c# - GetHashCode override of object containing generic array

Question

I have a class that contains the following two properties:

public int Id      { get; private set; }
public T[] Values  { get; private set; }

I have made it IEquatable<T> and overriden the object.Equals like this:

public override bool Equals(object obj)
{
    return Equals(obj as SimpleTableRow<T>);
}

public bool Equals(SimpleTableRow<T> other)
{
    // Check for null
    if(ReferenceEquals(other, null))
        return false;

    // Check for same reference
    if(ReferenceEquals(this, other))
        return true;

    // Check for same Id and same Values
    return Id == other.Id && Values.SequenceEqual(other.Values);
}

When having override object.Equals I must also override GetHashCode of course. But what code should I implement? How do I create a hashcode out of a generic array? And how do I combine it with the Id integer?

public override int GetHashCode()
{
    return // What?
}

Answer 1

Because of the problems raised in this thread, I'm posting another reply showing what happens if you get it wrong... mainly, that you can't use the array's GetHashCode(); the correct behaviour is that no warnings are printed when you run it... switch the comments to fix it:

using System;
using System.Collections.Generic;
using System.Linq;
static class Program
{
    static void Main()
    {
        // first and second are logically equivalent
        SimpleTableRow<int> first = new SimpleTableRow<int>(1, 2, 3, 4, 5, 6),
            second = new SimpleTableRow<int>(1, 2, 3, 4, 5, 6);

        if (first.Equals(second) && first.GetHashCode() != second.GetHashCode())
        { // proven Equals, but GetHashCode() disagrees
            Console.WriteLine("We have a problem");
        }
        HashSet<SimpleTableRow<int>> set = new HashSet<SimpleTableRow<int>>();
        set.Add(first);
        set.Add(second);
        // which confuses anything that uses hash algorithms
        if (set.Count != 1) Console.WriteLine("Yup, very bad indeed");
    }
}
class SimpleTableRow<T> : IEquatable<SimpleTableRow<T>>
{

    public SimpleTableRow(int id, params T[] values) {
        this.Id = id;
        this.Values = values;
    }
    public int Id { get; private set; }
    public T[] Values { get; private set; }

    public override int GetHashCode() // wrong
    {
        return Id.GetHashCode() ^ Values.GetHashCode();
    }
    /*
    public override int GetHashCode() // right
    {
        int hash = Id;
        if (Values != null)
        {
            hash = (hash * 17) + Values.Length;
            foreach (T t in Values)
            {
                hash *= 17;
                if (t != null) hash = hash + t.GetHashCode();
            }
        }
        return hash;
    }
    */
    public override bool Equals(object obj)
    {
        return Equals(obj as SimpleTableRow<T>);
    }
    public bool Equals(SimpleTableRow<T> other)
    {
        // Check for null
        if (ReferenceEquals(other, null))
            return false;

        // Check for same reference
        if (ReferenceEquals(this, other))
            return true;

        // Check for same Id and same Values
        return Id == other.Id && Values.SequenceEqual(other.Values);
    }
}