The Essence Pattern provides a way to enforce users of an object to enter valid properties for that object. A full description of the pattern can be found in the original paper here (PDF). In this article, I shall provide a C# implementation of this pattern.
If you are providing an external interface to your code either as an API or because your code forms a tier in an n-tier system, then the Essence Pattern can be used to force other programmers to put valid, required fields into an object before they start using that object. One common situation where this is required is for key fields to be saved to a database. The user of the object must not attempt to save that object to the database without the key properties, or the database will complain.
Of course, you could simply provide just one constructor that forces the programmer to enter all the required properties. In the simple case this is perfectly acceptable. However, if you are exposing an external interface, you may also want it to be COM wrappable. In this case, you can only have one, parameterless constructor which disallows that particular strategy. Also, what if there are a lot of required parameters, some of which need to be calculated or read from a data source? Are you going to make the user of your object store all these in local variables before they can construct your object?
The Example Code
The code sample with this article shows a very simple implementation of the Essence Pattern for a small class representing a contact. The
Contact class has 5 properties, three of which are required (
Email), and two of which are optional (
The first thing we do is create the
ContactEssence class as an
internal class of the
Contact class. The
ContactEssence contains only the required properties for the contact (
Email), and allows read and write access to them all.
One of the two important methods of the
ContaceEssence is the
Validate method (shown below). This method essentially checks that all the required parameters are present and correct. Only when the
Validate method returns
true can the class advance to the next stage.
private bool Validate()
if (mFirstName.Length == 0)
if (mLastName.Length == 0)
if (mEmail.Length == 0)
System.Text.RegularExpressions.Regex emailReg =
System.Text.RegularExpressions.MatchCollection results =
if (results.Count == 0)
The next stage is the creation of the real
Contact class, which is done through a call to the
public Contact CreateContact()
if (this.Validate() == true)
return new Contact(this);
As you can see, if the
ContactEssence is validated correctly, a
Contact is returned.
The final part of the puzzle is the
Contact class itself. The first area to look at are the constructors:
private Contact(ContactEssence xEssence)
mEmail = xEssence.Email;
mFirstName = xEssence.FirstName;
mLastName = xEssence.LastName;
"Default constructor not valid -- use " +
"ContactEssence to create the Contace object", true)]
Both constructors are private.
ContactEssence is an internal class to the
Contact, so it has access to the private constructors. The second constructor is marked with the obsolete attribute simply to prevent its use at all. This means that the
Contact class can only be created by the
Contact is created from the
ContactEssence, it simply copies all the required fields. Additional (optional) fields are left in their original initialised state.
Finally, in the
Contact class, the required fields are exposed as read-only properties, while the optional fields are read-write. This means that once the required fields are created, they cannot be changed.
So how do we use the
The example code shows the role of the
ContactEssence and the validation of required
Contact.ContactEssence essence = new Contact.ContactEssence();
essence.Email = "firstname.lastname@example.org";
Contact invalidContact = essence.CreateContact();
if (invalidContact == null)
Console.WriteLine("Created NULL contact!");
essence.FirstName = "Dermot";
essence.LastName = "Gubbins";
Contact validContact = essence.CreateContact();
if(validContact == null)
Console.WriteLine("Oops! This should be valid!");
validContact.PhoneNumber = "(+44) 444 444";
Console.WriteLine("\nContact Created ...\n");
Points of Interest
As explained in the original paper the Essence Pattern can be used in object creational patterns to create multiple valid classes by changing one or more of the required fields and then creating a new instance of the main class. Additionally, the Essence can also act as a builder class.
After ten years studying biology and evolution, I threw it all away and became a fully paid up professional programmer. Since 1990 I have done C/C++ Windows 3.1 API, Pascal, Win32, MFC, OOP, C#, etc, etc.
PS. In the picture, I'm the one in blue. On the right.