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:

Properties in EvaluationContext

EvaluationContext.Resource	Gets or sets the node returned by the %resource environment variable. Default is null.
EvaluationContext.RootResource	Gets or sets the node returned by the %rootResource environment variable. Default is null.
EvaluationContext.Tracer	A delegate that handles the output for the trace() function.
FhirEvaluationContext.ElementResolver	A delegate that resolves an uri to an instance of FHIR data (ITypedElement). This callback is used by the FHIR specific method resolve().
FhirEvaluationContext.Environment	A dictionary that can be used to store custom environment variables which will then be available in the FhirPath expression. These environment variables can be accessed using the %% prefix in the expression.

Note that FhirEvaluationContext is only used by the POCO extension methods for FhirPath, as it provides a property for setting the resolver.