c# - ASP.NET MVC InvalidOperationException was unhandled by user code while following book example -

namespace sportsstore.domain.entities {     public class product     {         [hiddeninput(displayvalue = false)]         public int productid { get; set; }          //[required(errormessage = "please enter product name")]         public string name { get; set; }         [datatype(datatype.multilinetext)]          //[required(errormessage ="please enter description")]         public string description { get; set; }          //[required]         //[range(0.01, double.maxvalue, errormessage = "please enter positive price")]         public decimal price { get; set; }          //[required(errormessage = "please specify category")]         public string category { get; set; }     } } 

this entity definition, , below action controller.

    public viewresult list(string category, int page = 1)     {         productslistviewmodel model = new productslistviewmodel         {             products = repository.products.where(p => category == null || p.category == category).orderby(p => p.productid).skip((page - 1) * pagesize).take(pagesize),              paginginfo = new paginginfo             {                 currentpage = page,                 itemsperpage = pagesize,                 totalitems = category == null ? repository.products.count() : repository.products.where(e => e.category == category).count()             },              currentcategory = category         };          return view(model);     } 

i invalidoperationexception complain visual studio when uncomment [required...] in product class. works fine is(commented).

i tried fix , googled can't solve problem myself.

thanks in advance.