c# linq with group by and join throwing exception

问题

(from p in this.m_dbContext.Patient
        join b in (from a in this.m_dbContext.Appointments
        join p in this.m_dbContext.Patient on a.Patientid equals 
        p.Patientid
        where a.Doctorid == doctorid && a.Clinicid == clinicid
        group a by a.Patientid)
        on p.Patientid equals b.FirstOrDefault().Patientid
        orderby p.Name
        select new
        {
          p.Patientid,
          p.Clinicid,
          p.Name,
          p.Mobilenumber,
          p.Gender,
          p.Dob,
          p.Age,
          p.Address,
          p.City,
          p.State,
          p.Pincode
       }).ToList().Count();

I get the below exception when i run, i use group by in order to remove the duplicates in the result set

Exception:

The LINQ expression 'FirstOrDefault(GroupByShaperExpression: KeySelector: a.patientid, ElementSelector:EntityShaperExpression: EntityType: Appointments ValueBufferExpression: ProjectionBindingExpression: EmptyProjectionMember IsNullable: False )' could not be translated. Either rewrite the query in a form that can be translated, or switch to client evaluation explicitly by inserting a call to either AsEnumerable(), AsAsyncEnumerable(), ToList(), or ToListAsync(). See https://go.microsoft.com/fwlink/?linkid=2101038 for more information.

回答1:

Perhaps your groupby could be changed to overcome this obstacle. The complexity of translating a GroupBy linq query to a proper sql query is still an ongoing development issue with dotnetcore.

Edit:
I see a closed issue on Github: Query with GroupBy or GroupJoin throws exception #17068
But I'm unsure whether the are still working on the GroupBy problem, or if it is fixed or if they will not do anything about it.

Perhaps you can change your query to the following: Note that I removed the .ToList() before the count, because that seems overkill to me if you just need the count.

var patientIds = (from a in this.m_dbContext.Appointments
                  join p in this.m_dbContext.Patient on a.Patientid equals p.Patientid
                  where a.Doctorid == doctorid && a.Clinicid == clinicid
                  select a.Patientid).Distinct();

var items = (from p in this.m_dbContext.Patient
             join patientId in patientIds on p.Patientid equals patientId
             orderby p.Name
             select new
             {
                 p.Patientid,
                 p.Clinicid,
                 p.Name,
                 p.Mobilenumber,
                 p.Gender,
                 p.Dob,
                 p.Age,
                 p.Address,
                 p.City,
                 p.State,
                 p.Pincode
             }).ToList();

Or if you just need the count:

var count = (from p in this.m_dbContext.Patient
             join patientId in patientIds on p.Patientid equals patientId
             orderby p.Name
             select new
             {
                 p.Patientid
             }).Count();

来源：https://stackoverflow.com/questions/61502253/c-sharp-linq-with-group-by-and-join-throwing-exception