You could say the same about any language

Coding in C# is like coding a suit, everything looks neat and tidy if you follow the conventions and standards. Otherwise your code will look like a fat cat.

That may be the gist of the problem, but the problem states 'The string has a length greater or equal to one and contains only letters from a to z.' Adding extra error handling for other symbols and capital letters is not optimal as it's assumed this is already handled before the string is passed in.

A detailed comment isn't necessary, just some mild familiarity with Linq and the string constructor.

Providing invalid characters to the input string is what the problem is all about

C# is not a good code golf language

Clever, but you mutated the list.

Nicely done

Really clever, but a bit hard to read. A few line breaks and spaces and it would be better. For performance of course not the best solution but from effort.

Smart approach

or extract "pure" factorial method, which can be made recursive

I like how peoples solving problems in one line thinking its a good practice, this is a nightmare for maintenance if you asked me, and this line is totally NOT CLEAR, a very detailed comments is required to understand whats going on in there for someone who doesn't know the context .

This code is not clean, nightmare for maintenance work