At one time, commercial software giant Microsoft was known as a company that was unfriendly to the open source software movement. Former CEO Steve Ballmer even referred to the popular open source Linux platform as a “cancer.” But now, Microsoft is crediting open source for the increased adoption of its .NET software development platform, in a blog post published this week.

Prior to open source, .NET ran only on Windows. “Now that it is open source, it runs in so many more locations,” said Claire Novotny, .NET Foundation executive director. Open source has made it easier for .NET to be cross-platform, with the company able to collaborate with Linux communities, added Dan Moseley, group manager for .NET libraries at Microsoft. Developers also gain access to the source files of .NET, making the platform easy to trust, he said.

Microsoft released its CoreCLR .NET execution engine as open source in 2015 and introduced the .NET Core open source runtime in 2016. With .NET 5, released in 2020, the company migrated .NET platform development away from the 20-year-old .NET Framework to open source .NET Core. But there have been challenges getting long-time Microsoft customers to adopt open source.

“Many .NET customers have historically composed their apps from Microsoft-supplied libraries, which were historically closed-source, and their own code, and are less comfortable depending on non-Microsoft libraries, which are typically open source,” Moseley said. Microsoft wants to make it easier for customers to trust libraries not coming from the .NET team.

Given that it’s increasingly common for language and runtime implementations to be open source, Microsoft would be conspicuous if it did not follow this pattern, said Kevin Pilch, Microsoft engineering manager for ASP.NET. Open source also opens up interesting collaborations involving individuals and other companies.