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Question

I have a database and connected to Visual Studio 2017 with Entity Framework as model. And on the database, there is some table which connect to other. Like table SELECTION_HISTORY to table USER and to CANDIDATE . But how to connect those tables in C#?

I was trying to connect 2 tables and it works, but when I try to connect 3 tables, there is some problem with my code.

List<CandidateInterviewDTO> CandidateInterview = db.CANDIDATEs.Join(db.POSITIONs,
                        candidate => candidate.JUDUL_POSISI,
                        position => position.POSITION_ID,
                        (candidate, position) =>
                            new CandidateInterviewDTO
                            {
                                AppliedPosition = db.POSITIONs.FirstOrDefault(p => p.POSITION_ID == candidate.JUDUL_POSISI).POSITION_NAME,
                                SuitablePosition = db.POSITIONs.FirstOrDefault(p => p.POSITION_ID == candidate.SUITABLE_POSITION).POSITION_NAME,
                                Name = candidate.NAMA_LENGKAP,
                                Source = db.SOURCEs.FirstOrDefault(s => s.SOURCE_ID == candidate.SOURCE_ID).SOURCE_NAME,
                                PhoneNumber = candidate.NOHP,
                                Email = candidate.EMAIL
// If i try until this, its work. But when i try the code on below, there is some error with my code. Is it correct ?
                            }).Join(db.SELECTION_HISTORY,
                            candidatePosisition => candidatePosisition,
                            sh => sh.PIC_ID, (candidatePosition, selection) => new CandidateInterviewDTO
                            {
                                Name = db.CANDIDATEs.FirstOrDefault(c => c.NAMA_LENGKAP == candidatePosition.Name).NAMA_LENGKAP,
                                CandidateId = db.SELECTION_HISTORY.FirstOrDefault(sh => sh.PIC_ID == selection.PIC_ID).PIC_ID,
                            });

The error is

CS0411 The type arguments for method 'Queryable.Join<TOuter, TInner, TKey, TResult>(IQueryable<TOuter>, IEnumerable<TInner>, Expression<Func<TOuter, TKey>>, Expression<Func<TInner, TKey>>, Expression<Func<TOuter, TInner, TResult>>)' cannot be inferred from the usage. Try specifying the type arguments explicitly

Answer 1

You are invoking Join , and then chaining the result to another Join invocation.

The first call is returning a result of type IQueryable<CandidateInterviewDTO> .

So when you invoke the second join, the second and third parameters are lambdas that return the keys that are being compared. They are returning different types. - One type is of type CandidateInterviewDTO - The other type is whatever PIC is.

It's typical to prefer call-chaining to LINQ Query Syntax. However,when it comes to joining, Query Syntax absolutely shines.

Here's how you can approach the three way join using Query Syntax:

var candidateInterviews = from candidate in db.CANDIDATES
    join position in db.POSITIONS 
        on candidate.JUDUL_POSISI equals position.POSITION_ID
    join selectionHistory in db.SELECTION_HISTORY 
        on position.POSITION_ID equals selectionHistory.PID_ID
    select new CandidateInterviewDTO
    {
        Name = candidate.NAMA_LENGKAP,
        (...)
    }