Domain Objects Persistence Pattern for .NET


Domain objects in an application represent the core data and business validation rules relating to it. And, domain objects are usually central to the entire application and used by most subsystems. Therefore, their good design is critical for a good application design that is robust, high performing, and yet flexible.

When it comes to developing object-oriented applications that use relational databases, the domain object design has to be consistent with the database design. This makes them easier to understand because they represent real-life “entities” and their relationships with each other. Therefore, in many situations, the domain objects are “mapped” to the relational database tables and relationships between tables. However, it is very easy to get this mapping wrong and end up with an undesirable domain object design. A good design for domain objects requires a solid understanding of object-oriented and relational fundamentals on the part of developers.

The Domain Objects Persistence Pattern attempts to provide a solution for domain object mapping to the relational databases that decouples the domain objects from the persistence logic. The domain objects in this pattern are unaware of the objects that persist them because the dependency is only one-way (from persistence objects to domain objects). This makes the domain objects’ design much simpler and easier to understand. It also hides the persistence objects from other subsystems in the application that are using the domain objects. This also works in distributed systems where only the domain objects are passed around. In this context, an attempt is made to incorporate the Factory Pattern into this pattern to help decouple domain objects and persistence logic.


Domain Objects, Persisting Domain Objects

Problem Definition

Domain objects form the backbone of any application. They capture the core data model from the database and also the business rules that apply to this data. It is very typical for most subsystems of an application to rely on these common domain objects. This means that the closer the domain objects map to the data model in the database, the easier it is for the application developers to understand and use them because they mimic real-life “entities” and “relationships” as represented in the database.

If domain objects are not separated from the rest of the application, you end up with duplication of code everywhere. Similarly, if domain objects are not separated from the persistence code, you face situations where any subsystem using the domain objects also knows and depends on the persistence objects. And, any change in persistence objects affects the entire application, hence a bad design.


One way to achieve the above-mentioned goals is to separate the domain objects into a separate subsystem and let the entire application use them wherever it needs domain data. Additionally, you should separate domain objects from the persistence code. This double-decoupling allows you on one hand to avoid code duplication and on the other to hide the persistence details from the domain objects and make them more flexible in case they need to change. The domain objects and the rest of the application are totally unaffected whether the data is coming from a relational database or any other source (for example, XML, flat files, or Active Directory/LDAP).

In separating the persistence logic from domain objects, you ensure that the domain objects have no dependency on the persistence code. This allows the domain objects to become available in environments where you don’t even want to expose your persistence code.

Sample Code

In this sample, you will look at a Customer object from the Northwind database mapped to the “Customers” table in the database.

public class Customer {
   // Private data members
   String      _customerId;
   String      _companyName;
   String      _contactName;
   String      _contactTitle;

   public Customer() {}

   // Properties for Customer object
   public String CustomerId {
      get { return _customerId; } set { _customerId = value;}

   public String CompanyName {
      get { return _companyName; } set { _companyName = value;}

   public String ContactName {
      get { return _contactName; } set { _contactName = value;}

   public String ContactTitle {
      get { return _contactTitle; } set { _contactTitle = value;}
public interface ICustomerFactory
   // Standard transactional methods for single-row operations
   void Load(Customer cust);
   void Insert(Customer cust);
   void Update(Customer cust);
   void Delete(Customer cust);

   // Query method to return a collection
   ArrayList FindCustomersByState(String state);


public class CustomerFactory : ICustomerFactory
   // Standard transactional methods for single-row operations
   void Load(Customer cust) { /* Implement here */ }
   void Insert(Customer cust) { /* Implement here */ }
   void Update(Customer cust) { /* Implement here */ }
   void Delete(Customer cust) { /* Implement here */ }

   // Query method to return a collection
   ArrayList FindCustomersByState(String state)
      { /* Implement here */ }

Below is an example of how a client application will use this code.

public class NorthwindApp
   static void Main (string[] args) {
      Customer cust = new Customer();
      CustomerFactory custFactory = new CustomerFactory();

      // Let's load a customer from the Northwind database.
      cust.CustomerId = "ALFKI";

      // Pass on the Customer object

      // custList is a collection of Customer objects
      ArrayList custList = custFactory.FindCustomersByState("CA");

As you can see above, the “load” method loads the Customer object from the database based on the CustomerId. Once the Customer is loaded, it can be passed on to any subsystem in the application without exposing the persistence code. Similarly, if you get an ArrayList of Customer objects, you can pass on the ArrayList, which has no persistence code dependency.


By using the Domain Objects Persistence pattern, you have extracted the persistence code out of the Customer object. This has made the Customer object more object-oriented and simpler to understand because its object model is closer to the data model in the database. And, finally, you have enabled the Customer object to be passed around to different parts of the application (or even to distributed applications through .NET Remoting) without exposing its persistence code.

More by Author

Must Read