Simulating a RequiredFieldValidator with RadComboBox

Telerik’s RadComboBox does not behave like a traditional DropDownList. Instead of a RequiredFieldValidator, you should use a CustomValidator. Here’s an implementation.

Step 1: Markup

<asp:Label ID="lblExample" runat="server" AssociatedControlID="rcbExample"
    Text="Example" />
<telerik:RadComboBox ID="rcbExample" runat="server" />
<asp:CustomValidator ID="cvlExample" runat="server" ControlToValidate="rcbExample"
    Text="*" ClientValidationFunction="cvlExampleValidate"
    OnServerValidate="cvlExample_ServerValidate" />

Step 2: Client-side validation

function cvlExampleValidate(source, args) {
    args.IsValid = radComboValidate("<%= rcbExample.ClientID %>");
}

function radComboValidate(controlName) {
    var combo = $find(controlName);
    var text = combo.get_text();

    if (text.length < 1) return false;
    var node = combo.findItemByText(text);
    if (node) {
        var value = node.get_value();
        return value.length > 0;
    }
    return false;
}

Step 3: Server-side validation

protected void cvlStatus_ServerValidate(object source, ServerValidateEventArgs args)
{
    args.IsValid = rcbStatus.SelectedValue.Length > 0;
}

Note (2025): This article is from 2010. Telerik’s RadControls have evolved significantly since then; check the current Telerik UI for ASP.NET AJAX documentation for modern practices.