LINQ to JSON in .NET

1. Yes, there is such a thing as a *.json file. The MIME type is application/json (source). JSON is a text-based format though, so you could hypothetically store JSON-formatted data in a text file with whatever extension you choose.

2. JSON can absolutely be used independently of JavaScript. In some cases, it's probably better suited to representing your data than XML. JSON.org has a great comparison page between JSON and XML.

3. JSON.org lists several JSON libraries for C# (for example, JSON.NET which you have already discovered), and most (if not all) of the collections that these libraries use should support LINQ. JSON.NET definitely does offer support for it. See here or here.

Everyone tends to stick with the JavaScriptSerializer (from the System.Web.Extensions library) when working with JSON in .NET. The handy part about this is the ability to create a custom JavaScriptConverter that will take custom objects and serialize them the way you chose. Likewise, you can make a deserialization method to receive in custom JSON formatting.

Though this of course depends on your application. Given that it's a Windows Forms application, is there any particular reason you'd chose JSON over storing the information natively or just use the XML format? If your application communicates with webpages, the JavaScriptSerializer is probably the best bet, though if you're using it to store/retrieve settings I'd use XML. And, if it's necessary to synchronize your application with a web-based one, just serialize to JSON when the time is ready.

You can deserialize your JSON file into C# objects. After that, you can query with LINQ on these objects.