Custom formatters (I am referring to Formatters section in https://github.com/dennisdoomen/fluentassertions/wiki#extensibility) are right now selected by exact match of types:

https://github.com/dennisdoomen/fluentassertions/blob/develop/Src/Core/Formatting/AttributeBasedFormatter.cs#L63

Given #308 I wonder if it would be better to go by isAssignableFrom() instead? It would then be possible to implement a catch all custom formatter with object as an argument and completely disable the default formatter that builds the object's graph.

Or maybe introduce an interface that formatters would need to implement to give them (the formatters) the ability to respond to CanHandle() ?