How to group a list of lists using LINQ (ex: List<List<int>>)

I know I can easily do this with some for loops, but wanted to see if there was a way to do it with fluent LINQ. I'm trying to find out how many of each sub-list I have.

I was looking at Enumerable.SequenceEqual but couldn't get it working with GroupBy()

Say I have a List<List<int> like this:

{
 {1,2}
 {2, 3, 4}
 {1,2}
 {1,3}
 {1,2}
}

and I want to group it by equal lists, like so

{
 <3, {1,2}>
 <1, {2, 3, 4>
 <1, {1,3}
}
Jon Skeet
people
quotationmark

You'd need to implement a IEqualityComparer<List<T>>, which you can then pass into GroupBy. For example:

public class ListEqualityComparer<T> : IEqualityComparer<List<T>>
{
    public bool Equals(List<T> lhs, List<T> rhs)
    {
        return lhs.SequenceEqual(rhs);
    }

    public int GetHashCode(List<T> list)
    {
        unchecked
        {
            int hash = 23;
            foreach (T item in list)
            {
                hash = (hash * 31) + (item == null ? 0 : item.GetHashCode());
            }
            return hash;
        }
    }
}

Then:

var counts = lists.GroupBy(x => x, 
                           (key, lists) => new { List = key, Count = lists.Count() },
                           new ListEqualityComparer<int>());

people

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