C#•2y ago

What's the meaning of CS0659?

'class' overrides Object.Equals(object o) but does not override Object.GetHashCode()

'class' overrides Object.Equals(object o) but does not override Object.GetHashCode()

I got this warning message but i cannot find what's the reason I should override GetHashCode. Also, how should I implement it?

15 Replies

TheRanger•2y ago

ur trying to make a class that implements the IEqualityComparer interface right?

hanu•2y ago

uhm wait

internal readonly struct Vector3D
{
    public double X { get; init; }
    public double Y { get; init; }
    public double Z { get; init; }

    public override bool Equals([NotNullWhen(true)] object? obj)
    {
        if (obj is Vector3D vector)
        {
            return X == vector.X && Y == vector.Y && Z == vector.Z;
        }
        return false;
    }
}

internal readonly struct Vector3D
{
    public double X { get; init; }
    public double Y { get; init; }
    public double Z { get; init; }

    public override bool Equals([NotNullWhen(true)] object? obj)
    {
        if (obj is Vector3D vector)
        {
            return X == vector.X && Y == vector.Y && Z == vector.Z;
        }
        return false;
    }
}

this is how it now looks like

TheRanger•2y ago

i copy pasted ur code, it does not give me ur error u sure the error is not somewhere else? anyway you can just override GetHashCode like you override any method

hanu•2y ago

hanu•2y ago

i'm on .net 6

TheRanger•2y ago

so it does implement the IEquatable interface

hanu•2y ago

does this matter? uh yeah i just added it to see if it solves the problem

Klarth•2y ago

CS0659 is a warning https://learn.microsoft.com/en-us/dotnet/csharp/misc/cs0659

hanu•2y ago

yeah it says "An override of Equals implies that you also want to override GetHashCode." but idk why

Klarth•2y ago

It's to ensure correct, consistent behavior when using both equals and getting a hash. eg. for Dictionary and HashSet. You can override GetHashCode and use https://learn.microsoft.com/en-us/dotnet/api/system.hashcode.combine?view=net-7.0 which is good enough. You probably shouldn't be creating your own vector type though.

hanu•2y ago

okay got it thanks uh wait so the 'correct, consistent behavior' means 'two hash codes are equal if and only if Equals() == true'?

Klarth•2y ago

It means that the equals and hash code calculation should both rely on the exact same subset of properties. Imagine if your Equals uses X and Y, but your hash code uses X, Y, and Z.

hanu•2y ago

i got it what would be a great alternative rather than manually implementing a vector type? Vector3?

Klarth•2y ago

Depends on what you're trying to accomplish, perf requirements, what libraries you are or can use, etc.

hanu•2y ago

reasonable thanks all, anyways!

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What's the meaning of CS0659?