DDD

Where does validation belong?

Request validation, business rules and data constraints are different kinds of checks. Putting each one in the right layer avoids both duplication and gaps.

Keep other systems' models out of yours

When a third-party API's field names and status codes spread through your code, every change on their side becomes a change on yours. An anti-corruption layer stops it at the boundary.

Domain events and integration events aren't the same thing

Both describe something that happened, but one stays inside your service and the other is a public contract. Mixing them up couples services you meant to keep apart.

Value objects with C# records

Replacing a decimal and a string with a Money type removes a whole class of bugs. Records make value objects short to write, with one catch around with expressions.