Business Objects and Value Objects

Encapsulation of data and behavior is one of the cornerstones of object-oriented programming. This basically means that a business object contains both the data and methods that manipulate the data. In the example below, a CreditCard object contains public method Authorize and public property AuthorizationCode. It's important for the object to establish a proper public interface. Authorization code value is returned by the payment processor; we wouldn't want clients to accidentally modify it. Therefore, I exposed a read-only property rather than a field.

public CreditCard
{
...
private string _AuthorizationCode;
public string AuthorizationCode
{
get { return _Authorizationcode; }
}

public bool Authorize(double amount) {...}
...
}

Object-oriented approach is great - within a single application tier. When designing a distributed application, we need to take other factors into consideration. Suppose, my application needs to display customer credit card data on a web page. Should I pass the CreditCard object to the web tier? Sure, it is possible, but the web tier only needs credit card data, not the behavior. Frankly, I wouldn't want web tier code to accidentally call Authorize() method. So, for the sake of security, we should somehow limit the objects. Performance is another concern, especially when passing objects between physical tiers. Regardless of which remote technology we use - web services, .NET Remoting, or COM+ - large objects with lots of methods and properties may not be ideal for this.

A simple and elegant solution is to combine essential business object data into a "value object". Individual data elements of the value object are exposed to business object's clients via public properties. Revised CreditCard class below demonstrates this approach. Note how overload of the constructor allows us to easily create a business object from a value object. We can extract value object just as easily and send it to another application tier.

public CreditCard
{
...
private CreditCardInfo _CCInfo;
public CreditCardInfo CCInfo
{
get { return _CCInfo; }
}

public CreditCard(CreditCardInfo info)
{
_CCInfo = info;
}

public string AuthorizationCode
{
get { return _CCInfo.Authorizationcode; }
}

public bool Authorize(double amount) {...}
...
}

public CreditCardInfo
{
...
public string CardNumber;
public DateTime ExpDate;
public string Authorizationcode;
public DateTime? LastTransactionDate;
...
}