SemWeb.NET is a Semantic Web/RDF library written in C# for Mono or Microsoft's .NET. The library can be used for reading and writing RDF (XML, N3), keeping RDF in persistent storage (memory, MySQL, etc.), querying persistent storage via simple graph matching and SPARQL, and making SPARQL queries to remote endpoints. Limited RDFS and general-purpose inferencing is also possible. SemWeb's API is straight-forward and flexible. (What is RDF?)
This is an open-source library, so patches are most welcome.
The latest version is 1.0.7 dated May 18, 2010. Thanks to Ruben Verborgh for a number of contributions including a rewritten Notation 3 writer that supports writing graphs.
See the ChangeLog for details.
As of May 19, 2009 I'm taking an indefinite hiatus from this project. That means that while I'll try to apply any patches to fix existing bugs, I won't be actively developing the library futher, and I won't be answering questions for help on the mail list. Over the last four years it's been fun to work on it, but I don't think there has been enough uptake of the Semantic Web in the .NET world (or otherwise) for me to justify spending more time on this when I have other things in life I'd rather be working on.
About the Library
SemWeb was first released in June 2005 and has been more recently tested with triple stores of over 1 billion statements (see see this). The core features, like reading/writing RDF in XML and N3, persistent SQL-backed storage, and basic SPARQL queries, are pretty solid. Peripheral features like RDFS reasoning and backward-chaining reasoning are working, but less tested and less complete. The library has no particular tools for OWL schema. It operates at the level of RDF triples only.
You may also be interested in LinqToRdf, a library that provides C# LINQ querying over RDF using this library.
License: SemWeb is licensed primarily under the terms of the GNU GPL (version 2 or later). Note that SemWeb includes some external components: the SPARQL engine (sparql-core.dll) is licensed under the GNU LGPL; the Euler proof mechanism (Euler.cs, which is a part of SemWeb.dll) is licensed under the W3C Software License; and IKVM (IKVM*.dll) is GPL-compatible. The source code which I wrote myself (which is most everything but the above, see the README for details) is dual-licensed under both the GPL and the Creative Commons Attribution license (which was the only license I listed from 2005 through 2007).
SemWeb was mentioned in this article in RedmondDeveloper.
SemWeb is used in ROWLEX, F-Spot (Gnome photo management), Beagle (Gnome desktop search) for its experimental RDF access layer, and (at least at one time) Sentient Knowledge Explorer (a commercial data visualizer).
Download, Documentation, Etc.
Source code, binaries (.dll assemblies, both .NET 1.1 and .NET 2.0 versions), and HTML documentation are included in the downloads:
The library is also packaged in Ubuntu as libsemweb1.0-cil but the latest version there is years old and has serious bugs.
LUBM Load Time and Query Time Benchmarks
I ran the following benchmarks with version 0.85 of this library. The benchmarks involved the LUBM benchmarks, with 50 universities, totaling 6.9 million statements spread accross 1,000 files totaling at 540MB. The benchmarks were performed on a single-processor 1.8GHz AMD with 1GB RAM, under the Mono 1.2.3 runtime in Fedora Core 6.
Loading The Data
Querying The Data
The LUBM suite contains 14 sample queries, of which only a subset can be answered by the library because the rest use reasoning not supported by the library. The RDFS class supports a subset of RDFS reasoning, and that was enough to answer queries 1, 3, 4, 5, 6, 7, 8, 9, and 14. Run against the MySQL data source, most of the queries take 2 seconds to answer, including program start-up time. The ones that have an enormous answer take 30-70 seconds.
Berlin SPARQL Benchmark
See this page for the results of the Berlin SPARQL Benchmark.