How to make Linq join using Group by on 2 different list of sharepoint

SPQuery querydossier = new SPQuery();
SPList sPlstapplied = web.Lists[lst_appliedproducts];
querydossier.Query = "" + 1 + "" + 1 + "";
SPListItemCollection oitmdossier = sPlstapplied.GetItems(querydossier);

var dossiserAll = from SPListItem itmdossier in sPlstapplied.GetItems(querydossier)
join SPListItem itmcountry in lstcountry.GetItems(querycountry)
on Convert.ToInt32(Convert.ToString(itmdossier[p_CountryFk]).Split(';')[0].ToString()) equals Convert.ToInt32(itmcountry[p_CountryFk].ToString().Split(';')[0].ToString())
// equals new SPFieldLookupValue(itmcountry[p_CountryFk] as string).LookupValue

group itmdossier by new { resDosParent = Convert.ToInt32(itmdossier[_DossierParentID])} into dossierRegisterFinal
// group itm by new { grp_dossierParent = Convert.ToInt32(itm["DossierParentID"]) } into gDosser

select new { m_registerFinaloutput = dossierRegisterFinal,m_lstREgisterDossier= dossierRegisterFinal.ToList() };
foreach (var oitm in dossiserAll)
{
if (oitm.m_registerFinaloutput.Count() > 0)

foreach (var childitem in oitm.m_lstREgisterDossier)
{
if (!childitem[_DossierType].Equals(1))
{
countryID = +';' + Convert.ToString(childitem[_DossierParentID]);
oDossier.registercount = +Convert.ToInt32(childitem[_DossierParentID]);
}
}
}