Welcome to the .NET Framework Compatibility Blog. We're launching this with .NET Framework 4 Beta 1 to provide you with the latest information on .NET Framework compatibility through the .NET Framework 4 cycle and beyond.

This post gives you a quick overview of .NET Framework 4 compatibility and some information on how to test your applications for compatibility. .NET Framework 4 is compatible with previous versions of the .NET Framework. Most applications or components from previous versions will run on .NET Framework 4 with no issues. That said, there are a few things you should be aware of:

• In many cases, running applications built for previous versions of the .NET Framework on version 4 requires you to opt-in, typically by adding an entry to your application configuration file. Requiring this opt-in provides end users with the most reliable experience and requires developers to test their applications on .NET Framework 4 before end users run the applications on .NET Framework 4. In some rare cases, applications take dependencies on implementation details of the .NET Framework, such as the exact order in which static constructors are called, or the exact timing behavior of thread pool threads. Behaviors like these may change from version to version, and applications which take dependencies on them may run into issues when running on new versions. Because of this, it's important to test your applications and libraries on .NET Framework 4 before releasing them to end users.
• .NET Framework 4 is still Beta software. We will have bugs that prevent code built against previous versions from working. We want your help to find an fix these bugs, preferably by filing Connect items, or posting comments to this blog. Below you'll find a link to a walkthrough that covers how to do this testing.
• There are some changes in .NET Framework 4 that could affect applications. The list of known migration issues for Beta 1 is attached to this post. Please let us know if you run into one of these issues, preferably by filing a Connect bug, or by posting a comment on this blog.

This walkthrough explains how to test your code for compatibility. Please try the scenarios listed in the walkthrough and file any bugs you find with Connect.

Thanks for trying out .NET Framework 4 and helping us make it the most compatible release possible!