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PODomain Objects and Services
 

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    <p>In <a href="https://stackoverflow.com/questions/428859/what-goes-in-your-domain-model-objects-and-what-goes-in-your-services">this</a> question someone <a href="https://stackoverflow.com/questions/428859/what-goes-in-your-domain-model-objects-and-what-goes-in-your-services#428902">replies</a> "You never let the domain object implementations call services by themselves!". Is this statement a hard fast rule of DDD or does it depend on your own application and architecture?</p> <p><strong>Contrived example:</strong></p> <p>As an example lets suppose we have a <code>UserImage</code> object in our model that gets populated from an uploaded image by a user. And then lets suppose that we can submit this image to a 3rd party service that can identify thumb prints and return a <code>Guid</code> if a match is found.</p> <pre><code>public IThumbPrintService { Guid FindMatch(Bitmap image); } public class UserImage { public Bitmap Image {get; set;} public Guid ThumbPrintId {get; set;} public bool FindThumbPrintMatch() { // Would you call the service from here? ThumbPrintId = _thumbPrintService.FindMatch(this.Image); return ! ThumbPrintId.CompareTo(Guid.Empty); } } public class RoboCopUserImageService : IUserImageService { // Or move the call to a service method // since it depends on calling a separate service interface public bool FindThumbPrintMatch(UserImage userImage) { userImage.ThumbPrintId = _thumbPrintService.FindMatch(userImage.Image); return !userImage.ThumbPrintId.CompareTo(Guid.Empty); } } </code></pre> <p>What is avoided or gained by not letting domain objects call services themselves?</p> <p><strong>EDIT:</strong> Are there any good online articles that discuss this specific topic?</p>
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