Get selected (checked) CheckBox values as Comma Separated String in Controller in ASP.Net MVC

hello,

i have this checkbox list bind with list working fine

on button submit event i want to get all the selected checkboxlist value as comma seperated in my string on my controller 

@model IEnumerable<MVCApp.Models.FirstCategories>
@using (@Html.BeginForm("CheckboxListTopcategoryMenu_post", "Application", FormMethod.Get))
{  
foreach (var item in Model)
{
    @Html.CheckBoxFor(m => item.isSelected)
    @Html.DisplayFor(m => item.Title)
    @Html.HiddenFor(m => item.id)
}
<input id="Button2" type="submit" class="btn red" value="Search" />
}

but it gives error pls advice

public ActionResult CheckboxListTopcategoryMenu_post(List<FirstCategories> categoryselected)
{
    string _searches = "";
    foreach (FirstCategories cat in categoryselected)
    {
        _searches = cat.isSelected.ToString();
    }
    return View();
}