LINQ的左外部联接运行时错误

Question

I'm trying to write a left outer join query with Linq on 2 tables, but getting NULL reference exception during runtime, when there are null values in the right table.

For all MatchID values as on tbl_Match table tbl_UserBets table does not have values for all. Hence when NULL value comes up, I'm getting run time exception

PFB my LINQ query,

string userID = "dfa3c0e7-2aa3-42ee-a7d3-803db902dc56";
var res2 = dbEntity.tbl_Match.Select(m => new
            {
                MatchID = m.MatchID,
                Team1 = m.Team1,
                Team2 = m.Team2,
                UserForTeam1 = dbEntity.tbl_UserBets.Where(b => b.UserForTeam1 == userID).FirstOrDefault(b => b.MatchID == m.MatchID),
                UserForTeam2 = dbEntity.tbl_UserBets.Where(b => b.UserForTeam2 == userID).FirstOrDefault(b => b.MatchID == m.MatchID)
            });

            foreach (var item in res2)
            {
                Console.WriteLine(item.MatchID + " " + item.Team1 + " vs " + item.Team2 + " " + item.UserForTeam1 == null ? " NA " : item.UserForTeam1.UserForTeam1);
            }

SQL Table design for tbl_Match table:

Create Table tbl_Match(
MatchID int primary key Identity,
TournamentID int Foreign key references tbl_Tournament(TournamentID),
Team1 int Foreign key references tbl_TournamentTeams(TeamID),
Team2 int Foreign key references tbl_TournamentTeams(TeamID),
StartTime DateTime not null,
MatchBetAmount int not null
);

SQL Table design for tbl_UserBets table:

Create Table tbl_UserBets(
UserBetSlNo int primary key identity,
TournamentID int Foreign key references tbl_Tournament(TournamentID),
MatchID int Foreign key references tbl_Match(MatchID),
UserForTeam1 nvarchar(128) Foreign key references AspNetUsers(Id),
UserForTeam2 nvarchar(128) Foreign key references AspNetUsers(Id),
UserForNoBets nvarchar(128) Foreign key references AspNetUsers(Id)
);

With the below query in SQL i'm able to get the results properly, Need to do the same with LINQ.

select DISTINCT(tbl_Match.MatchID),tbl_Match.Team1,tbl_Match.Team2,tbl_Match.StartTime,tbl_Match.MatchBetAmount,tbl_UserBets.UserForTeam1,tbl_UserBets.UserForTeam2,tbl_UserBets.UserForNoBets from tbl_Match left outer join tbl_UserBets on tbl_Match.MatchID = tbl_UserBets.MatchID and (tbl_UserBets.UserForTeam1 = 'dfa3c0e7-2aa3-42ee-a7d3-803db902dc56' or tbl_UserBets.UserForTeam2 = 'dfa3c0e7-2aa3-42ee-a7d3-803db902dc56')

Please let me know, what changes i should be doing to fix the issue. Thanks.

Answer 1

try merging the where condition with the first or default

i'm guessing the problem is when the where returns null the first or default will cause the exception as stated by msdn doc

var res2 = dbEntity.tbl_Match.Select(m => new
            {
                MatchID = m.MatchID,
                Team1 = m.Team1,
                Team2 = m.Team2,
                UserForTeam1 = dbEntity.tbl_UserBets.FirstOrDefault(b => b.UserForTeam1 == userID && b.MatchID == m.MatchID),
                UserForTeam2 = dbEntity.tbl_UserBets.FirstOrDefault(b => b.UserForTeam2 == userID && b.MatchID == m.MatchID)
            });