C5 is a generic collection library whose functionality, efficiency, and quality meets or exceeds what is available for similar contemporary programming platforms. Influenced by the collection libraries for Java and Smalltalk and their published critiques, C5 contains functionality and a regularity of design that considerably exceeds that of the standard libraries for existing platforms. C5 is designed in keeping to a policy of designing toward interface rather than implementation.

C5 works with Microsoft .NET 2.0 and later and Mono 1.2 and later.