GetHashCode() override coliding way to often

Question

I'm using unity, and unity does not have a tuple in it, so I created my own tuple class to work since I needed it for my Dictionary.

Dictionary <Tuple<int,int>, Tile>

Tile class that I created and isn't really relevant to solve this problem(at least I think it wont help).

But the problem is that I'm using both negative and positive integer in my tuples, and when I use my current GetHashCode() with the Tuples , sometimes I get the same HashCode, for example Tuple<-10, 8> and Tuple<-9,-10> both gives -172 when I return the hashcode.

Is there any good GetHashCode that wouldn't get me conflicts? To be honest I'm only using the operator == , because I need to check if both tuples have the same integers inside of them, if I could get a operator == that only collides when both integer are the same and in the same order, it would solve my problem.

Some other minor problems, I can't get to understand the Equals override, as it is, it is working, but I don't know how well it works, since I kind of changed every single thing until it worked.

public class Tuple<T1, T2>
{
    public T1 First { get; private set; }
    public T2 Second { get; private set; }

    public Tuple(T1 _First, T2 _Second)
    {
        First = _First;
        Second = _Second;
    }
    public override int GetHashCode()
    {
        int hash = 0;

        hash = First.GetHashCode() * 17 + Second.GetHashCode() + First.GetHashCode();

        return hash;
    }
    public static bool operator==(Tuple<T1, T2> obj1, Tuple<T1, T2> obj2)
    {
        if (ReferenceEquals(null, obj2))
            return false;
        return (obj1.GetHashCode() == obj2.GetHashCode());
    }
    public static bool operator!=(Tuple<T1, T2> obj1, Tuple<T1, T2> obj2)
    {
        if (ReferenceEquals(null, obj2))
            return true;
        return !(obj1.GetHashCode() == obj2.GetHashCode());
    }
    public bool Equals(Tuple<T1, T2> other)
    {
        if (other == null)
            return false;

        if (GetHashCode() == other.GetHashCode())
            return true;
        else
            return false;

    }
    public override bool Equals(object obj)
    {
        if (ReferenceEquals(null, obj))
        {
            return false;
        }
        if (ReferenceEquals(this, obj))
        {
            return true;
        }

        Tuple<T1, T2> other = obj as Tuple<T1, T2>;

        return obj.GetType() == GetType() && Equals(other);
    }
}
public static class Tuple
{
    public static Tuple<T1, T2> New<T1, T2>(T1 first, T2 second)
    {
        var tuple = new Tuple<T1, T2>(first, second);
        return tuple;
    }
}

Answer 1

GetHashCode() isn't supposed to be collision free. You should use it to determine if two things might be the same objects, and then you have to actually do a thorough check to see if they are.

For example, your == method should be written more like this:

public static bool operator==(Tuple<T1, T2> obj1, Tuple<T1, T2> obj2)
{
    if (ReferenceEquals(null, obj2))
        return false;

    if (obj1.GetHashCode() != obj2.GetHashCode())
    {
        return false;
    }
    return DefaultComparer<T1>.Equals(obj1.First, obj2.First) && DefaultComparer<T2>.Equals(obj1.Second, obj2.Second);
}

Also, don't forget to consider the case where obj1 and obj2 are both null .

If you're implementing your own Tuple , you might consider just stealing Microsoft's from the Reference Source repository, or at least use it as a base for your own.

Answer 2

I'm using unity, and unity does not have a tuple in it

It supports Tuple if you have Unity 2017 and above.

Go to Edit --> Project Settings --> Player --> Other Settings --> Configuration --> Scripting Runtime Version --> .NET 4.x Equivalent .

Reload or restart Visual Studio and you should be able to use Tuple . If you are not using Unity 2017 and above and also don't want to update then see John's answer .

Answer 3

This is what resharper generates for you automatically. Just note how they do the GetHashCode() and Equals.

private class Tuple<T1,T2> : IEquatable<Tuple<T1, T2>>
{
    public T1 First {get;}
    public T2 Second {get;}

    public Tuple(T1 first, T2 second)
    {
        First = first;
        Second = second;
    }

    public bool Equals(Tuple<T1, T2> other)
    {
        if (ReferenceEquals(null, other)) return false;
        if (ReferenceEquals(this, other)) return true;
        return EqualityComparer<T1>.Default.Equals(First, other.First) && EqualityComparer<T2>.Default.Equals(Second, other.Second);
    }

    public override bool Equals(object obj)
    {
        if (ReferenceEquals(null, obj)) return false;
        if (ReferenceEquals(this, obj)) return true;
        if (obj.GetType() != this.GetType()) return false;
        return Equals((Tuple<T1, T2>) obj);
    }

    public override int GetHashCode()
    {
        unchecked
        {
            return (EqualityComparer<T1>.Default.GetHashCode(First) * 397) ^ EqualityComparer<T2>.Default.GetHashCode(Second);
        }
    }

    public static bool operator ==(Tuple<T1, T2> left, Tuple<T1, T2> right)
    {
        return Equals(left, right);
    }

    public static bool operator !=(Tuple<T1, T2> left, Tuple<T1, T2> right)
    {
        return !Equals(left, right);
    }
}

Answer 4

I have found PropertyCompare (*) to be useful in auto-generating Equals implementations. It will automatically compare all public properties (so if you add a new public property you don't need to change anything except GetHashCode (and even that is technically optional).

It uses Cache to be reasonably performant - it takes a one-off hit (per type) to generate the appropriate expressions for the comparisons.

using System;
using System.Linq.Expressions;

namespace YourApp
{
    public class Tuple<T1, T2>
    {
        public T1 First { get; private set; }
        public T2 Second { get; private set; }

        public Tuple(T1 _First, T2 _Second)
        {
            First = _First;
            Second = _Second;
        }
        public override int GetHashCode()
        {
            var hash = 0;

            // Implement this however you like
            hash = First.GetHashCode() * 17 + Second.GetHashCode() + First.GetHashCode();

            return hash;
        }
        public static bool operator ==(Tuple<T1, T2> x, Tuple<T1, T2> y)
        {
            return PropertyCompare.Equal(x, y);
        }
        public static bool operator !=(Tuple<T1, T2> x, Tuple<T1, T2> y)
        {
            return !PropertyCompare.Equal(x, y);
        }
        public bool Equals(Tuple<T1, T2> other)
        {
            return PropertyCompare.Equal(this, other);

        }
        public override bool Equals(object obj)
        {
            return PropertyCompare.Equal(this, obj);
        }
    }
    public static class Tuple
    {
        public static Tuple<T1, T2> New<T1, T2>(T1 first, T2 second)
        {
            var tuple = new Tuple<T1, T2>(first, second);
            return tuple;
        }
    }

    public class Program
    {
        public static void Main()
        {
            var bob1 = Tuple.New("a", 1);
            var bob2 = Tuple.New("a", 1);

            Console.WriteLine(bob1 == bob2);
            Console.ReadLine();
        }
    }

    public static class PropertyCompare
    {
        public static bool Equal<T>(T x, object y) where T : class
        {
            return Cache<T>.Compare(x, y as T);
        }

        public static bool Equal<T>(T x, T y)
        {
            if (x == null)
            {
                return y == null;
            }

            if (y == null)
            {
                return false;
            }

            return Cache<T>.Compare(x, y);
        }

        private static class Cache<T>
        {
            internal static readonly Func<T, T, bool> Compare;
            static Cache()
            {
                var props = typeof(T).GetProperties();
                if (props.Length == 0)
                {
                    Compare = delegate { return true; };
                    return;
                }
                var x = Expression.Parameter(typeof(T), "x");
                var y = Expression.Parameter(typeof(T), "y");

                Expression body = null;
                for (var i = 0; i < props.Length; i++)
                {
                    var propEqual = Expression.Equal(
                        Expression.Property(x, props[i]),
                        Expression.Property(y, props[i]));
                    if (body == null)
                    {
                        body = propEqual;
                    }
                    else
                    {
                        body = Expression.AndAlso(body, propEqual);
                    }
                }
                Compare = Expression.Lambda<Func<T, T, bool>>(body, x, y)
                              .Compile();
            }
        }
    }
}

(*) I found it online somewhere, alas I can't remember where and Google is failing me here.