Make TextBox Required if Other CheckBox is Checked using ASP.Net Custom Validator

Hi Thuyai,

Check this example. Now please take its reference and correct your code.

HTML

<div>
    Select Fruits<br />
    <asp:CheckBoxList ID="chkFruits" runat="server">
        <asp:ListItem Text="Mango" />
        <asp:ListItem Text="Apple" />
        <asp:ListItem Text="Banana" />
        <asp:ListItem Text="Pineapple" />
        <asp:ListItem Text="Guava" />
        <asp:ListItem Text="Grapes" />
        <asp:ListItem Text="Papaya" />
        <asp:ListItem Text="Other" />
    </asp:CheckBoxList>
    <asp:CustomValidator ID="CustomValidator1" ErrorMessage="Please select at least one item."
        ForeColor="Red" ClientValidationFunction="ValidateCheckBoxList" runat="server" />
    <br />
    <asp:TextBox ID="txtOtherfruit" Rows="5" TextMode="multiline" runat="server" />
    <asp:RequiredFieldValidator ID="rfvOtherFruit" ErrorMessage="Required" ControlToValidate="txtOtherfruit"
        Enabled="false" runat="server" ForeColor="Red" />
    <br />
    <br />
    <asp:Button ID="btnSubmit" Text="Submit" runat="server" /><br />
    <script type="text/javascript">
        function ValidateCheckBoxList(sender, args) {
            var checkBoxList = document.getElementById("<%=chkFruits.ClientID %>");
            var checkboxes = checkBoxList.getElementsByTagName("input");
            var isValid = false;
            for (var i = 0; i < checkboxes.length; i++) {
                if (checkboxes[i].checked) {
                    if (checkboxes[i].value == 'Other') {
                        ValidatorEnable(document.getElementById("rfvOtherFruit"), true);
                        isValid = true;
                    }
                    else {
                        ValidatorEnable(document.getElementById("rfvOtherFruit"), false);
                    }
                    isValid = true;
                }
            }
            args.IsValid = isValid;
        }
    </script>
</div>

Screenshot