Using the Namespace Mapper

Namespaces are a concept from XML which are used in the Semantic Web as the standard way of simplifying URIs into QNames. A QName (Qualified Name) is a way of abbreviating a URI in a simple readable form which makes it easier for humans to understand the data while still being machine readable. e.g.

http://www.w3.org/1999/02/22-rdf-syntax-ns#type can be represented as rdf:type

More generally in RDF there is the concept of a PName (Prefixed Name), this is an extension to XML QNames which is more flexible and allows more abbreviations since it does not have to conform to the strict XML naming rules.

A Prefixed Name is composed of two parts:

• The Namespace Prefix appears before the colon and may be empty.
• The Local Name appears after the colon and may be empty.

Note: Exact definitions of what is and isn't a valid Prefixed Name vary depending on the serialization syntax being used.

A prefixed name is resolved into a URI by concatenating the Namespace URI with the Local Name. A Namespace Prefix is used to refer to a Namespace URI, prefixes must be associated with URIs before they can be used. The following example shows a fragment of Turtle which defines some namespaces and then uses them to define a Triple.

@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>.
@prefix ex: <http://example.org/>.

ex:this rdf:type ex:Example.

The above represents the following Triple:

<http://example.org/this> <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://example.org/Example>

Empty Namespace Prefix

The empty namespace prefix has a special meaning, it either refers to the default Namespace (a Namespace defined with an empty prefix) or if no default Namespace is defined it may refer to the Base URI of the Graph. The usage of the empty namespace prefix does vary between different serialization syntaxes.

The Namespace Mapper

In order that prefixed names can be resolved into URIs various parts of the library must maintain a table of associations of Namespace Prefixes and their corresponding Namespace URIs. This function is provided by the VDS.RDF.INamespaceMapper interface.

The most common place to encounter this interface is when working with graphs, any implementation of the VDS.RDF.IGraph interface provides a property called NamespaceMap which returns the INamespaceMapper associated with the graph.

The INamespaceMapper can be used to introduce new Namespaces into the graph to allow you to more easily define URI Nodes by using QNames e.g.

IGraph g = new Graph();

//Define the Namespaces we want to use
g.NamespaceMap.AddNamespace("rdf", new Uri("http://www.w3.org/1999/02/22-rdf-syntax-ns#"));
g.NamespaceMap.AddNamespace("ex", new Uri("http://example.org"));

//Define the same Triple as the previous example
UriNode exThis = g.CreateUriNode("ex:this");
UriNode rdfType = g.CreateUriNode("rdf:type");
UriNode exExample = g.CreateUriNode("ex:Example");
g.Assert(new Triple(exThis, rdfType, exExample));

You can also use the GetNamespaceUri(String prefix) and GetPrefix(Uri uri)) methods to retrieve Namespace URIs based on a Prefix and vice versa.

Another important feature of the INamespaceMapper is it's ability to reduce URIs to prefixed names via it's ReduceToQName(String uri, out String qname) function. This function allows you to take a URI and attempts to turn it into a prefixed name returning true if this succeeds and setting the out variable qname to be the prefixed name. All of the writer classes provided in the library make use of this function to help them generate prefixed names for output.

Merging Namespace Maps

If you wish to combine two namespaces maps then you may wish to use the Import(INamespaceMapper nsmap) method which imports namespaces from one map into another.