Hi
No the code behind won't need changed. You can implement validation using just asp.net tags which validate the field contains data.
This is my example of how to add a field validation to the company name and the phone fields.
<tr>
<td><asp:Label id="lblCompany" CssClass="storeQuoteFormLabels" Runat="server" resourcekey="lblCompany"/></td>
<td>
<asp:TextBox ID="txtCompany" Columns="40" runat="server"></asp:TextBox>
<asp:RequiredFieldValidator
ID="rfvCompany" runat="server" ControlToValidate="txtCompany" errorMessage="<br/>Company is required" Display="Dynamic"></asp:RequiredFieldValidator>
</td>
</tr>
<tr>
<td><asp:Label id="lblPhone" CssClass="storeQuoteFormLabels" Runat="server" resourcekey="lblPhone"/></td>
<td>
<asp:TextBox ID="txtPhone" Columns="40" runat="server"></asp:TextBox>
<asp:RequiredFieldValidator
ID="rfvPhone" runat="server" ControlToValidate="txtPhone" errorMessage="<br/>Phone is required" Display="Dynamic"></asp:RequiredFieldValidator>
</td>
</tr>
I've marked my changes in bold.
You could apply a similar change to the comment field.
Regards
Mark