How to Perform a Left Outer Join in LINQ Without Using Join-On-Equals-Into Clauses?

C# LINQ Left Outer Join: An Alternative Approach

This article demonstrates how to perform a left outer join in LINQ to Objects without using the traditional join-on-equals-into clause. We leverage the from and join operators in conjunction with DefaultIfEmpty() for a concise and efficient solution.

Here's the alternative syntax:

var q =
    from l in lefts
    join r in rights on l.Key equals r.Key into ps_jointable
    from p in ps_jointable.DefaultIfEmpty()
    select new JoinPair { LeftId = l.Id, RightId = p?.Id ?? 0 };

This LINQ query executes a left outer join on the lefts and rights collections, generating a new collection of JoinPair objects. For each item in lefts, it attempts to find a matching item in rights based on the Key property. If a match exists, RightId in JoinPair will hold the Id of the matched rights item. If no match is found, RightId defaults to 0 using the null-conditional operator (?.) and null-coalescing operator (??).

The DefaultIfEmpty() method is essential. It ensures a result is produced even when no matches are found in the rights collection. In the absence of a match, DefaultIfEmpty() returns a default value (null in this case), which is then handled gracefully by the null-conditional and null-coalescing operators. This approach provides a more readable and potentially more performant alternative to the standard join-on-equals-into syntax for left outer joins in LINQ.

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