How can I group by a list of elements?

.net, c#, group-by, linq

Solution

You can implement `IEqualityComparer<List<B>>` and use it in the other GroupBy overload.

public class ListOfBEqualityComparer : IEqualityComparer<List<B>>
{
    public bool Equals(List<B> x, List<B> y)
    {
        // you can also implement IEqualityComparer<B> and use the overload
        return x.SequenceEqual(y);
    }

    public int GetHashCode(List<B> obj)
    {
        //implementation of List<T> may not work for your situation
        return obj.GetHashCode();
    }
}

Then you can use the overload

var groupedExampleList = exampleList.GroupBy(x => x.ListProp, 
                                             new ListOfBEqualityComparer());

Problem

I run into this issue again and again: how can I group a list of objects by a containing list of other objects? I have a list of objects of type `A` and each of these objects has an property (lets call it `ListProp`) which is a list also. `ListProp` has elements of the type `B`. There are multiple elements of type `A` with identically `B`-objects in `ListProp`, but the `ListProp` property reference differs from element to element. How can I group these `A`-objects the fastest way, where the `B`-objects in `ListProp` are identically? Sample code: ``` class Program { static void Main(string[] args) { var exampleList = new List<A> { // Should be in first group new A { ListProp = new List<B> { new B { Prop = new C { Number = 0 }}, new B { Prop = new C { Number = 1 }} }}, // Should be in first group new A { ListProp = new List<B> { new B { Prop = new C { Number = 0 }}, new B { Prop = new C { Number = 1 }} }}, // Should be in second group new A { ListProp = new List<B> { new B { Prop = new C { Number = 0 }}, new B { Prop = new C { Number = 1 }}, new B { Prop = new C { Number = 1 }} }}, // Should be in third group new A { ListProp = new List<B> { new B { Prop = new C { Number = 0 }}, new B { Prop = new C { Number = 0 }} }} }; // Doesn't work because the reference of ListProp is always different var groupedExampleList = exampleList.GroupBy(x => x.ListProp); } } class C { public int Number { get; set; } public override bool Equals(object o) { if (o is C) return Number.Equals(((C)o).Number); else return false; } } class B { public C Prop { get; set; } } class A { public IList<B> ListProp { get; set; } } ```

Original source