Working with CQL

As a result of the cooperation between Firely and NCQA, the SDK contains a full CQL Engine and a tool to convert ELM-based CQL logic to C#.

To start working with CQL, you need to do two things:

  • Install the CQL Packager .NET tool, which will allow you to read ELM files, turn them into C# and compile them. This is done by running dotnet tool install Hl7.Cql.Packager --global (add --prerelease if you are feeling adventurous).

  • Add a reference to the main CQL package to your project.

The packager tool will generate the C# that the engine can run using the engine. For example, if we had compiled a library called BCSEHEDISMY2022 (version 1.0.0), the packager will create a class called BCSEHEDISMY2022_1_0_0 that can be invoked like this:

using Hl7.Cql.Fhir;
using Hl7.Cql.Packaging;
using Hl7.Cql.Primitives;
using Hl7.Fhir.Model;

// Set up the parameters for the CQL library
private readonly IDictionary<string, object> MY2023 =
  new Dictionary<string, object>
          "Measurement Period",
                new CqlInterval<CqlDateTime>(
                new CqlDateTime(2023, 1, 1, 0, 0, 0, 0, 0, 0),
                new CqlDateTime(2023, 12, 31, 0, 0, 0, 0, 0, 0),

var patientEverything = new Bundle();  // add some data
var context = FhirCqlContext.ForBundle(patientEverything, MY2023);
var bcs = new BCSEHEDISMY2022_1_0_0(context); // instantiate the library
var numerator = bcs.Numerator(); // invoke one of the definition in the library

More information can be found in the GitHub repository for the CQL Engine.