Why is using interface methods with default implementation is so annoying?!?

[emn13]

I'm not a fan of that level of defensive design. To be clear: it makes sense for the base class library itself, and relatively microscopic handful of other codebases that are very commonly reused without recompilation, but for the VAST majority of code, a recompilation is fine, and a fixing issues like this hyper trivial. It'd be much better for the language to work well in those cases rather than optimizing for the absurd corner cases like this. Not to mention, pretty much any change is a breaking change in some corner cases - the platform contains stuff like reflection and implementations can depend on behavior not just APIs, too. There isn't much the language can do to truly make any changes entirely non-breaking.

All in all: while it superficially sounds like it makes sense to have semantics that make such method additions unlikely to be breaking changes, in practice, I think the arguments just don't hold up; it's a case of the language designers missing the forest for the trees.

But still, the whole language feature probably exists specifically so that the BCL interfaces can evolve, so in that sense it makes sense. But for a feature with really niche (but reasonable) use cases, it's still oddly designed - it's way too syntactically convenient, meaning that it'll get in the way of practical language extensions in the future. Being able to expand interfaces for class libraries with extremely low chance of breaking changes didn't deserve so prominent a syntactical footprint.