Where to put global rules validation in DDD

Question

I\'m new to DDD, and I\'m trying to apply it in real life. There is no questions about such validation logic, as null check, empty strings check, etc - that goes directly to

Answer 1

Create a method, for example, called IsUserNameValid() and make that accessible from everywhere. I would put it in the user service myself. Doing this will not limit you when future changes arise. It keeps the validation code in one place (implementation), and other code that depends on it will not have to change if the validation changes You may find that you need to call this from multiple places later on, such as the ui for visual indication without having to resort to exception handling. The service layer for correct operations, and the repository (cache, db, etc.) layer to ensure that stored items are valid.