The xBIM Toolkit (eXtensible Building Information Modelling) is a .NET open-source software development BIM toolkit that supports complete IFC2x3 and IFC4 data models.
xBIM allows .NET developers to read, create and view Building Information (BIM) Models in the IFC format. There is full support for geometric, topological operations and visualisation (both desktop and web). In addition xBIM supports bi-directional translation between IFC and COBie formats. Core libraries for data manipulation are all written in C#, core of geometry engine is written in C++.
Case studies and high overview can be found here, code examples and documentation here and source code on GitHub