriff · Answer 1 · 2021-10-31T22:53:00

If you don't understand an error message, google it: Controls collection cannot be modified because the control contains code blocks - Google Search[^]
The top hit explains the problem: c# - "The Controls collection cannot be modified because the control contains code blocks" - Stack Overflow[^]

Richard Deeming · Answer 2 · 2021-11-02T01:23:00

You can simplify your code by using the AppSettingsExpressionBuilder class[^]:

JavaScript

var key1 = '<%$ AppSettings:AESKey %>';

Since you're no longer using the <%= ... %> construct, this should also resolve the error.

NB: You need to be absolutely sure that the appSettings value will not contain a single-quote character ('); otherwise, your Javascript will not be valid.

How to resolve the exception the controls collection cannot be modified because the control contains code blocks (i.e. <% ... %>). While reading from the config key to javascript?

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