Internal abstract methods in public abstract class prevent deriving

[ynsehoornenborg]

Given this API:

public abstract class ExampleBase
{
internal abstract void ImplementMe();
}

When a external developer whats to use this from a different assembly and tries to implement the ExampleBase class, he/she is unable to implement the internal ImplementMe method. This fails compilation, effectively blocking an outside developer from deriving from ExampleBase.

I think this is worth writing an Analyzer for, to guard that we don't write such an API in the first place.