Serialization with POCOs and System.Text.Json

With the introduction of the NET 5.0 target of the SDK, we have added a serializer based on the new System.Text.Json namespace. These serializers serialize POCO’s directly to Utf8JsonWriter objects, achieving higher throughput than the existing serializers with a much smaller memory footprint.

The functionality is exposed through JsonFhirConverter, which is an implementation of System.Text.Json’s JsonConvert<T>. To use it, set up a new JsonSerializerOptions to add this converter, and then call the JsonSerializer:

Patient p = new() {  };
var options = new JsonSerializerOptions().ForFhir(typeof(Patient).Assembly).Pretty();
string patientJson = JsonSerializer.Serialize(p, options);

The ForFhir() method initializes the options to use the FHIR Json converter. Additionally, we have specified that we want indented output. This method has an overload that takes an Assembly as an argument, which is the assembly where the SDK’s POCO classes can be found. This determines which version of FHIR to use for serialization and is used by deserializer to locate the classes to instantiate when parsing FHIR data. If you are working with one specific version of FHIR (i.e. you are using a NuGet assembly for R4), there will be an overload that does not require the Assembly argument, and it will default to the version of FHIR you have included in your project.

NB: It is very important that you reuse instances of JsonSerializerOptions across all your deserialization calls, otherwise performance will degrade tremendously.

More details can be found on the section on deserialization.

Note that the serializer will not validate the data passed to it, so it can easily be driven to produce Json that is incorrect, e.g. this code:

FhirBoolean b = new() { ObjectValue = "treu" };
Patient p = new() { Contact = new() [ new Patient.ContactComponent() ], ActiveElement = b };

will produce the following invalid FHIR Json:

  "resourceType": "Patient",
  "active": "treu",
  "contact": [{}]

This is by design, as this is useful for round-tripping incorrect stored historical data. It avoids duplicate validation work and increases performance as well. To ensure correct json output, you should validate the instances first.

If you do not want to set up a converter, it is possible to invoke the serializer directly by instantiating a JsonFhirDictionarySerializer, and then calling its Serialize() method. The JsonFhirDictionarySerializer can be subclassed to change the way primitives are serialized in the virtual method SerializePrimitiveValue(). This will become especially useful in .NET 6, which adds functionality to allow the user to write “raw” json to the output stream. An example is overriding the handling of very large decimals.

Generating a summary

The serializer can be told to serialize only parts of an object’s tree, thus generating a summary view of the FHIR data`.

To enable summary generation, create a concrete subclass of the SerializationFilter class, there are several pre-defined factory methods that you can use:

  • SerializationFilter.ForSummary() - returns a new serialization filter to support _summary=true

  • SerializationFilter.ForText() - returns a new serialization filter for _summary=text

  • SerializationFilter.ForData() - returns a new serialization filter for _summary=data

  • SerializationFilter.ForElements() - returns a new serialization filter for _elements=

The other summary forms mentioned in the FHIR specification do not need specific support from the serializer and can be constructed by hand.

Once created, such a filter must then be passed to one of the serialization methods, like so:

var summaryFilter = SerializationFilter.ForSummary();
var options = new JsonSerializerOptions().ForFhir(typeof(Patient).Assembly, summaryFilter).Pretty();

The filters are highly configurable and it is possible to write your own by subclassing SerializationFilter. Please refer to the (pretty trivial) implementation of the current summary filters (e.g. ElementMetadataFilter.cs and BundleFilter.cs) in the source code for more information.