sql 转 linq， join group by 一起用的问题

SELECT o.OrderType, o.InsCompanyID, o.CorpCompanyID, item.ContractorID, COUNT(o.ID) FROM MobileOrder o
INNER JOIN MobileOrderServiceItem item ON o.ID=item.MobileOrderID
GROUP BY o.OrderType, o.InsCompanyID, o.CorpCompanyID, item.ContractorID
把这个转成 linq怎么写