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. :-) I have 2 lists that I need to join together: ``` // list1 contains ALL contacts for a customer. // Each item has a unique ID. // There are no duplicates. ContactCollection list1 = myCustomer.GetContacts(); // list2 contains the customer contacts (in list1) relevant to a REPORT // the items in this list may have properties that differ from those in list1. /*****/// e.g.: /*****/ bool SelectedForNotification; /*****/// may be different. ContactCollection list2 = myReport.GetContacts(); ``` I need to create a third ContactCollection that contains all of the contacts in `list1` but with the properties of the items in `list2`, if the item is in the list[2] (`list3.Count == list1.Count`). I need to replace all items in `list1` with the items in `list2` where items in `list1` have the IDs of the items in `list2`. The resulting list (`list3`) should contain the same number of items at…

Accepted Answer

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.

LINQ Join 2 List<T>s

Solution

Problem