The IResourceResolver interface

The IResourceResolver interface is used to resolve conformance resources from the specification or an implementation guide (e.g. StructureDefinition, ValueSet, CodeSystem). Newer implementations use the async version of this interface:

public interface IAsyncResourceResolver
{
   /// <summary>Find a resource based on its relative or absolute uri.</summary>
   Task<Resource> ResolveByUriAsync(string uri);

   /// <summary>Find a (conformance) resource based on its canonical uri.</summary>
   Task<Resource> ResolveByCanonicalUriAsync(string uri); // IConformanceResource
}

Note that the principle method is ResolveByCanonicalUriAsync which resolves a canonical resource. The ResolveByUriAsync method is used by some implementations to resolve non-conformance resources, like example instances of resources but is otherwise not widely supported and normally is just hardwired to use the ResolveByCanonicalUriAsync method. These methods will return the resolved resource or null if the resource is not found.

There are a number of useful extension methods that can be used to simplify the use of the IAsyncResourceResolver interface:

• FindStructureDefinitionAsync(this IAsyncResourceResolver resolver, string uri) - resolves a StructureDefinition,

• FindStructureDefinitionForCoreTypeAsync(this IAsyncResourceResolver resolver, string coreType) - resolves a StructureDefinition for a named FHIR resource or datatype.

• FindValueSetAsync(this IAsyncResourceResolver resolver, string uri) - resolves a ValueSet

• FindCodeSystemAsync(this IAsyncResourceResolver resolver, string uri) - resolves a CodeSystem

These methods will return the resolved resource or null if the resource is not found or if the resolved resource is not of the requested type.

There is a related interface IConformanceSource, which extends IResourceResolver. This interface has methods to find specific types of conformance resources using criteria other than the canonical uri:

public interface IConformanceSource : ICommonConformanceSource
{
    /// <summary>
    /// Find a <see cref="CodeSystem"/> resource by a <see cref="ValueSet"/> canonical url that contains all codes from that codesystem.
    /// </summary>
    CodeSystem FindCodeSystemByValueSet(string valueSetUri);

    /// <summary>List all resource uris for the resources managed by the source, optionally filtered by type. (these are not Canonical Uris)</summary>
    IEnumerable<string> ListResourceUris(ResourceType? filter = null);

    /// <summary>Find <see cref="ConceptMap"/> resources which map from the given source to the given target.</summary>
    IEnumerable<ConceptMap> FindConceptMaps(string? sourceUri = null, string? targetUri = null);

    /// <summary>Finds a <see cref="NamingSystem"/> resource by matching any of a system's UniqueIds.</summary>
    NamingSystem? FindNamingSystem(string uniqueId);
}

This interface is not supported by all implementations of IResourceResolver.