You're going to have to be more specific. There is no System.Data.SemanticWeb namespace in C#. The so called "semantic web" is really just a large idea, with much of it having not been realized. There is no pre-defined way of interacting with the semantic web (the semantic web is not even a concretely defined concept). There are certain formats and such that are being developed, but their use in the wild is little at best. If you want to make a semantic search engine, much of the implementation details will be up to you to decide. One thing to look into is the XML format (which C# can readily output and parse). But there are other formats not built into the .Net Framework that you should look into. Read more about the semantic web on
Wikipedia.