LINQ Join 2 Lists

Question

Preface: I don\'t understand what this does:

o => o.ID, i => i.ID, (o, id) => o

So go easy on me. :-)

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I have 2 lis

Answer 1

Joins are not so difficult, but your problem could probably use some further explanation.

To join two lists, you could do something like

var joined = from Item1 in list1
             join Item2 in list2
             on Item1.Id equals Item2.Id // join on some property
             select new { Item1, Item2 };

this will give an IEnumerable<'a>, where 'a is an anonymous type holding an item from list1 and its related item from list2. You could then choose which objects' properties to use as needed.

To get the result to a concrete list, all that is needed is a call to .ToList(). You can do that like

var list3 = joined.ToList();
// or
var list3 = (from Item1 in list1
             join Item2 in list2
             on Item1.Id equals Item2.Id // join on some property
             select new { Item1, Item2 }).ToList();

To do a left join to select all elements from list1 even without a match in list2, you can do something like this

var list3 = (from Item1 in list1
             join Item2 in list2
             on Item1.Id equals Item2.Id // join on some property
             into grouping
             from Item2 in grouping.DefaultIfEmpty()
             select new { Item1, Item2 }).ToList();

This will give you a list where Item1 equals the item from the first list and Item2 will either equal the matching item from the second list or the default, which will be null for a reference type.