How can get value of an item based on ID in cshtml?

Question

Let says I have a select tag with a specific id and multiples option. Then below, I want to display partial view base on the selected . I think of using something as if else statement but I don't know how to gain the value of that select with c#.

My thought about this is like this

    <select id="selectItem">
        <option value="A">A</option>
        <option value="B">A</option>
        <option value="C">A</option>
    </select>
    
    @if ('selectItem' == "A"){
        <partial name="..."/>
    }

Answer 1

you can do it with js,here is a demo: Controller(Test):

public IActionResult Index()
        {
            return View();
        }
        public IActionResult Partial()
        {
            return PartialView("Partial1");
        }

Partial1:

    <h1>Partial1</h1>
Index.cshtml:
<select id="selectItem">
    <option value="A">A</option>
    <option value="B">B</option>
    <option value="C">C</option>
</select>
<div id="PartialContent"></div>

js:

<script>
        $(function () {
            getPartial();
        });
        $("#selectItem").change(function () {
            getPartial();
        });
        function getPartial() {
            if ($("#selectItem").val() == "C") {
                $.ajax({
                    type: 'GET',
                    url: "/Test/Partial",
                    dataType: 'html', //dataType - html
                    success: function (result) {
                        //Create a Div around the Partial View and fill the result
                        $('#PartialContent').html(result);
                    }
                });
            }else {
                     $('#PartialContent').html("");
             }
        }
    </script>

result: