Question

I have an ASP.NET usercontrol that implements the ValidationProperty attribute. This attribute successfully makes it possible for me to use a RequiredFieldValidator for my custom control, however on validation it causes a full postback rather than using client side javascript based validation.

Is there a way to prevent this and enable client side validation without using a custom validator?

This is the what my UserControl looks like.

<%@ Control Language="C#" AutoEventWireup="true" CodeBehind="ucBooleanRadio.ascx.cs" Inherits="MyCompany.Web.UserControls.ucBooleanRadio" %>

<div class="BooleanRadio">
    <input runat="server" id="radTrue" type="radio" name="BooleanRadio" value="True" /> Yes
    <input runat="server" id="radFalse" type="radio" name="BooleanRadio" value="False" /> No
</div>

[ValidationProperty("Checked")]
public partial class ucBooleanRadio : System.Web.UI.UserControl
{
    public Nullable<bool> Checked
    {
        get
        {
            if (radTrue.Checked || radFalse.Checked)
                return radTrue.Checked;
            else
                return null;
        }
        set
        {
            radTrue.Checked = value != null ? value.Value : false;
            radFalse.Checked = value != null ? !value.Value : false;                    
        }
    }
}

And this is how it is being used

<uc1:ucBooleanRadio ID="ucAgree" runat="server" />
<asp:RequiredFieldValidator ID="RequiredFieldValidator6" runat="server" CssClass="Validator" Display="Dynamic" ControlToValidate="ucAgree" InitialValue="" ErrorMessage="You must agree to continue."></asp:RequiredFieldValidator>

Page.Validate();
if (Page.IsValid)
{
    //Do stuff
}

Answer 1

Turns out, ASP.NET don’t event care about element tag.
I’ve just looked through validation code and found this

function ValidatorGetValue(id) {
    var control;
    control = document.getElementById(id);
    if (typeof(control.value) == "string") {
        return control.value;
    }
    return ValidatorGetValueRecursive(control);
}

So

<div class="BooleanRadio" id="<%= ClientID %>" value="<%= radTrue.Checked? "true" : radFalse.Checked? "false" : "" %>">
    <% radTrue.Attributes["onclick"] = "document.getElementById('" + ClientID + "').value='true'"; %>
    <% radFalse.Attributes["onclick"] = "document.getElementById('" + ClientID + "').value='false'"; %>
    <input runat="server" id="radTrue" type="radio" name="BooleanRadio" value="True" /> Yes
    <input runat="server" id="radFalse" type="radio" name="BooleanRadio" value="False" /> No
</div>

actually works :\, as does

<div class="BooleanRadio" id="<%= ClientID %>">
    <input runat="server" id="radTrue" type="radio" name="BooleanRadio" value="True" /> Yes
    <input runat="server" id="radFalse" type="radio" name="BooleanRadio" value="False" /> No
</div>

And there is events autohooking – validated element (one with ClientID) and its children are wired to cause validation automatically (look ValidatorHookupControl).
That may result in:
1. user does something
2. validation is performed
3. value to validate is updated (after validation!)
First example with value on div behaves this way.