Question

MyObject()
{
     String dept;
     List<int> id;
     Object obj;
}

Using LINQ, how can I return a list of the above objects organized as follows:

Group all of the obj objects by [ department and EQUAL id list ]. The list being considered equal if it contains the same numbers, not necessarily the same order(a set).

Answer 1

GroupBy has an overload that accepts a custom IEqualityComparer<MyObject>. Write one that regards two objects as equal when dept is equal and id is set-equal, and pass it as an argument.

A convenient way to implement set equality is to write

new HashSet(x.id).SetEquals(new HashSet(y.id))

although this will end up being inefficient and probably not the best idea if there are lots of comparisons to make.