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Fhir-net-api

The official .NET API for HL7 FHIR

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Searching for Resources

FHIR has extensive support for searching resources through the use of the REST interface. Describing all the possibilities is outside the scope of this document, but much more details can be found online in the specification.

The FHIR client has a few operations to do basic search.

Searching within a specific type of resource

The most basic search is the client’s Search<T>(string[] criteria = null, string[] includes = null, int? pageSize = null) function. It searches all resources of a specific type based on zero or more criteria. Criteria must conform to the parameters as they would be specified on the search URL in the REST interface, so for example searching for all patients named ‘Eve’ would look like this

Bundle results = client.Search<Patient>(new string[] { "family:exact=Eve" });

The search will return a Bundle containing entries for each resource found. It is even possible to leave out all criteria, effectively resulting in a search that returns all resources of the given type. Additionally, there is a Search() overload that does not use the generic T argument, you can pass the type of resource as a string in the first parameter instead.

Searching for a resource with a specific id

In some cases you may already have the id of a specific resource (e.g. an Observation with logical id 123, corresponding to the url Observation/123). In this case you can use SearchById<T>(string id, string[] includes = null, int? pageSize = null).

Note that this function still returns a Bundle. The operation differs from a Read<T>() operation because it can return included resources as well. E.g. given an id 123 for an Observation, you can ask a FHIR server to not only look for the indicated Observation but to return the associated subject as well:

var incl = new string[] { "Observation.subject" };
Bundle results = client.SearchById<Observation>("123", incl);

Some servers allow you to execute searches across all resource types. This would use FhirClient’s WholeSystemSearch(string[] criteria = null, string[] includes = null, int? pageSize = null).

Doing this search:

Bundle results = client.WholeSystemSearch(new string[] { "name=foo" });

would then not only return Patients with “foo” in their name, but Devices named “foo” as well.

Complex searches

An alternative way to specify a query is by creating a Query resource and pass this to the client’s Search(Query q) overload. The Query resource has a set of fluent calls to allow you to easily construct more complex queries:

var q = new Query()
         .For("Patient").Where("name:exact=ewout")
         .OrderBy("birthDate", SortOrder.Descending)
         .SummaryOnly().Include("Patient.managingOrganization")
         .LimitTo(20);

Bundle result = client.Search(q);
 ```

Note that unlike the search options shown before, you can specify search ordering and the use of a summary result. As well, this syntax avoids the need to create arrays of strings as parameters and tends to be more readable. 

### Paged Results
Normally, any FHIR server will limit the number of search results returned. In the previous example, we explicitly limited the number of results per page to 20.

The FhirClient has a `Continue` function to browse a search result after the first page has been received using a `Search`:

```csharp
var result = client.Search(q);

while( result != null )
{
	// Do something useful
	result = client.Continue(result);
}

Note that Continue supports a second parameter that allows you to browse forward, backward, or go immediately to the first or last page of the search result.

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