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);
            }  
        });
});

Answer 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");

Answer 2

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