Passing parameters between a Controller and JavaScript in an ASP.NET Core MVC app?

Question

I have an ASP.NET Core MVC application with a view, controller and a small JavaScript file. A JavaScript function gets called when the user clicks on an item in a listbox. The JavaScript calls a function on the controller, the controller creates a SelectedListItem list and sets the values for 'Selected', 'Value' and 'Text'. 'Selected' is set to true.

What I noticed is when the JavaScript function receives the data, everything is correct but the 'Selected' field is set to false' I am not sure how that can be. Thank you for your help!

View

        <div class="form-group">
          @Html.LabelFor(x => Model.SelectedBaselines, htmlAttributes: new { @class = "col-form-label" })
          @Html.ListBoxFor(x => Model.SelectedBaselines, new SelectList(Model.BaselinesList, "Value", "Text"), htmlAttributes: new { @class = "form-control", id = "baselines", @multiple = true, size = "10", style = "width:200px;" })
          @Html.ValidationMessageFor(x => Model.SelectedBaselines, "", new { @class = "text-danger" })
        </div>

Controller

public IEnumerable<SelectListItem> GetSelectedBaselines(string meterId)
{
  IQueryable<SelectListItem> baselines =
    from mlookup in _dbContext.MeterBaselineLookup.AsNoTracking()
    join baseline in _dbContext.MeterBaseline on mlookup.MeterBaselineId equals baseline.Id
    where mlookup.MeterId == Convert.ToInt32(meterId)
    orderby baseline.Id
    select new SelectListItem
    {
      Selected = true,
      Value = baseline.Id.ToString(),
      Text = baseline.Name
    };

  return new SelectList(baselines, "Value", "Text");
}

JavaScript

$('#meters').change(function() {
    var url = "/Meters/GetSelectedBaselines";

    $.getJSON(url,
        { meterId: $("#meters option:selected").val() },
        function(data) {

            $('#baselines option').prop('selected', false); // Clear selected baselines

            var len = $('#baselines option').length;

            for (var i = 1; i <= len; i++) {  
                $("#baselines option[value=" + i + "]").attr("selected", true);
            }  
        });
});

There is not point setting the 4th parameter in the SelectList constructor. It's ignored when binding to a model property because internally the ListBoxFor() method builds its own IEnumerable<SelectListItem> and sets the Selected property based on the value of the property your binding to. Therefore send a value as a 4th parameter that would be a condition and based on that, you can mark your item as selected. — Rahul Sharma
– Rahul Sharma, Commented Apr 4, 2019 at 16:47
Please do not repost your same question in different posts. I've closed your previous one as a duplicate of this one, since this one has a bit more information associated with it. In the future, edit your existing question to add information. — Chris Pratt
– Chris Pratt, Commented Apr 4, 2019 at 16:55
Chris, they are not duplicates but two completely different issues. — PKonstant
– PKonstant, Commented Apr 4, 2019 at 17:47

Xueli Chen · Accepted Answer · 2019-04-05 06:46:32Z

1

Try just return below

public IEnumerable<SelectListItem> GetSelectedBaselines(string meterId)
{
 IQueryable<SelectListItem> baselines =
from mlookup in _dbContext.MeterBaselineLookup.AsNoTracking()
join baseline in _dbContext.MeterBaseline on mlookup.MeterBaselineId equals baseline.Id
where mlookup.MeterId == Convert.ToInt32(meterId)
orderby baseline.Id
select new SelectListItem
{
  Selected = true,
  Value = baseline.Id.ToString(),
  Text = baseline.Name
};

return baselines ;

}

Reference link for related questions：

https://stackoverflow.com/a/25714016/10201850 https://forums.asp.net/t/1382564.aspx?SelectListItem+losting+select+value+in+View+page

edited Apr 5, 2019 at 6:46

answered Apr 5, 2019 at 6:38

Xueli Chen

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Comments

user10954641 · Accepted Answer · 2019-04-04 16:56:36Z

1

The 4th parameter is meant to store the value being selected, kindly try:

return new SelectList(baselines, "Value", "Text",baselines[0].baseline.Id);

Or just this:

return new SelectList(baselines, "Value", "Text");

edited Apr 4, 2019 at 16:56

answered Apr 4, 2019 at 16:42

user10954641

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1 Comment

PKonstant Over a year ago

I removed the fourth parameter and makes no difference. (I actually added the fourth parameter just to see if it would work.)

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