Other Features of the POCOs
Fluent Initializers
The SDK provides additional initialization methods for several data types. These methods are designed to simplify object construction using fluent notation. Visual Studio’s IntelliSense can guide you through the available options as you type. Alternatively, you can explore the Hl7.Fhir.Model namespace in the Object Browser to review the methods and their attributes.
For example, the HumanName data type includes methods that allow you to construct a name in a single statement:
pat.Name.Add(new HumanName().WithGiven("Christopher").WithGiven("C.H.").AndFamily("Parks"));
If you need to populate fields beyond Given and Family, you can first create a HumanName instance using this approach and then add additional details later. Alternatively, you can skip the fluent notation and populate all fields as described in other sections.
Annotations
All POCOs in the SDK support annotations, which allow you to attach arbitrary data to a POCO instance without modifying its class definition. This feature is particularly useful for storing metadata or other information relevant to the instance or its processing, but not part of the FHIR specification:
var patient = new Patient();
patient.AddAnnotation(new MyCustomMetadata { ProcessedBy = "SystemX", Timestamp = DateTime.UtcNow });
if (patient.TryGetAnnotation<MyCustomMetadata>(out var metadata))
{
Console.WriteLine($"Patient processed by {metadata.ProcessedBy} at {metadata.Timestamp}");
}
The IIdentifiable<T> Interface
FHIR resources with an Identifier element have corresponding POCOs that implement the IIdentifiable<T> interface, where T represents the type of the Identifier property. In practice, T is either Identifier or List<Identifier>. The IIdentifiable<T> interface inherits from the marker interface IIdentifiable.
Extension methods such as GetIdentifier() and TryGetIdentifier() are available to quickly retrieve an identifier by its system from a POCO implementing IIdentifiable.
The ICode<T> Interface
Note
This is an advanced feature that is mostly used for model binding within the CQL engine, but can also be useful in other scenarios.
The ICode<T> interface is used to represent the “primary code” element of a resource, as defined by the FHIR model for Clinical Quality Language (CQL). While this element is often named “Code,” it may have a different name depending on the POCO design. The SDK uses the CQL binding to identify these elements.
Resources with a primary code implement ICode<T>, where T can be one of the following types:
CodeCodeableConceptList<CodeableConcept>FhirStringDataType
The DataType type (an abstract class) is used when the coded property is a choice, such as elements related to Medication that can be either Coding or Reference. Although the current set of types is limited, future changes to the CQL binding may expand this list. Implementers should be prepared to handle all coded types (Code, Coding, CodeableConcept, Quantity) and data types (string, uri).
The ICode<T> interface extends the ICode interface, which includes a single method, ToCodings(). This method returns the coded contents as a list of Coding instances. The conversion of coded types to Coding follows the guidelines outlined in the FHIR Terminologies documentation.