Using FhirPath

The SDK contains a compiler and runtime for FhirPath. FhirPath is an extraction and navigation language and you can execute FhirPath expression both on FHIR POCO classes and on ITypedElement-based data. For both, the following (extension) methods are available:

Functions to evaluate FhirPath

Select()	Returns the nodes produced by the expression.
Scalar()	Returns a single scalar value produced by the expression.
Predicate()	Returns true if the expression evaluates to true or {} (empty) and false otherwise.
IsTrue()	Returns true if the expression evaluates to true and false otherwise.
IsBoolean()	Determines whether the expression evaluates to a given boolean value.

These functions exist as extension methods, so they can conveniently be called on either a POCO or ITypedElement:

Patient p = new() {...}
var hasName = p.IsTrue("Patient.name.exists()");

ITypedElement ite = FhirXmlNode.Parse(...).ToTypedElement(...);
hasName = ite.IsTrue("Patient.name.exists()");

The Select() method will return a list of nodes - this is often used to navigate within the object and select a subset of the nodes within it:

Patient p = new() {...}
var extensions = p.Select("Patient.extension");

When you run the Select method on a POCO, the nodes you will get returned are POCO’s, in this example they would be of type Hl7.Fhir.Model.Extension. When running FhirPath on ITypedElement, the nodes will be of type ITypedElement (and its InstanceType in this case would be Extension).

Select() consistently returns a list of nodes, even on a non-repeating element of a primitive type, p.Select("Patient.active") would therefore return an IEnumerable<Base>, for which the only member is a POCO of type FhirBoolean, not a string (true/false). It’s worth noting that this function will still return .NET primitive values (as in .NET int32 etc) when these are encountered in the FhirPath expression, e.g. Select("4+5"). If you want to avoid this, you can use the convenience method ToFhirValues(), which will translate all such .NET primitive values to their equivalent FHIR primitive types:

Patient p = new() {...}
var t = p.Select("true").ToFhirValues().Single();
Assert.IsTrue(t is FhirBoolean);