Microsoft has announced an end-of-life schedule for .NET 4.0 thru 4.5.1. After January 12 of 2016, all technical support, including security and non-security updates, will be discontinued. Developers and users will need to either go back to.NET 3.5 SP 1 or upgrade to 4.5.2.

Previews of .NET 4.0 since 2009, with the first production release in April of 2010. Along with new and updated libraries, this marked the first revision of the CLR since version 2 in 2006. Since then the 4.0.x series saw three updates, April/October of 2011 and March of 2012.

Based on the same CLR, the 4.5 series began in August of 2012 for Windows Vista and later. Its updates were in October of 2013 and May of 2014. This last version, .NET 4.5.2, is what Microsoft would like developers and users to all be on in January of 2016.

This end of life announcement comes on the heels of announcing the end of support for non-current versions of Internet Explorer. While their exact motivations have not been revealed, it’s clear why they need to go in this direction. If there is a major security issue today, Microsoft would need to implement, test, and deploy a patch for .NET 4.0, 4.0.1, 4.0.2, 4.0.3, 4.5, 4.5.1, and 4.5.2.

As we mentioned in the introduction, Microsoft will continue to service .NET 3.5 SP 1. This version is tied to the lifespan of Windows 7 and Windows Server 2008 R2, which it currently set as 2020.

Deadline Summary