List.Intersect results changing after a foreach

Question

I have a problem with this piece of code, when I do my intersect method everything works fine.

When I do the count on my intersection before the foreach I have 1.

After the foreach, if do the count again after the foreach I have 0, why is this happening? It should always be 1...

var matchedRoles = roles.Intersect(user.Roles);
int before = matchedRoles.Count();

foreach (var matchedRole in matchedRoles)
{
    user.Roles.Remove(matchedRole);
}

int after = matchedRoles.Count();
if (matchedRoles.Any())
{
    accountRepository.Update(user);   
}

Answer 1

It happens because LINQ queries are lazy-evaluated: the result is not produced until it needs to be (which is whenever you call Count). It stands to reason that if you modify user.Roles in the meantime, the Count computed after the modification will be different.

If you want to "fix" the result, then you have to force LINQ to make a local copy of the results, e.g. like this:

// Here, ToArray() forces LINQ to immediately produce the results
var matchedRoles = roles.Intersect(user.Roles).ToArray();

This way, any further operations on matchedRoles will work on a fixed "snapshot" and produce the same results as long as you do not modify matchedRoles itself.

Answer 2

Intersect uses deferred execution. That means, every time you enumerate the result, the code is executed.

So, your second call to Count enumerates the matchedRoles enumerable a second time, causing a second execution of roles.Intersect(user.Roles). Because you removed the role from the user, it now returns no items.

To avoid this, enumerate the result once using ToList and work with the enumerated values from there:

var matchedRoles = roles.Intersect(user.Roles).ToList();

Answer 3

Linq uses deferred execution: it essentially does the intersect twice. Once with the matched role still in user.Roles, and once after your call to remove the role.

Answer 4

your LINQ gets executed again at the moment you access it the second time. If you want to get a copy of the references you could use the ToList method and iterate over its items