LEFT OUTER JOIN in LINQ

Question

How to perform left outer join in C# LINQ to objects without using join-on-equals-into clauses? Is there any way to do that with where clause? Corr

Answer 1

Extension method that works like left join with Join syntax

public static class LinQExtensions
{
    public static IEnumerable LeftJoin(
        this IEnumerable outer, IEnumerable inner, 
        Func outerKeySelector, 
        Func innerKeySelector, 
        Func resultSelector)
    {
        return outer.GroupJoin(
            inner, 
            outerKeySelector, 
            innerKeySelector,
            (outerElement, innerElements) => resultSelector(outerElement, innerElements.FirstOrDefault()));
    }
}

just wrote it in .NET core and it seems to be working as expected.

Small test:

        var Ids = new List { 1, 2, 3, 4};
        var items = new List>
        {
            new Tuple(1,"a"),
            new Tuple(2,"b"),
            new Tuple(4,"d"),
            new Tuple(5,"e"),
        };

        var result = Ids.LeftJoin(
            items,
            id => id,
            item => item.Item1,
            (id, item) => item ?? new Tuple(id, "not found"));

        result.ToList()
        Count = 4
        [0]: {(1, a)}
        [1]: {(2, b)}
        [2]: {(3, not found)}
        [3]: {(4, d)}