Checkbox list values empty when ModelState is not valid and ASP.NET Core does return Page() - asp.net-core

Question

I created an ASP.NET Core 3.1 project. I have a form in it with several checkbox lists. I can get the values into the properties in POST and they are correctly filled in (e.g. List SelectedItems). However for some custom fields I have to do a validation in OnPost() method and if the conditions are not met or a ModelState is not valid, it return Page(). Normally I would expect that every property that was filled in in the form is still filled in, but the checkboxes are always empty and not a single one is checked. The other data (radio buttons, textboxes, etc.) are still filled in.
I even tried to put the values within the Razor page, but even then neither of the checkboxes was checked.
Here is an example of one of the checkboxes:
In Razor page:
#for (var i = 1; i <= 10; i++){
<input name="AreChecked" type="checkbox" id="#i" value="#i" /> #i<br />
<input type="hidden" value="true" id="#i" name="AreChecked" />}
Behind code:
[BindProperties]
public class TestFormModel : PageModel
{
[BindProperty]
public List<int> AreChecked { get; set; }}
public IActionResult OnPost()
{
//some other form check statements here
//...
if (ModelState.IsValid)
{
//process data code...
}
return Page();
}
Can someone help me with this?

Answer 1

You could use JQuery to achieve as shown :
#page
#model RazorPages3_1.AddimgModelModel
<div class="row">
<div class="col-md-4">
<form enctype="multipart/form-data" method="post">
<div asp-validation-summary="ModelOnly" class="text-danger"></div>
<div class="form-group">
<label asp-for="Petimg.PetName" class="control-label"></label>
<input asp-for="Petimg.PetName" class="form-control" />
<span asp-validation-for="Petimg.PetName" class="text-danger"></span>
</div>
<div class="form-group">
<input asp-for="Uploads" class="form-control" />
</div>
#for (var i = 1; i <= 10; i++)
{
<input name="AreChecked" type="checkbox" id="#i" value="#i" /> #i<br />
<input type="hidden" value="true" id="#i" name=""AreChecked" />
}
<div class="form-group">
<input type="submit" value="Create" class="btn btn-primary" />
</div>
</form>
</div>
</div>
<div>
<a asp-page="Index">Back to List</a>
</div>
#section Scripts {
<script>
var checkedlist = #Html.Raw(Json.Serialize(Model.AreChecked));;
if (checkedlist.length > 0) {
$.each(checkedlist, function (index, value) {
$('input[type=checkbox]').each(function () {
var id = $(this).attr("id");
if (id == value) {
$(this).attr('checked', 'checked');
}
})
});
}
</script>
}
Result