ASP.Net Linq query Error: Only primitive types or enumeration types are supported in this context

If we want to join varable list with table using linq we have to make variable of that table which we have to join. I got the solution and it is working also. below i have attached  working code which i have solved:

var cf = from a in ent.Forms
            select new
                {
                    a.TaxFormID,
                    a.TaxFormName,
                    Name = a.TaxFormName,
                    a.Common
                };
var custForms = cf.ToList();

var cmf = from a in entEP.FormsEPs
            select new
            {
                a.TaxFormID,
                a.TaxFormName,
                Name = a.TaxFormName,
                a.Common
            };
var commonForms = cmf.ToList();

var formdoc = commonForms.Concat(custForms);

var alert = from a in ent.AlertNotifications
            select new
            {
                a.AlertID,
                a.AlertOnly,
                a.EmailAdmin,
                a.FormID,
                a.Acknowledgment,
                a.SignatureRequired,
                a.NoNotification,
                a.IsCommon
            };
var alertno = alert.ToList();

var dt = from a in alertno
            join b in formdoc on a.FormID equals b.TaxFormID
            where a.IsCommon == b.Common
            select new
            {
                a.AlertID,
                a.AlertOnly,
                a.EmailAdmin,
                a.FormID,
                b.TaxFormName,
                a.Acknowledgment,
                a.SignatureRequired,
                a.NoNotification
            };
Radgridformsdocuments.DataSource = dt.ToList();
DataTable d2 = ToDataTable(dt.ToList());
ViewState["Radgridformsdocuments"] = d2;