@{
    ViewData["Title"] = "DishUpdate";
}
<div class="text-center">
    <h2 class="display-4">Данные блюда</h2>
</div>
<form method="post">
    <div class="row">
        <div class="col-4">Блюдо:</div>
        <div class="col-8"><select id="Id" name="Id" class="form-control" onchange="populateFields()" asp-items="@(new SelectList(@ViewBag.DishList, "Id", "DishName"))"></select></div>
    </div>
    <div class="row">
        <div class="col-4">Название:</div>
        <div class="col-8"><input type="text" name="name" value="" /></div>
    </div>
    <div class="row">
        <div class="col-4">Цена:</div>
        <div class="col-8"><input type="text" name="price" value="" /></div>
    </div>
    <div class="row">
        <div class="col-8"></div>
        <div class="col-4"><input type="submit" value="Сохранить" class="btn btn-primary" /></div>
    </div>
</form>
<script>
    function populateFields() {
        var selectedDishId = document.getElementById("dish").value;
        var dishes = @Json.Serialize(ViewBag.CookList);
        var selectedDish = dishes.find(function (dish) {
            return dish.Id === selectedDishId;
        });

        if (selectedDish) {
            document.getElementsByName("name")[0].value = selectedDish.DishName;
            document.getElementsByName("price")[0].value = selectedDish.Price;
        }
    }
</script>