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You install a software update for the .NET Framework that uninstalls previously installed software updates for the .NET Framework
Consider the following scenario. You install a software update for the Microsoft .NET Framework on a computer that is running previously installed software updates for the .NET Framework. The previously installed software updates include the same files as the new software update that you will install. When you install the new software update, the files for the previously installed software updates are removed and are replaced by the files of the new software update.
Note This scenario rarely occurs. Microsoft software updates are cumulative. This means that each new software update includes the fixes that were included in prior software updates.
RESOLUTION
Before you install a software update, we recommend that you open Add/Remove Programs in Control Panel. Note the software updates for the .NET Framework that are currently installed. After you have installed the new software update, open Add/Remove Programs in Control Panel again. Determine the software updates for the .NET Framework that are no longer listed. The software updates that are no longer listed are the software updates that you must reinstall. Remove the new software update. Then, obtain and reinstall the previously installed software updates.