Evaluation Contexts

The extension methods and the CompiledExpression all take an expression (as a string) and a second parameter, the EvaluationContext. The context can normally be ignored, but is used to set specific environment-variables in case the defaults don’t work out:

Property	Purpose
EvaluationContext.Resource	Gets or sets the PocoNode returned by the %resource environment variable. Default is null.
EvaluationContext.RootResource	Gets or sets the PocoNode returned by the %rootResource environment variable. Default is null.
EvaluationContext.Environment	A dictionary of custom environment variables, made available in the expression under the % prefix followed by the variable name.
EvaluationContext.Tracer	A delegate that handles the output for the trace() function.
FhirEvaluationContext.ElementResolver	A delegate that resolves a uri to an instance of FHIR data (a PocoNode). This callback is used by the FHIR-specific method resolve().
FhirEvaluationContext.TerminologyService	The terminology service used by the FHIR-specific method memberOf().

Note that FhirEvaluationContext (a subclass of EvaluationContext) is only used by the POCO extension methods for FhirPath, as it provides the properties for the FHIR-specific resolve() and memberOf() functions.