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.