Model Classes

For each resource type and data type in FHIR, the SDK contains a corresponding public class. These are often referred to as “POCOs” (Plain Old C# Objects) or “model classes.”

Creating a new instance of any resource or data type from the FHIR specification is straightforward:

var pat = new Patient();
var id = new Identifier();

Tip

When creating an instance of a resource or data type, refer to the FHIR specification to identify mandatory elements for that type and understand how each element is used.

Elements

The SDK classes include a property for each element in the resource or data type model. For example, the Patient resource has an active element:

Patient with some elements

The Patient class in the SDK has a property called Active that corresponds to this element:

pat.Active = true;

List Elements

Repeating elements are represented in the class as a .NET List of the element’s type:

Identifier with cardinality

For example, to add the Identifier we created earlier to the Identifier property of the Patient instance, you can do the following:

pat.Identifier.Add(id);

Nullability

You may have noticed that you did not need to initialize the Identifier list property in the previous example before adding elements to it. For list properties, the SDK automatically initializes the list on first use.

However, non-list properties will be null by default, so you must initialize them before use.

To assist with this, the model classes are generated with nullability annotations. This ensures the compiler warns you if you attempt to access or set a property that has not been initialized.

Note

Even if the FHIR specification indicates that an element is mandatory (i.e., has a cardinality greater than 0), the POCO will still allow you to set it to null. We have also not used the required keyword in C# to enforce this. This design choice allows you to create objects even if the required data is unavailable at creation time. However, exchanging incomplete data may result in non-conformant FHIR usage. Ensuring your POCOs are valid is discussed in the section on validation.

Element.id and Extension.url

The FHIR specification describes two special elements: Element.id and Extension.url. While these elements are typed as normal FHIR strings in the specification, they are actually special properties that can only hold primitive values. For consistency, the model classes use the standard FHIR data types. Treat these elements as primitive data types and avoid adding extensions to them.

Note that Element.id is generated as the ElementId property to avoid confusion with other elements named id.