[Bug]: Mutability of record GetHashCode

Type of issue

Spec incorrect

Description

Specification states that record GetHashCode uses only positional properties (i.e. the ones specified in default constructor).
But in current implementation it uses at least public properties as well. Here is the failing test

namespace RecordGetHashCodeMutability;

public record A(int X)
{
    public int Y { get; set; }
}

public record B(int X)
{
    public int Y { get; set; }

    public override int GetHashCode() => X.GetHashCode();
}

[TestClass]
public sealed class Test1
{
    [TestMethod]
    public void Given_Record_When_Modify_Non_Positional_Property_Then_HashCode_Shouldnt_Change()
    {
        var a = new A(default);
        var hashCode1 = a.GetHashCode();
        a.Y = 2;
        var hashCode2 = a.GetHashCode();
        Assert.AreEqual(hashCode1, hashCode2, "Hash codes should be equal even after mutating the record's non-positional property.");
    }

    [TestMethod]
    public void Given_Record_With_Overriden_GetHashCode_When_Modify_Non_Positional_Property_Then_HashCode_Shouldnt_Change()
    {
        var b = new B(default);
        var hashCode1 = b.GetHashCode();
        b.Y = 2;
        var hashCode2 = b.GetHashCode();
        Assert.AreEqual(hashCode1, hashCode2, "Hash codes should be equal even after mutating the record's non-positional property.");
    }

    [TestMethod]
    public void Given_Dictionary_Containing_Record_When_Modifying_Non_Positional_Property_Then_Should_Still_Contains_Key()
    {
        var a = new A(default);
        var d = new Dictionary<A, int>
        {
            [a] = 1
        };
        Assert.IsTrue(d.ContainsKey(a), "Dictionary should contain the key after adding it.");
        a.Y = 2;
        Assert.IsTrue(d.ContainsKey(a), "Dictionary should still contain the key after modifying the non-positional property.");
    }
}

Page URL

https://github.com/dotnet/csharplang/blob/main/proposals/csharp-9.0/records.md#equality-members

Content source URL

https://github.com/dotnet/csharplang/blob/main/proposals/csharp-9.0/records.md#equality-members

I had to add Page Urls and Content source Urls, as it wouldn't allow create issue with out it. Even though Issue creating form says that they would be autofilled.

Record hasn't been specificized at all. The specification is still a draft at dotnet/csharpstandard#983 .
The linked document you are referencing is under the "rejected" folder. It isn't the version that serves for implementation. The accepted version is at https://github.com/dotnet/csharplang/blob/main/proposals/csharp-9.0/records.md .

Record hasn't been specificized at all. The specification is still a draft at dotnet/csharpstandard#983 . The linked document you are referencing is under the "rejected" folder. It isn't the version that serves for implementation. The accepted version is at https://github.com/dotnet/csharplang/blob/main/proposals/csharp-9.0/records.md .

Thank you, updated links, though question still remains.

This is indeed confusing, but the spec/proposal allows it:

The synthesized override of GetHashCode() returns an int result of combining the following values:

For each instance field fieldN in the record type that is not inherited, the value of
System.Collections.Generic.EqualityComparer<TN>.Default.GetHashCode(fieldN) where TN is the field type, and

If there is a base record type, the value of base.GetHashCode(); otherwise
the value of System.Collections.Generic.EqualityComparer<System.Type>.Default.GetHashCode(EqualityContract).

Since properties are backed by fields, point 1 applies.

IMHO, there should be a compiler-generated warning saying you should implement GetHashCode() manually.

Specification states that record GetHashCode uses only positional properties (i.e. the ones specified in default constructor).
But in current implementation it uses at least public properties as well. Here is the failing test

No. Records use all backing fields in the type for equals/gethashcode. This is the design we wanted (as these represent value types, and all fields in a record are part of its value), and what hte impl does.

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[Bug]: Mutability of record GetHashCode #9595

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[Bug]: Mutability of record GetHashCode #9595

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YakudzaBY commented on Aug 8, 2025

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