I'm not sure, whe had to count only letters so.

I did the other way, because I liked how it looked.

But yours surely looks way, waaay better. Well done fella!

it could be so easy...great

Need better tests as others said and I really dislike the class without an interface for the Mock implementation, the code should provide us with an interface (pretty much what you wrote on the description) and the user should be forced to implement a mock version (and if he wants a real one too). The tests should verify that it all works.

None of the requirements are listed in the description.

Needs Random Tests.

It's not incorrect. It's just a proposal to create similar but a little more tricky kata. As far as I remember. And this example should give false result in this alternative quiz.

I think your definition is incorrect, for example:
B I T
I C E
T E N

Yes, there is mine.

Excellent solution! Just:
char.IsLower(c) && char.IsLetter(c) can be replaced by char.IsLower only, isn't it?

Thanks for your replies :)