Question

Is it possible to use Except() for two List’s that have two different classes but a common field? I have List<User1> and List<User2> collections. They have different properties except Id column and I want to find the different records between them using this Id column. I’m trying to use List<>.Except() but I’m getting this error:

The type arguments for method ‘System.Linq.Enumerable.Except(System.Collections.Generic.IEnumerable, System.Collections.Generic.IEnumerable)’ cannot be inferred from the usage. Try specifying the type arguments explicitly.

Here’s what I’m trying:

List<User1> list1 = List1();
List<User2> list2 = List2();
var listdiff = list1.Except(list2.Select(row => row.Id));

What am I doing wrong?

Answer 1

List1 contains instances of User1 and List2 contains instances of User2.

What type of instance should be produced by list1.Except(list2.Select(row => row.Id))?
In other words if type inference was not available, what would you replace var with?

If User1 and User2 inherit from the same ancestor (with ID), use List<User> instead.

Otherwise:

var list2Lookup = list2.ToLookup(user => user.Id);
var listdiff = list1.Where(user => (!list2Lookup.Contains(user.Id))