On a webform there is a radiobutton list. The user must select at least one of the options for the form to be valid.

<div>
	<asp:Label ID="Label37" runat="server" Text="Is Sales Item:" AssociatedControlID="rblSaleItem" />
	<asp:RadioButtonList ID="rblSaleItem" runat="server" ClientIDMode="Static" TabIndex="36"
		RepeatDirection="Horizontal" RepeatLayout="Flow" CssClass="checkbox-list">
		<asp:ListItem Text="Yes" Value="1"></asp:ListItem>
		<asp:ListItem Text="No" Value="0"></asp:ListItem>
	</asp:RadioButtonList>
	<asp:CustomValidator ID="IsSaleItemCustomValidator" runat="server" Text="Is Sales Item: selection is required"
		Display="Dynamic" ClientIDMode="Static" ClientValidationFunction="IsSalesItemValidator"
		OnServerValidate="IsSalesItemValidation" ValidationGroup="FORMGROUP" CssClass="field-validation-error"></asp:CustomValidator>
</div>

Here we have the javascript validation. The radiobutton list and validator are in the same div element, so from the validation control triggered we go up to the parent to find the radiobutton list. Each radio button in the list is tested to see if it is selected.

// Javascript client side validation
function IsSalesItemValidator(sender, args) {
    args.IsValid = false;

    $("#" + sender.id).parent().find(".checkbox-list").find(":radio").each(function () {
        if ($(this).is(":checked")) {
            args.IsValid = true;
            return;
        }
    });
}

On the server side the validation works in a similar way.

// C# Server side validation
protected void IsSalesItemValidation(object source, ServerValidateEventArgs args)
{
    // has a sale item check box been selected
    foreach (ListItem li in rblSaleItem.Items)
    {
        if (li.Selected)
        {
            args.IsValid = true;
            break;
        }
    }
}

I actually do not like the idea of iterating through the radiobutton list to find if one is checked. Normally the radiobutton lists are quite short on a page so it is probably not a big overhead, but it is too old school lazy coding for my liking. In .Net3.5 the Enumerable class was introduced in the System.Linq namespace. Unsing the Enumerable.Cast method we can construct linq queries to retrieve the selected item from the radiobutton list instead of iterating all the items.

protected void IsSalesItemValidation(object source, ServerValidateEventArgs args)
{
    var selectedItem = (from li in rblSaleItem.Items.Cast<ListItem>()
                        where li.Selected == true
                        select li).FirstOrDefault();
    /*
    // Alternatively if you prefer inline linq
    var selectedItem = rblSaleItem.Items.Cast<ListItem>()
                        .Where(item => item.Selected == true).FirstOrDefault();
    */
    
    args.IsValid = SelectedItem != null && SelectedItem.Selected;
}

Using the FirstOrDefault() method will return null if there are not selected items in the radiobutton list. If we were to use the First() method an InvalidOperationException would be thrown if there are no elements that meet the query conditions.
It is safe to use the Cast method in this situation because it is guaranteed all the items in the collection are of the ListItem type. If the Cast method was used on a collection containing different objects an InvalidCastException will be thrown. If the collection object itself is null an ArgumentNullException will be thrown.