using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;
using System.Runtime.CompilerServices;

[MemoryDiagnoser]
[DisassemblyDiagnoser]
public class Program
{
    static void Main(string[] args) => BenchmarkSwitcher.FromAssemblies(new[] { typeof(Program).Assembly }).Run(args);

    private S _s = default(S);
    private C _c = new C();
    private S? _ns = (S?)default(S);

    [Benchmark] public int Struct() => CallM(_s);
    [Benchmark] public int Class() => CallM(_c);
    [Benchmark] public int Nullable() => CallM(_ns);
    [Benchmark] public int NullableSpecialized() => CallMSpecial(_ns);

    static int CallM<T>(T t)
    {
        if (t is IMyInterface)
        {
            return ((IMyInterface)t).M();
        }

        return 0;
    }

    static int CallMSpecial<T>(T? t) where T : struct
    {
        if (t.HasValue)
        {
            return CallM(t.GetValueOrDefault());
        }

        return 0;
    }
}

interface IMyInterface
{
    int M();
}

struct S : IMyInterface
{
    public int M() => 42;
}

class C : IMyInterface
{
    public int M() => 42;
}