Mono is calling derived class function instead of base class function when base's function is invoked using delegate.

[alhad-deshpande]

Description

Mono is calling derived class function instead of base class function when base's function is invoked using delegate. This is occurring on x86 Mono, ppc64le and s390x mono versions. The coreclr x86 version calls base class function correctly.

Reproduction Steps

Following is the program:

using System;

class A
{
    protected virtual void F() { Console.WriteLine(nameof(A)); }
}
class B : A
{
    protected override void F() { Console.WriteLine(nameof(B)); }
    static void Invoke(Delegate d) { d.DynamicInvoke(); }
    void M()
    {
        Invoke(base.F);
    }
    static void Main()
    {
        new B().M();
    }
}

Compile and Run the program on x86/ppc64le/s390x using Mono SDK.

Expected behavior

"Expected output for the program is "A"

Actual behavior

Actual output on x86 using mono sdk and on ppc64le/s390x using mono is "B".

x86_64 machine output:
[root@dotnet-x86-cent-8-stream-node5 AandBProgram]# dotnet build
MSBuild version 17.10.0-preview-24101-01+07fd5d51f for .NET
Restore complete (0.7s)
You are using a preview version of .NET. See: https://aka.ms/dotnet-support-policy
AandBProgram succeeded (4.1s) → bin/Debug/net9.0/AandBProgram.dll

Build succeeded in 5.8s
[root@dotnet-x86-cent-8-stream-node5 AandBProgram]# dotnet run
Restore complete (0.7s)
You are using a preview version of .NET. See: https://aka.ms/dotnet-support-policy
AandBProgram succeeded (0.3s) → bin/Debug/net9.0/AandBProgram.dll

Build succeeded in 1.9s
B

[root@dotnet-x86-cent-8-stream-node5 AandBProgram]# arch
x86_64

[root@dotnet-x86-cent-8-stream-node5 AandBProgram]# dotnet --version
9.0.100-preview.1.24101.1

Power machine output:
(base) root@p006vm101-ub20:~/AandBProgram# dotnet build
MSBuild version 17.8.3+195e7f5a3 for .NET
Determining projects to restore...
All projects are up-to-date for restore.
AandBProgram -> /root/AandBProgram/bin/Debug/net8.0/AandBProgram.dll

Build succeeded.
0 Warning(s)
0 Error(s)

Time Elapsed 00:00:09.91
(base) root@p006vm101-ub20:/AandBProgram# dotnet run
B
(base) root@p006vm101-ub20:/AandBProgram# arch
ppc64le
(base) root@p006vm101-ub20:~/AandBProgram# dotnet --version
8.0.100

Regression?

No response

Known Workarounds

No response

Configuration

No response

Other information

No response

Cc: @uweigand

[Neme12]

I would assume this is the expected behavior on every platform. I remember at one point having made a delegate over a virtual method and using that as a cheap-enough check as to whether the method is overridden in a derived class or not by checking the target object.

[MichalStrehovsky]

I would assume this is the expected behavior on every platform. I remember at one point having made a delegate over a virtual method and using that as a cheap-enough check as to whether the method is overridden in a derived class or not by checking the target object.

The base.F syntax in C# translates to using ldftn IL instruction instead of ldvirtftn that C# normally emits for delegate creation. The delegate should point to the base method directly, not to the result of virtual resolution. The CoreCLR behavior is expected/correct, Mono behavior looks like a bug.

[janani66]

Any update on this issue?

[lambdageek]

@steveisok