After a longtime partnership, Microsoft is finally buying Xamarin, a platform provider for mobile app development, executive vice president Scott Guthrie said in a blog post Wednesday. The terms of the deal were not disclosed.

In conjunction with the Redmond, Wash. giant’s Visual Studio, Xamarin provides a development offering that enables programmers to build mobile apps using C# and deliver fully native mobile app experiences to all major operating systems — including iOS, Android, and Windows, Guthrie said.

Xamarin has more than 15,000 customers in 120 countries, including more than 100 Fortune 500 companies. Microsoft has had a longstanding partnership with Xamarin, and has jointly built Xamarin integration into Visual Studio, Microsoft Azure, Office 365, and its Enterprise Mobility Suite to provide developers with an end-to-end workflow for native, secure apps across platforms.

“With today’s acquisition announcement we will be taking this work much further to make our world class developer tools and services even better with deeper integration and enable seamless mobile app dev experiences,” Guthrie wrote. “The combination of Xamarin, Visual Studio, Visual Studio Team Services, and Azure delivers a complete mobile app dev solution that provides everything a developer needs to develop, test, deliver and instrument mobile apps for every device.”

Fortune wrote, “So, what took so long?”

Sravish Sridar, chief executive of Kinvey, another mobile development specialist, told Fortune that Xamarin provided Microsoft .NET developers with an “excellent” toolset to deliver cross-platform native mobile apps.

“One of the weaker parts of Microsoft’s mobile-first strategy was making it easy for developers to build cross-platform apps, and with the acquisition of Xamarin, Microsoft has shown that it is trying bring value to developers, irrespective of [the] mobile OS platform they are building apps on,” Sridhar said.