How do I get a left outer join in LINQ.

[abhijit]

You use the DefaultIfEmpty method on the right-hand list to perform a left outer join:

http://www.hookedonlinq.com/OuterJoinSample.ashx

That would work very well with two tables, but with three lists, I run into a bit of a problem.

Here's my code to join the three lists.

var leftJoin = from s in students
                           join e in scores on s.StudentId equals e.StudentId into joinedScores
                           from jS in joinedScores.DefaultIfEmpty()
                               select new
                                   {
                                     score = jS ==null ? 0 :  jS.Score,
                                     testDate = jS == null ? "Did Not Take Test" : jS.TestDate.ToString("yyyy-dd-MM"),
                                     testId = jS == null ? 0 :jS.TestId,
                                     studentName = s.Name
                                   };
 
            var secondLeft = from t in exams join lj in leftJoin on t.TestId equals lj.testId into joinedTests
            from jt in joinedTests.DefaultIfEmpty()
                        select new 
                                {
                                    testName = t.Name,
                                    testId = t.TestId,
                                    score = jt == null ? 0 : jt.score,
                                    testDate = jt == null ? "Did not take test" : jt.testDate,
                                    studentName = jt == null ? "Not available" : jt.studentName
                                };
 
            
            foreach (var lJ in secondLeft)
            {
                Console.WriteLine("{3} \t{0}\t{1}\t{2}", lJ.score, lJ.studentName, lJ.testDate, lJ.testName);
            }

here is the output it produces

English         9.11    Steyn Jacob     2013-01-01
English         9.11    Elisha Martin   2013-01-01
English         9.11    Jim Martin      2013-01-01
Math    8.11    Steyn Jacob     2013-01-02
Science         8.11    Elisha Martin   2013-01-03
Geography       0       Not available   Did not take test
French  0       Not available   Did not take test
History         0       Not available   Did not take test 

I would instead prefer to have a listing that has all the students as well as all the tests.