Using single ClientValidationFunction for similar validations using custom validator

Here is an example of how to write a common client validation function while using similar kind of validations on more than one asp.net control.
This example is showing how to validate two drop down list using a single javascript function for required
ASP.NET Markup

<asp:DropDownList ID="ddlCountry" runat="server" AutoPostBack="True" 
                onselectedindexchanged="ddlCountry_SelectedIndexChanged" CausesValidation="false">
            </asp:DropDownList>
            <asp:CustomValidator ID="cvCountry" runat="server" ControlToValidate="ddlCountry"
                EnableClientScript="true" ValidationGroup="REG" Display="None" ErrorMessage="Country is required" Text=" "
                ClientValidationFunction="validateCbo"></asp:CustomValidator>
<asp:DropDownList ID="ddlState" runat="server">
            </asp:DropDownList>
            <asp:CustomValidator ID="cvState" runat="server" ControlToValidate="ddlState"
                EnableClientScript="true" ValidationGroup="REG" Display="None" ErrorMessage="State is required" Text=" "
                ClientValidationFunction="validateCbo"></asp:CustomValidator>

Javascript client validation function:

function validateCbo(source, args) {
        var ddl = null;
        if(source.controltovalidate.indexOf('Country')>-1)
            ddl = document.getElementById('<%=ddlCountry.ClientID %>');
        else
            ddl = document.getElementById('<%=ddlState.ClientID %>');
        if (ddl != null) {
            args.IsValid = !(ddl.options[ddl.selectedIndex].value == 0);            
        }
        else
            args.IsValid = true;
    }