A cycle was detected in a LINQ expression exception

Question

I get the error :

A cycle was detected in a LINQ expression.

in ToList() while trying to do the following:

Answer 1

The behavior seems strange because you are not considering the closure semantics correctly. See the comments below:

private IEnumerable<int> FilterIdsByClient(IEnumerable<int> entityIds) 
{ 
    // The variable entityIds points to whatever was passed in: A List, according to the edited question.

    entityIds =                                    //this is an assignment, changing the referent of entityIds
        MyObjectContext.CreateObjectSet<TEntity>() 
            .Where(x => x.ClientId == _clientId) 
            .Where(x => entityIds.Contains(x.Id))  //this lambda closes over the variable entityIds
            .Select(x => x.Id); 

    // The query now has a reference to the *variable* entityIds, not to the object that entityIds pointed to originally.
    // The value of entityIds has been changed; it now points to the query itself!
    // The query is therefore operating on itself; this causes the "cycle detected" message.
    // Because of delayed execution, the query is not executed until the next line of code:

    return entityIds.ToList();
}

Answer 2

Why are you assigning to your parameter? Why not

private IEnumerable<int> FilterIdsByClient(IEnumerable<int> entityIds) 
{ 
    return
        MyObjectContext.CreateObjectSet<TEntity>() 
            .Where(x => x.ClientId == _clientId) 
            .Where(x => entityIds.Contains(x.Id)) 
            .Select(x => x.Id)
            .ToList(); 
} 

Answer 3

The answer is to not assign the LINQ query to entityIds. See @Stu's answer for a solution.

Answer 4

Of course there is a cycle. You are using entityIds in the Where Linq Extension method and it is the query being constructed itself. Instead of modifying the inputted IEnumerable, return a new query as follows:

private IEnumerable<int> FilterIdsByClient(IEnumerable<int> entityIds)
{
    var query =
        MyObjectContext.CreateObjectSet<TEntity>()
            .Where(x => x.ClientId == _clientId)
            .Where(x => entityIds.Contains(x.Id))
            .Select(x => x.Id);

    return query.ToList();
}