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

I see no jQuery there. The error means selectedOptions is undefined. Where do you set it? — epascarello
– epascarello, Commented Oct 15, 2015 at 16:04
Now that you changed it, there is no selectedOptions in JavaScript for a select. Hence undefined. — epascarello
– epascarello, Commented Oct 15, 2015 at 16:09
Sorry, that got left out. I dont understand the downvote, this is a valid question. I got the selectedOptions property by viewing another question on SO. Maybe the question should be reworded to what should I use instead? — quirky purple
– quirky purple, Commented Oct 15, 2015 at 16:12
Well that is not a valid property so it is wrong. You need to look at the link I posted to count the options that were selected. If you are actually using jQuery it is easy. — epascarello
– epascarello, Commented Oct 15, 2015 at 16:15

epascarello · Accepted Answer · 2015-10-15 16:19:08Z

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;
}

answered Oct 15, 2015 at 16:19

epascarello

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1 Comment

quirky purple Over a year ago

That worked. Sorry if my wording was confusing, I often don't know how best to word to depict the problem.

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