C# : using lambda to GroupBy every collection elements

Question

I have a collection of Component and these collections have a collection of Pax:

public class Component
{
    public IEnumerable<Pax> Paxes { get; set; }  
}

public class Pax
{
    public int Number { get; set; }  
    public int Room { get; set; }
}

I would like to group my Component collection (which is an IEnumerable<Component>) by the list of passenger.

var groups = components.GroupBy(c => c.Paxes);

Of course, those groups are not expected. Paxes repeated in components have not the same object reference, so I have to compare their properties in the group expression and this is where I'm stuck.

score 4 · Accepted Answer · edited May 23 '17 at 12:00

4

You must implement an IEqualityComparer for it:

List<Component> components = new List<Component>();
var result = components.GroupBy(c => c.Paxes, new PaxesComparer());

public class PaxesComparer : IEqualityComparer<IEnumerable<Pax>>
{
    public bool Equals(IEnumerable<Pax> x, IEnumerable<Pax> y)
    {
        throw new NotImplementedException();
    }

    public int GetHashCode(IEnumerable<Pax> obj)
    {
        throw new NotImplementedException();
    }
}

For implementation of Equals - You can check this question to check that the collections are identical.
You'd probably also want to override Equals and GetHashCode of Pax class.
Make sure you implement GetHashcode - cause if hashes are different equals will not be called - Kirill's comment below

edited May 23 '17 at 12:00

Community

1
1

answered Aug 12 '16 at 12:40

Gilad Green

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It is critical to correctly implement GetHashCode method cause if hashes are different equals will not be called. – Kirill Bestemyanov Aug 12 '16 at 12:46
@KirillBestemyanov - Added your comment to the answer too - good one – Gilad Green Aug 12 '16 at 12:48

Magnus · Answer 2 · 2016-08-12T13:12:22.250

You can use this generic collection equality comparer.

public class CollectionEqualityComparer<T> : IEqualityComparer<IEnumerable<T>>
{
    public int GetHashCode(IEnumerable<T> obj)
    {
        unchecked
        {
            return obj.Aggregate(17, (current, item) => current * 31 + item.GetHashCode());
        }
    }

    public bool Equals(IEnumerable<T> x, IEnumerable<T> y)
    {
        return x.SequenceEqual(y);
    }
}

And then

var q = components.GroupBy(c => c.Paxes, new CollectionEqualityComparer<Pax>()));

This of course requires the Pax object to override Equals and GetHashCode.

C# : using lambda to GroupBy every collection elements

2 Answers2