我用entity framework 4的code first 在两表关联时如果一个字段为guid一个字段为string用ToString()方法强转时会报错，请问如何关联不同数据类型的字段。如下在on new { EmpGuid = emps.EmpGuid.ToString(), AreaCode = areaCode } equals new { EmpGuid = flowCode.ObjectId, AreaCode = flowCode.AreaCode }时报错了，数值型与字符型转换可以用SqlFunctions的方法实现，非数值型呢？

DbSet<tb_Emp_Manager_Zbdl> dbEmp = zbdlcontext.Emp_Manager;
            DbSet<Tb_Corp_CompanyInfo_Zbdl> dbCompany = zbdlcontext.Corp_CompanyInfos;
            DbSet<Tb_FlowCode_Zbdl> dbFlowCode = zbdlcontext.FlowCodes;
            DbSet<Tb_Emp_Post_Zbdl> dbEmpPost = zbdlcontext.Emp_Post;
            var query = from emps in dbEmp
                        join companies in dbCompany on emps.CorpGuid equals companies.CorpUserGuid
                        join flowCode in dbFlowCode on new { EmpGuid = emps.EmpGuid.ToString(), AreaCode = areaCode } equals new { EmpGuid = flowCode.ObjectId, AreaCode = flowCode.AreaCode } into jFlowCodes
                        from flowCodes in jFlowCodes.DefaultIfEmpty()
                        join post in dbEmpPost on emps.EmpGuid equals post.EmpGuid into jPosts
                        from posts in jPosts.DefaultIfEmpty()
                        select new tb_Emp_Manager()
                        {
                            EmpGuid = emps.EmpGuid,
                            AreaCode = emps.AreaCode,
                            FlowCode = flowCodes.FlowCode,
                            corpName = companies.CorpName
                        };
            return query.ToList();