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    <p><strong>Edit:</strong> Judging from the other answers, the correct name for such a 'domain service' is <em>specification</em>. I've updated my answer to reflect this, including a more detailed code sample.</p> <p>I'd go with option 3; create a <strike>domain service</strike> specification which encapsulates the actual logic that performs the validation. For example, the specification initially calls a repository, but you could replace it with a web service call at a later stage. Having all that logic behind an abstract specification will keep the overall design more flexible.</p> <p>To prevent someone from editing the name without validating it, make the specification a required aspect of editing the name. You can achieve this by changing the API of your entity to something like this:</p> <pre><code>public class User { public string Name { get; private set; } public void SetName(string name, ISpecification&lt;User, string&gt; specification) { // Insert basic null validation here. if (!specification.IsSatisfiedBy(this, name)) { // Throw some validation exception. } this.Name = name; } } public interface ISpecification&lt;TType, TValue&gt; { bool IsSatisfiedBy(TType obj, TValue value); } public class UniqueUserNameSpecification : ISpecification&lt;User, string&gt; { private IUserRepository repository; public UniqueUserNameSpecification(IUserRepository repository) { this.repository = repository; } public bool IsSatisfiedBy(User obj, string value) { if (value == obj.Name) { return true; } // Use this.repository for further validation of the name. } } </code></pre> <p>Your calling code would look something like this:</p> <pre><code>var userRepository = IoC.Resolve&lt;IUserRepository&gt;(); var specification = new UniqueUserNameSpecification(userRepository); user.SetName("John", specification); </code></pre> <p>And of course, you can mock <code>ISpecification</code> in your unit tests for easier testing.</p>
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    1. POWhere to put global rules validation in DDD
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    1. USNiels van der Rest
    1. USNiels van der Rest
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    1. COSo, calling code should have access to repository? Thus caller should be domain or application service. So, what the benefit from injecting specification to user entity, if calling code could just call userRepository.IsNameUnique("John")?
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      1. USSergey Berezovskiy
    2. CO@lazyberezovsky: By making the specification part of the signature of `SetName`, the calling code can immediately see that the name has to follow a certain specification. If you depend on the 'outer' code to call `userRepository.IsNameUnique("John")`, you can't ensure that the name is actually unique, because the outer code can easily omit the call the the repository.
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      1. USNiels van der Rest
    3. COI really like this approach! It's the perfect blend of being explicit enough to let consuming code understand the dependency on UniqueUserNameSpecification when called User.SetName, the ability to mock out the specification, and "safe" enough to know the entity is still taking care of the validation of username without relying on some other code/layer to call it. For example. relying on the Service Layer to call the specification during a .Create() method call. This way, your entity will never be allowed to get into an invalid state, which is the goal.
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      1. USMichael McCarthy
 

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