Hi nauna,
First of all you don't need to enable and disable requiredfieldvalidator, only you have to set initial value of you dropdownlist.
Refer below sample.
HTML
WebUserControl.ascx
<%@ Control Language="C#" AutoEventWireup="true" CodeFile="WebUserControl.ascx.cs"
Inherits="WebUserControl" %>
<asp:DropDownList ID="ddlItems" runat="server">
<asp:ListItem Value="none">Select</asp:ListItem>
<asp:ListItem Value="Item A">Item A</asp:ListItem>
<asp:ListItem Value="Item B">Item B</asp:ListItem>
<asp:ListItem Value="Item C">Item C</asp:ListItem>
</asp:DropDownList>
<asp:RequiredFieldValidator ID="RequiredFieldValidator1" runat="server" ControlToValidate="ddlItems" InitialValue="none"
ErrorMessage="RequiredFieldValidator" Display="Dynamic" ForeColor="Red"></asp:RequiredFieldValidator>
CS.aspx
<%@ Page Language="C#" AutoEventWireup="true" CodeFile="CS.aspx.cs" Inherits="CS" %>
<%@ Register Src="~/WebUserControl.ascx" TagName="ControlA" TagPrefix="ctrlA" %>
<!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></title>
</head>
<body>
<form id="form1" runat="server">
<div>
<ctrlA:ControlA ID="cA" runat="server" />
<asp:Button Text="Submit" runat="server" OnClick="Save" />
</div>
</form>
</body>
</html>
Code
C#
WebUserControl.ascx.cs
public RequiredFieldValidator ControlB_RC
{
get
{
return this.RequiredFieldValidator1;
}
}
CS.aspx.cs
protected void Save(object sender, EventArgs e)
{
DropDownList ddl = this.FindControl("cA").FindControl("ddlItems") as DropDownList;
}
Screenshot
