.. _vonk_reference_api_resourceresolver:

Resource resolver for conformance resources
===========================================

The HL7 .NET SDK defines the ``IAsyncResourceResolver``. In Firely Server an implementation of this class is available to access conformance resources.
Because ``IAsyncResourceResolver`` is bound to a single FHIR Version, Firely Server actually has as many instances as there are FHIR versions loaded (see :ref:`feature_multiversion`).
You can simply have it injected, and Firely Server will provide the implementation that fits the currently applicable information model (a.k.a. FHIR version).
Using a primary constructor this could look like:

.. code-block:: csharp

    internal class MyCustomPluginService(IAsyncResourceResolver conformanceResolver)
    {}
    
You can also get access to conformance resources directly through the ``IAdministrationSearchRepository``. However, using the ``IAsyncResourceResolver`` implementation provides extras:

- Conformance resources are cached in memory.    
- StructureDefinition resources have a freshly calculated snapshot. This is important for correct validation.
- When a ValueSet is expanded (e.g. during a validation), the expansion is cached along with the ValueSet itself. ValueSets are not expanded by Firely Server by default, but they may have an expansion already present when loaded into the administration database.