Required Field Validation based on condition

I have a scenario, where user has to select any one role from radiobuttonlist to register himself as below

• Administrator
• Shop floor User
• Key User
• Others

If he selects Role "Others" from radio-button list, a text box will be appeared to enter the role manually. Can I have a RequiredFieldValidator for text-box if only user selects role "Others" else if he selects other roles validation is not required.

<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="Default.aspx.cs" Inherits="Required_Field_Validator_based._Default" %>

<!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 runat="server">
<title>Untitled Page</title>
</head>
<body>
<form id="form1" runat="server">
<div>
<table align="center">
<tr>
<td colspan="2">
<h3>
Registration Form</h3>
</td>
</tr>
<tr>
<td>
Name:</td>
<td>
<asp:TextBox ID="txtName" runat="server"></asp:TextBox>
</td>
<td><asp:RequiredFieldValidator ID="RequiredFieldValidator2" runat="server" ErrorMessage="Enter Name" ControlToValidate="txtName"></asp:RequiredFieldValidator> </td>
</tr>
<tr>
<td>
Email</td>
<td>
<asp:TextBox ID="txtEmail" runat="server"></asp:TextBox>
</td>
<td><asp:RequiredFieldValidator ID="RequiredFieldValidator3" runat="server" ErrorMessage="Enter Email" ControlToValidate="txtEmail"></asp:RequiredFieldValidator> </td>
<td> <asp:RegularExpressionValidator ID="RegularExpressionValidator1" runat="server" ErrorMessage="Invaid Email" ValidationExpression="\w+([-+.']\w+)*@\w+([-.]\w+)*\.\w+([-.]\w+)*" ControlToValidate="txtEmail"></asp:RegularExpressionValidator> </td>
</tr>
<tr>

<td>
Role</td>
<td>
<asp:RadioButtonList ID="rbtRole" runat="server" RepeatDirection="Vertical"
AutoPostBack="True" onselectedindexchanged="rbtRole_SelectedIndexChanged">
<asp:ListItem>Administrator</asp:ListItem>
<asp:ListItem>Shop floor User</asp:ListItem>
<asp:ListItem>Key User</asp:ListItem>
<asp:ListItem>Others</asp:ListItem>
</asp:RadioButtonList>
</td>
<td><asp:RequiredFieldValidator ID="RequiredFieldValidator4" runat="server" ErrorMessage="Select role" ControlToValidate="rbtRole"></asp:RequiredFieldValidator> </td>
</tr>
<asp:Panel ID="panell" runat="server" Visible="false">
<tr>
<td></td><td><asp:TextBox ID="TextBox1" runat="server"></asp:TextBox></td>
<td><asp:RequiredFieldValidator ID="RequiredFieldValidator1" runat="server" ErrorMessage="Enter role" ControlToValidate="TextBox1"></asp:RequiredFieldValidator> </td>

</tr>
</asp:Panel>
<tr>
<td align="center" colspan="2">
<asp:Button ID="btn_register" runat="server" Text="Register"
onclick="btn_register_Click" />

</td>
</tr>
<tr>
<td align="center" colspan="2">
<asp:Label ID="lblmsg" runat="server"></asp:Label>
</td>
</tr>
</table>
</div>
</form>
</body>
</html>

using System;
using System.Collections;
using System.Configuration;
using System.Data;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Xml.Linq;
using System.Data.SqlClient;
namespace Required_Field_Validator_based
{
public partial class _Default : System.Web.UI.Page
{
string strConnString = ConfigurationManager.ConnectionStrings["ConnectionString"].ConnectionString;
SqlCommand com;

protected void btn_register_Click(object sender, EventArgs e)
{
if (rbtRole.Text == "Others")
{
SqlConnection con = new SqlConnection(strConnString);
com = new SqlCommand();
com.Connection = con;
com.CommandType = CommandType.Text;
com.CommandText = "insert into login values(@Name,@Email,@role)";
com.Parameters.Clear();
com.Parameters.AddWithValue("@Name", txtName.Text);
com.Parameters.AddWithValue("@Email", txtEmail.Text);
com.Parameters.AddWithValue("@role", TextBox1.Text);
if (con.State == ConnectionState.Closed)
con.Open();
com.ExecuteNonQuery();
con.Close();
lblmsg.Text = "Data entered successfully!!!";
clear();
}
else
{
SqlConnection con = new SqlConnection(strConnString);
com = new SqlCommand();
com.Connection = con;
com.CommandType = CommandType.Text;
com.CommandText = "insert into login values(@Name,@Email,@role)";
com.Parameters.Clear();
com.Parameters.AddWithValue("@Name", txtName.Text);
com.Parameters.AddWithValue("@Email", txtEmail.Text);
com.Parameters.AddWithValue("@role", rbtRole.SelectedValue);
if (con.State == ConnectionState.Closed)
con.Open();
com.ExecuteNonQuery();
con.Close();
lblmsg.Text = "Data entered successfully!!!";
clear();
}

}

protected void rbtRole_SelectedIndexChanged(object sender, EventArgs e)
{
lblmsg.Text = "";

if (rbtRole.SelectedValue == "Others")
{
panell.Visible = true;
TextBox1.CausesValidation = true;
}
else
{
panell.Visible = false;
TextBox1.CausesValidation = false;
}
}
private void clear()
{
TextBox1.Text = "";
txtEmail.Text = "";
txtName.Text = "";
rbtRole.ClearSelection();
}
}
}