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POShould domain objects be allowed to be temporarily invalid, and how does that decision affect validation techniques?
 

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  1. POShould domain objects be allowed to be temporarily invalid, and how does that decision affect validation techniques?
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    <p>I'm writing a VB.NET Winforms project based on MVVM (using Winforms binding). My instinct is to never allow a domain entity be in an invalid state. This requires that I do validation checks in the constructor for new entities and in each setter for existing entities:</p> <pre><code>Public Class Product Public Sub New(ProductID as Integer, Name as String) If ProductID &gt; 0 AndAlso Name.Length &gt; 5 Then _ProductID = ProductID _Name = Name Else Throw New InvalidProductException End If End Sub Private _ProductID as Integer Public Property ProductID as Integer Set(value as String) If value &gt; 0 then _ProductID = value Else Throw New InvalidProductException End If End Set End Property 'Same principle as above for Name setter. End Class </code></pre> <p>Then I ran across Data Annotations, which seemed pretty slick. I noticed that most people using Data Annotations allow the domain entity to become invalid temporarily, and then validate the entity at some later point with a call to Validate.ValidateObject. By this point the entity is invalid and the original state has been lost unless you have some other mechanism to roll it back.</p> <p>Two Questions:</p> <p>1) Do you allow domain entities to become temporarily invalid?</p> <p>2) Based on your answer to #1, what techniques do you use to validate the entity?</p>
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    1. COYou could use DataAnnotations (or anything else) on the ViewModels as a first line of defense, and exceptions from the Domain as the last.
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    2. COYou read my mind, I was just thinking that I may go that route.
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