As promised earlier this year, Microsoft has completed its previously announced acquisition of Xamarin, the San Francisco-based mobile app development tool company.

The closing of the deal was quietly revealed by Xamarin co-founder Miguel de Icaza on Twitter:

Xamarin was founded in 2011 and since then over 1.3 million unique developers have used its tools to make mobile apps with C# that allow for cross-platform support for iOS, Android and Windows operating systems. Microsoft did not disclose the specific financial details of their Xamarin acquisition, but one rumor claims it paid about $400 million to buy the company.