RuntimeHelpers.Equals difference between CoreCLR and Mono

[filipnavara]

The following program:

using System;
using System.Text;
using System.Runtime.CompilerServices;

public class C {
    struct S {
        public int a;
        public string b;
    };
    
    public static void Main() {
        var b1 = 42.ToString();
        var b2 = 42.ToString();
        var s1 = new S { a = 42, b = b1 };
        var s2 = new S { a = 42, b = b2 };
        Console.WriteLine(object.ReferenceEquals(b1, b2));
        Console.WriteLine(s1.Equals(s2));
        Console.WriteLine(RuntimeHelpers.Equals(s1, s2));
    }
}

prints

False
True
False

on CoreCLR.

On Mono it prints

False
True
True

The difference is in how ValueTypes are treated by RuntimeHelpers.Equals. CoreCLR does memcmp of the contents, Mono does field-by-field comparison. Which one is right and which one is wrong?

[filipnavara]

I think https://github.com/dotnet/corefx/issues/13692#issuecomment-263101741 partially answers that. Follow up question though, is there any incentive to get this fixed and align the behavior? Is it emitted by the compiler in some cases?

[joperezr]

cc: @jkotas do you mind answering the above question? Also, do we want to either fix this or document it for 5.0?

[jkotas]

Is it emitted by the compiler in some cases?

I am not aware of any compilers that emit calls to this API.

https://apisof.net/catalog/System.Runtime.CompilerServices.RuntimeHelpers.Equals(Object,Object) says that the usage is 0.7%. It is a lot for a broken slow API. It may be a good idea to understand where this API is actually used before deciding what to do here.

is there any incentive to get this fixed and align the behavior?

Aligning the behavior between CoreCLR and Mono sounds good to me.

Also, the documentation for the API is wrong. It says "true if the o1 parameter is the same instance as the o2 parameter, or if both are null, or if o1.Equals(o2) returns true; otherwise, false.". This description does not match neither CoreCLR nor Mono implementations.

[danmoseley]

Not required to fix this to ship the product.