Lucene.Net is a source code, class-per-class, API-per-API and algorithmatic port of the Java Lucene search engine to the C# and .NET platform utilizing Microsoft .NET Framework.
Lucene.Net sticks to the APIs and classes used in the original Java implementation of Lucene. The API names as well as class names are preserved with the intention of giving Lucene.Net the look and feel of the C# language and the .NET Framework. For example, the method Hits.length() in the Java implementation now reads Hits.Length() in the C# port.
In addition to the APIs and classes port to C#, the algorithm of Java Lucene is ported to C# Lucene. This means an index created with Java Lucene is back-and-forth compatible with the C# Lucene; both at reading, writing and updating. In fact a Lucene index can be concurrently searched and updated using Java Lucene and C# Lucene processes.
Go to http://incubator.apache.org/lucene.net/
See also these related projects.