For NetSim v8 (8.0, 8.3) and v9 (9.0, 9.1) MS Visual Studio 2010 is recommended.
For NetSim v10 and v11 (11.0, 11.1), MS Visual Studio 2015/2017 is recommended.
For NetSim v12 (12.0, 12.1), MS Visual Studio 2017/2019 is recommended.
For NetSim v13 (13.0, 13.1), MS Visual Studio 2019/20 is recommended.
Visual Studio Community edition and free and can be used for editing protocol source codes in NetSim as long as “C++ Cross-Platform Library Development” module is present.
Related Knowledgebase Articles:
What components of Visual Studio Community 2017 to install and configure to work with NetSim source codes?
What is the development environment settings for NetSim v10/v11/v12?