Get count of selected items from asp listbox using jquery

Question

I am trying to get the count of selected items from an asp listbox using jquery but can't figure out how to do it. this is how I have it as of now, but I get "Cannot get property length of undefined or null reference" on selectedOptions property. var lb fills with objectHtmlSelectElement, so that seems right.

function CheckSelectionCount(sender, args) {
            var lb = document.getElementById(sender.controltovalidate);
            var count = lb.selectedOptions.length;
            args.IsValid = count >= 1 && count <= 5;
}

Many similar questions remedy this using a selector combined with the :selected attribute, but I think I need to leverage the sender argument and store it in a variable.

I'm sure this will be easy for the experts on here! Thanks in advance

Answer 1

jQuery to get the count

function CheckSelectionCount(sender, args) {
    var count = $("#" + sender.controltovalidate + " option:selected").length;
    args.IsValid = count >= 1 && count <= 5;
}

Without jQuery

function CheckSelectionCount(sender, args) {
    var lb = document.getElementById(sender.controltovalidate);
    var opts = lb.options;
    var count = 0;
    for(var i=0; i<opts.length; i++) {
        if(opts[i].selected) count++;
    }
    args.IsValid = count >= 1 && count <= 5;
}