Migration Guide to the .NET Framework 4.5
If you created your application using an earlier version of the .NET Framework, you can generally upgrade it to the .NET Framework 4.5 easily. Open your project in Visual Studio 2012. If your project was created in an earlier version, the Project Compatibility dialog box automatically opens. For more information about upgrading a project, see How to: Troubleshoot Unsuccessful Visual Studio Project Upgrades.
However, some changes in the .NET Framework require changes to your code. You may also want to take advantage of functionality that is new in the .NET Framework 4.5. Making these types of changes to your application for a new version of the .NET Framework is typically referred to as migration. If your application does not have to be migrated, you can run it in the .NET Framework 4.5 without recompiling it.
Review the following documents before you migrate your application from the .NET Framework 4 to the .NET Framework 4.5:
Review Application Compatibility in the .NET Framework 4.5 to determine the technologies that may cause migration issues for your application.
Review What's Obsolete in the .NET Framework Class Library to determine any types or members in your code that have been made obsolete, and the recommended alternatives.
See What's New in the .NET Framework 4.5 for descriptions of new features that you may want to add to your application.