Functional Principles in C# – method should be honest

Below are the key ideas of how to apply functional principles in C# explained in Pluralsight course by Vladimir Khorikov.

1. Immutable Architecture
   Separate domain logic, Infrastructure(Persister), Service logic. Make the domain logic immutable and functional(input -> output) so as to easily unit test it. Make the infrastructure and service logic as simple as possible.
2. Exception
   Use return value over an exception if it’s a known exception.
   Fail Fast
   General exception catch should be in a single place to catch all unexpected exception.
3. Avoid primitive obsession
   Utilize value object which may contain validation logic as a single place to check input data.
4. Avoid null
   Use ‘NullGuard’ library with [assembly: NullGuard(ValidationFlags.All)] option to check not only public method but also private method and properties.
   Use Maybe<T> around a domain model which is separated from the presentation layer.
5.  Handling Failures and Input Errors
   Use value object
   Try catch at the lowest level
   Use Railway-oriented Programming => Result<T> and Maybe<T> => Monad.