In the Primitive data types paragraph, we have mentioned that both resource and data types can be extended in FHIR. To add an extension to your data, you will have to fill in both a URL identifying the extension and a value that is valid according to the definition of the extension.

The following code example adds a place and time of birth to our Patient instance defined by standard extensions in the FHIR specification. The URL and the type of value you can fill in are listed in the definition of the extension. The definition will also tell you on what field the extension can be added.

var birthplace = new Extension();
birthplace.Url = "http://hl7.org/fhir/StructureDefinition/birthPlace";
birthplace.Value = new Address() { City = "Seattle" };

var birthtime = new Extension("http://hl7.org/fhir/StructureDefinition/patient-birthTime",
                               new FhirDateTime(1983,4,23,7,44));

The extension for birthPlace is pretty straightforward to add. The URL is taken from the extension definition. The value is of type Address, and the extension can be added to the top-level of the Patient instance.
The birthTime extension is a little more complex. This extension takes a dateTime value, and has to be added to the BirthDate field. For this field the API provides you with the easy way to fill it, by allowing you to set the value of BirthDate as a string – internally converting this to the Date type. This means you will have to use the [fieldname]Element construction to add extensions to the field.