Partial default interface members are not virtual

[jjonescz]

using System;

M(new C1());
M(new C2());

static void M(I x)
{
    x.M1();
    x.M2();
}

partial interface I
{
    public void M1()
    {
        Console.Write(1);
    }

    public partial void M2();
    public partial void M2()
    {
        Console.Write(2);
    }
}

class C1 : I;

class C2 : I
{
    public void M1()
    {
        Console.Write(3);
    }
    
    public void M2()
    {
        Console.Write(4);
    }
}

Expected behavior: Outputs 1234.
Actual behavior: Outputs 1232. That is, the partial method is not virtual unlike the non-partial method.
Notes: Partial properties and events behave similarly.

[MauNguyenVan]

IMO, we should fix for both the method behavior and other members, it keeps language behavior is consistency for multiple types(property, indexer, method, event ...). We should add the breaking change for docs,

I guess, very less code base used/got it (this issue) because this is high-level feature, if has, they realized and maybe report a bug or ask it in StackOverflow, or at least they don't use it any more (it can be realize the expect behavior when read the code base, but the actual result is not).

Suppose that a dev decided to change the logic from method to property (or vice versa), cause their behaviour them are different, so Bomb, still silly errors remain

You see, C# was designed so good, and after 20+ years, the language stills keeps the good design, I don't want C# to become C++ like, to much rules and tricks, inconsistency.

[MauNguyenVan]

Inconsistency will make the language increasingly complex, especially when a developer refactors code from a method to a property—it can introduce potential bugs that are very hard to detect. Or, if in a team some people use it as a method while others use it as a property, it can lead to incorrect logic execution without the developers even realizing it.

With this fix, I don't think the issue is actually resolved. Instead, it still introduces potential hidden bugs.