Question

I have an array-list that contains some UserID.
I need a query like this:

vat tmp= users.select(a=> a.UserID in (arraylist));

what can I do?

Answer 1

If it’s actually in an ArrayList, you should create a List<T> or array first. Then you can use Contains:

// Use the appropriate type, of course.
var ids = arraylist.Cast<string>().ToList();
var tmp = users.Select(a => ids.Contains(a.UserID));

While using Contains on the plain ArrayList may well compile, I would expect it to fail at execution time, assuming users is an IQueryable<>.