DeveloperGuide SPARQL Operators

Home > Developer Guide > SPARQL Engine > SPARQL Operators

SPARQL Operators

SPARQL Operators are an extension to SPARQL permitted by the specification which allow for implementations to extend the definition of certain operators - namely =, !=, <, <=, >, >=, +, -, * and /.

Currently dotNetRDF only supports extending the arithmetic operators i.e. +, -, * and /. In the future we will likely extend this functionality to allow any operator to be overloaded.

Extended Operators

Right now the only operators which dotNetRDF provides extensions for are + and - when used with xsd:dateTime, xsd:duration, xsd:dayTimeDuration. This means that when using our SPARQL engine you can add/subtract durations from date times and add/subtract durations from each other.

Implementing Custom Operators

Custom operators are implemented by implementing the VDS.RDF.Query.Operators.ISparqlOperator interface and registering your implementation with the VDS.RDF.Query.Operators.SparqlOperators registry.

The ISparqlOperator Interface

The ISparqlOperator interface is relatively simple containing a single property and two methods. Once this is implemented you register your implementation like so:

SparqlOperators.AddOperator(new MyCustomOperator());

Operator

The Operator property returns a VDS.RDF.Query.Operators.SparqlOperatorType which indicates which operator your implementation overloads.

IsApplicable()

The IsApplicable() method takes in the proposed arguments for the operator and determines whether the operator can be applied to those arguments e.g. are the argument types compatible for the implementation.

Apply()

The Apply() method takes the arguments for the operator and applies the operator returning either the resulting value or throwing a VDS.RDF.Query.RdfQueryException if an error occurs.