vs2012报这个错啥意思

------解决方案--------------------
var reportQuery = from a in examReportTable.AsEnumerable()
                              join b in rvuTable.AsEnumerable()
                         on new{exam=a.Field<int>("exam"), examtype=a.Field<string>("examtype")} equals new{exam=b.Field<int>("rvuName"),examtype=b.Field<string>("rvuType")}
                         group b by new{ a.reportDoc,a.examtype } into c                      
                        select new YourClassName
                        {                        
                        };