How to modify nodes selected by FHIRPath "Select" function

[BoyaWu10]

Hi,

We have a question about IEnumerable<ITypedElement> Select(this ITypedElement input, string expression, EvaluationContext ctx = null).

After parsing the input FHIR resource to ITypedElement:

var provider = new PocoStructureDefinitionSummaryProvider();
var rootNode = FhirJsonNode.Parse(inputFileContent).ToTypedElement(provider);

we could select the target node by FHIRPath using above function:

var matchNode = rootNode.Select("Patient.identifier").FirstOrDefault();

Matched node with instance type ScopedNode would be returned.

If we want to modify this node using Add/Remove/Replace functions in ElementNode, is there any method to convert this ScopedNode to ElementNode safely?

Can we achieve this by converting the input to ElementNode before Select, and then get it from ScopedNode.Current?

// Convert to ElementNode
var rootNode = ElementNode.FromElement(FhirJsonNode.Parse(inputFileContent).ToTypedElement(provider));
var matchNode = rootNode.Select("Patient.identifier").FirstOrDefault();
if (matchNode != null)
{
    // Will this always work?
    var elementNode = (ElementNode)((ScopedNode)matchNode).Current;
}

Thanks.

[ewoutkramer]

Yes, that is the approach I would take as well. Navigating a ScopedNode using FhirPath will generally just call Children() on the ScopedNode, which will do this:

        public IEnumerable<ITypedElement> Children(string? name = null) =>
            Current.Children(name).Select(c => new ScopedNode(this, ParentResource, c, _fullUrl));

This fragment shows that while navigating down the tree using FhirPath, new ScopedNodes are created using a private constructor, which just wraps the ElementNode children (c) of the current ElementNode, and they will be exposed through Current in the child nodes.

So this should work!

[BoyaWu10]

Thanks so much, Ewout! And why do we choose to wrap FhirPath input in ScopedNode before evaluating (FirelyTeam/firely-net-common#116)? When should we consider to use ScopedNode instead of other nodes? In previous versions of the library, if we pass an ElementNode to function Select(), it would return ElementNode as well.

[ewoutkramer]

This is because some functions in FhirPath require awareness of more context than ITypedElement is capable of providing. For example, ScopedNode keeps track of the contained resources that are encountered while walking the tree, so that FhirPath's resolve() function is able to locate contained resources. The same holds for the FhirPath variables %resource and %rootResource. ScopedNode allows code to walk up in the hierarchy so these variables can be determined.

[BoyaWu10]

I see. Thanks so much for your kind help!