Left join on two Lists and maintain one property from the right with Linq

Question

I have 2 lists of the same type. The left list:

var leftList = new List();
leftList.Add(new Person {Id = 1, Name = \"John\", Chang         


        

Answer 1

This looks like a pretty standard left outer join scenario.

I always keep this extension method handy for left outer joins so I don't have to look up how to use the nasty query syntax (or remember wtf a GroupJoin is)...

public static class LinqEx
{
    public static IEnumerable LeftOuterJoin(
        this IEnumerable outer, 
        IEnumerable inner, 
        Func outerKeySelector, 
        Func innerKeySelector, 
        Func resultSelector)
    {
        return outer
            .GroupJoin(inner, outerKeySelector, innerKeySelector, (a, b) => new
            {
                a,
                b
            })
            .SelectMany(x => x.b.DefaultIfEmpty(), (x, b) => resultSelector(x.a, b));
    }
}

Now you can:

leftList.LeftOuterJoin(
     rightList, 
     lft => lft.Id,
     rgt => rgt.Id,
     (lft, rgt) => new Person{Id = lft.Id, 
                              Name = lft.Name, 
                              Changed = rgt == null ? lft.Changed : rgt.Changed})