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<html>
<head>
<title>SemWeb: Docs: Hello World</title>
<link rel="stylesheet" type="text/css" href="stylesheet.css"/>
</head>
<body>
<p><a href="index.html">SemWeb Documentation</a></p>
<h1>SemWeb Hello World</h1>
<p>Here's an example of using SemWeb to construct an RDF/XML or Notation 3 file.</p>
<p>Create a new file called <tt>helloworld.cs</tt>. If you're using Visual Studio, create this in a new project and reference <tt>SemWeb.dll</tt> provided in the download package.</p>
<p>First, use the relevant namespaces and create a simple class. Also add a constant for the RDF namespace.</p>
<pre class="code">
using System;
using SemWeb;
public class Example {
const string RDF = "http://www.w3.org/1999/02/22-rdf-syntax-ns#";
public static void Main() {
}
}
</pre>
<p>In <tt>Main()</tt>, create a new <tt>MemoryStore</tt>, which is an in-memory store of RDF statements:</p>
<pre class="code">
MemoryStore store = new MemoryStore();
</pre>
<p>The next thing to do is create some entities for the things you will be asserting statements about. You can do that simply by creating a new <tt>Entity</tt> object:</p>
<pre class="code">
Entity computer = new Entity("http://example.org/computer");
Entity description = new Entity("http://example.org/description");
</pre>
<p>The <tt>Entity</tt> class has an implicit conversion operator from strings, which means you can just assign a string to an entity variable to save typing:</p>
<pre class="code">
Entity says = "http://example.org/says";
Entity wants = "http://example.org/wants";
</pre>
<p>Blank nodes, a.k.a. anonymous entities, are created using the <tt>BNode</tt> class, which is a subclass of <tt>Entity</tt>.</p>
<pre class="code">
Entity desire = new BNode();
</pre>
<p>Next, we create statements using these entities. To create a statement, use the <tt>Statement</tt> constructor, which takes a subject, predicate, and object. Note that <tt>Statement</tt>s are <i>structs</i>, not classes, but this probably won't affect you.</p>
<pre class="code">
Statement assertion = new Statement(computer, says, new Literal("Hello world!"));
</pre>
<p>Another conversion operator is defined to make it easy for you to create Literals out of strings.
Unlike the one that is implict for Entities, this one is <i>explicit</i>, which means you
always need to write out the cast. We could have written the previous line like this insted:
<pre class="code">
Statement assertion = new Statement(computer, says, (Literal)"Hello world!");
</pre>
<p>Statements have to be put into a <tt>Store</tt> like this:</p>
<pre class="code">
store.Add(assertion);
</pre>
<p>I'll condense that into this:</p>
<pre class="code">
store.Add(new Statement(computer, says, (Literal)"Hello world!"));
store.Add(new Statement(computer, wants, desire));
store.Add(new Statement(desire, description, (Literal)"to be human"));
store.Add(new Statement(desire, RDF+"type", (Entity)"http://example.org/Desire"));
</pre>
<p>A store is a collection of statements. In true RDF, the order and number of occurrences of a statement doesn't matter because a graph is simply a set of statements. Some SemWeb stores act like sets, rather than collections. The memory store is not one of these.</p>
<p>Lastly, we want to write out the contents of the store to an RDF/XML file. We do this by creating a new <tt>RdfXmlWriter</tt> object, sending the store's contents to the writer, and then <b>disposing the writer</b>. It's very important to dispose of writers so they can finish, so you should always wrap writers with the <i>using</i> C# directive.</p>
<pre class="code">
using (RdfWriter writer = new RdfXmlWriter(Console.Out)) {
writer.Write(store);
}
</pre>
<p>If you're using Mono, like I do, to compile and run the program, run the following commands. Be sure to put <tt>SemWeb.dll</tt> in the current directory so Mono can find it at compile time, and then at run time.</p>
<pre class="code">
mcs helloworld.cs -r:SemWeb.dll
mono helloworld.exe
</pre>
<p>If you're using VisualStudio, put this in a new project, reference <tt>SemWeb.dll</tt>
and compile and run it.</p>
<p>Here's the output:</p>
<pre class="code"><?xml version="1.0"?>
<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns:exampleorg="http://example.org/">
<rdf:Description rdf:about="http://example.org/computer">
<exampleorg:says>Hello world!</exampleorg:says>
<exampleorg:wants>
<exampleorg:Desire>
<exampleorg:description>to be human</exampleorg:description>
</exampleorg:Desire>
</exampleorg:wants>
</rdf:Description>
</rdf:RDF></pre>
<p>We didn't provide the writer with any namespace prefixes, so it made one up. To provide a prefix for a namespace, use the <tt>Namespaces</tt> property of the writer:</p>
<pre class="code">
using (RdfWriter writer = new RdfXmlWriter(Console.Out)) {
writer.Namespaces.AddNamespace("http://example.org/", "ex");
...
</pre>
<p>You need to set the namespaces before any statements are streamed to the writer. Here's the final output:</p>
<pre class="code" file="../examples/helloworld.output.rdf"><?xml version="1.0"?>
<rdf:RDF xmlns:ex="http://example.org/" xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#">
<rdf:Description rdf:about="http://example.org/computer">
<ex:says>Hello world!</ex:says>
<ex:wants>
<ex:Desire>
<ex:description>to be human</ex:description>
</ex:Desire>
</ex:wants>
</rdf:Description>
</rdf:RDF></pre>
<p>To write out the statements in Notation 3 format, just use the <tt>N3Writer</tt> class instead. It produces this output:</p>
<pre class="code">
@prefix ex: <http://example.org/> .
ex:computer ex:says "Hello world!" ;
ex:wants _:bnode0 .
_:bnode0 ex:description "to be human" ;
<http://www.w3.org/1999/02/22-rdf-syntax-ns#type> ex:Desire .
</pre>
<p>Here's the complete program:</p>
<pre class="code" file="../examples/helloworld.cs">// This example creates a few RDF statements and adds
// them to a MemoryStore. Then it writes out the
// statements in RDF/XML format to the console. Note
// that the implicit string-to-Entity and string-to-
// Literal conversion operators are being used.
using System;
using SemWeb;
public class Example {
const string RDF = "http://www.w3.org/1999/02/22-rdf-syntax-ns#";
public static void Main() {
MemoryStore store = new MemoryStore();
Entity computer = new Entity("http://example.org/computer");
Entity says = "http://example.org/says";
Entity wants = "http://example.org/wants";
Entity desire = new BNode();
Entity description = new Entity("http://example.org/description");
store.Add(new Statement(computer, says, (Literal)"Hello world!"));
store.Add(new Statement(computer, wants, desire));
store.Add(new Statement(desire, description, (Literal)"to be human"));
store.Add(new Statement(desire, RDF+"type", (Entity)"http://example.org/Desire"));
using (RdfWriter writer = new RdfXmlWriter(Console.Out)) {
writer.Namespaces.AddNamespace("http://example.org/", "ex");
writer.Write(store);
}
}
}
</pre>
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</html>
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