It's not Monitor itself that's problematic. It's that every object is implicitly associated with one, and anyone who holds a reference to an object can lock it. It doesn't matter if the type is internal - it can still be upcast to System.Object and leaked that way.
In practice this means that unless you can guarantee that you never, ever leak a reference anywhere, you don't know who else might be locking it. Which makes it impossible to reason about possible deadlocks. So the only sane way to manage it is to have a separate object used just for locking, which is never ever passed outside of the object that owns the lock.
And yes, this is absolutely bad design. There's no reason why every object needs a lock, for starters - for the vast majority of them, it's just unnecessary overhead (and yes, I know the monitors are lazily created, but every object header still needs space to store the reference to it). Then of course the fact that it's there means that people take the easy path and just lock objects directly instead of creating separate locks, just because it's slightly less code - and then things break. It's almost always the wrong granularity, too.
Thing is, I haven't seen this design anywhere outside of Java and .NET (which copied it from Java along with so many other bad ideas). Everybody else uses the sane and obvious approach of creating locks explicitly if and when they are needed.
In practice this means that unless you can guarantee that you never, ever leak a reference anywhere, you don't know who else might be locking it. Which makes it impossible to reason about possible deadlocks. So the only sane way to manage it is to have a separate object used just for locking, which is never ever passed outside of the object that owns the lock.
And yes, this is absolutely bad design. There's no reason why every object needs a lock, for starters - for the vast majority of them, it's just unnecessary overhead (and yes, I know the monitors are lazily created, but every object header still needs space to store the reference to it). Then of course the fact that it's there means that people take the easy path and just lock objects directly instead of creating separate locks, just because it's slightly less code - and then things break. It's almost always the wrong granularity, too.
Thing is, I haven't seen this design anywhere outside of Java and .NET (which copied it from Java along with so many other bad ideas). Everybody else uses the sane and obvious approach of creating locks explicitly if and when they are needed.