How to use System.HashCode.Combine with more than 8 values?

Question

.NET Standard 2.1 / .NET Core 3 introduce System.HashCode to quickly combine fields and values to a hash code without having to care about the underlying implementation.

However, it only provides Combine method overloads for up to 8 values. What do I do if I have a class with 9 values (3x3 matrix) or even 16 values (4x4 matrix)?

Should I simply add together the results of two Combine calls, passing as many values as possible in each?

public override int GetHashCode()
    => HashCode.Combine(M11, M12, M13, M21, M22, M23, M31, M32) + HashCode.Combine(M33);

Looking at the source, I cannot completely argue if this may have implications I don't know of.

Ray · Accepted Answer · 2024-03-16 14:46:56Z

68

As stated in the System.HashCode documentation, adding together hashes returned by successive HashCode.Combine calls is NOT the solution.

While the static HashCode.Combine method overloads only allow up to 8 values, these are just convenience methods - to combine more, instantiate the HashCode class itself and use it as follows:

public override int GetHashCode()
{
    HashCode hash = new();
    hash.Add(M11);
    hash.Add(M12);
    hash.Add(M13);
    hash.Add(M21);
    hash.Add(M22);
    hash.Add(M23);
    hash.Add(M31);
    hash.Add(M32);
    hash.Add(M33);
    return hash.ToHashCode();
}

~~It does make me wonder why there is no HashCode constructor accepting a params object[] values so you could do all that in one line, but there are probably reasons I didn't think of this quickly.~~ (S. the comments why such an overload does not exist.)

edited Mar 16, 2024 at 14:46

answered Dec 17, 2019 at 13:18

Ray

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6 Comments

germi Over a year ago

It might have something to do with allocating an array, which can be a performance drag. Since .NET Core is more performance oriented, I would think that that's the reason.

freakish Over a year ago

Not only array allocation but also boxing of every value type. Horribly inefficient.

Vlad Over a year ago

It neither does boxing nor array allocation

niproblema Over a year ago

@Vlad Why not? Surely it has to do with performance considerations..

Ray Over a year ago

@Vlad I think the concerns about boxing and array allocation were made about a constructor accepting a params object[] values which absence I mentioned in my answer, not the calls to HashCode.Add.

|

Ber'Zophus · Accepted Answer · 2025-07-31 00:06:25Z

3

If you happen to be able to use C# 13 (.NET 9), and all your types are the same, you can write a simple generic helper method that avoids boxing as well as the array allocation.

This example uses C# 14 in order to make it a static extension method off HashCode, but if you can only use C# 13, you can just make this a static method in a helper class.

extension(HashCode)
{
    public static int Create<T>(params ReadOnlySpan<T> values)
    {
        var hashCode = new HashCode();
        foreach (var value in values)
        {
            hashCode.Add(value);
        }

        return hashCode.ToHashCode();
    }
}

Usage:

HashCode.Create(M11, M12, M13, M21, M22, M23, M31, M32, M33);

Behind the scenes, the compiler will make use of InlineArray in order to avoid the allocation.

Of note, if this is absolutely performance critical, it will not beat @Ray's answer for speed. But we're only talking on the order of single-digit ns slower.

answered Jul 31 at 0:06

Ber'Zophus

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1 Comment

Ray Jul 31 at 8:23

Thanks for the update, wish I could accept multiple answers.

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