Serialization with POCOs and XmlWriter

With the FhirXmlPocoDeserializer have added a serializer that serializes POCO’s directly to XmlWriter objects, achieving higher throughput than the existing serializers with a much smaller memory footprint.

To use it, we first create an Xmlwriter and a FhirXmlPocoSerializer, and then use the Serialize function to serialize a POCO to the XmlWriter. Which you can later use to get you the actual xml string.

Patient p = new() {  };
var sb = new StringBuilder();
using (var w = XmlWriter.Create(sb))
{
   var serializer = new FhirXmlPocoSerializer(Specification.FhirRelease.STU3);
   serializer.Serialize(p, w);
}
var xmlString = sb.ToString();

When initializing the FhirXmlPocoSerializer you have to specify which FHIR version you are using for serialization.

Note that the serializer will not validate the data passed to it, so it can easily be driven to produce XML 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 Xml:

<Patient xmlns="http://hl7.org/fhir">
    <active value = "treu"/>
    <contact></contact>
</Patient>

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 xml output, you should validate the instances first.

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 the serialize method, like so:

var summaryFilter = SerializationFilter.ForSummary();
Patient p = new() {  };
var sb = new StringBuilder();
using (var w = XmlWriter.Create(sb))
{
   var serializer = new FhirXmlPocoSerializer(Specification.FhirRelease.STU3);
   serializer.Serialize(p, w, summaryFilter);
}

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.