Edition 0 Updated to asp. Net core 0

Validate entities by implementing the Specification pattern and the Notification 
pattern 
Finally, a more elaborate approach to implementing validations in the domain model is by 
implementing the Specification pattern in conjunction with the Notification pattern, as explained in 
some of the additional resources listed later. 
It is worth mentioning that you can also use just one of those patterns
—
for example, validating 
manually with control statements, but using the Notification pattern to stack and return a list of 
validation errors. 
Use deferred validation in the domain 
There are various approaches to deal with deferred validations in the domain. In his book 
Implementing Domain-Driven Design
, Vaughn Vernon discusses these in the section on validation. 
Two-step validation 
Also consider two-step validation. Use field-level validation on your command Data Transfer Objects 
(DTOs) and domain-level validation inside your entities. You can do this by returning a result object 
instead of exceptions in order to make it easier to deal with the validation errors. 

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CHAPTER 6 | Tackle Business Complexity in a Microservice with DDD and CQRS Patterns 
Using field validation with data annotations, for example, you do not duplicate the validation 
definition. The execution, though, can be both server-side and client-side in the case of DTOs 
(commands and ViewModels, for instance). 
Additional resources 
•
Rachel Appel. Introduction to model validation in ASP.NET Core MVC
https://learn.microsoft.com/aspnet/core/mvc/models/validation
 
•
Rick Anderson. Adding validation
https://learn.microsoft.com/aspnet/core/tutorials/first-mvc-app/validation
 
•
Martin Fowler. Replacing Throwing Exceptions with Notification in Validations
https://martinfowler.com/articles/replaceThrowWithNotification.html
 
•