Invoking the FhirPath Compiler directly

The FhirPath compiler is just another public class in the Hl7.FhirPath namespace. It has a constructor which takes an argument of type SymbolTable - the key to full control over the installed dialect:

var symbolTable = new SymbolTable()
      .AddStandardFP()
      .AddFhirExtensions();
var newCompiler = new FhirPathCompiler(symbolTable);

You can now use the compiler to:

• Compile() an expression to a ready-to-execute delegate (called CompiledExpression)

• Parse() an expression to an abstract symbol tree, for display or debugging purposes

Invoking the CompiledExpression is equivalent to using the Select() function described above. The other functions, like IsBoolean are also available (as extension methods).

Note that the built-in extension methods (Select(), Scalar(), etc.) don’t recompile the expression on every call: internally they use a FhirPathCompilerCache, which pairs a configured FhirPathCompiler with a cache of already-compiled expressions (by default holding up to 500 entries). If you invoke your own FhirPathCompiler directly, you bypass this cache — so for expressions you evaluate repeatedly, either cache the resulting CompiledExpression yourself or create your own FhirPathCompilerCache (its constructor takes an optional FhirPathCompiler and cache size).