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Projects and Backward Compatibility
When a project created in Visual Studio .NET 2002 is opened in Visual Studio .NET 2003 or later, you are prompted with a warning that the project file will be updated. After you open the project, you will no longer be able to open it in the earlier version.
The versions of Visual Studio .NET rely on different versions of the common language runtime — for example, 1.0 for Visual Studio .NET 2002 and 1.1 for Visual Studio .NET 2003. After an application has been built against version 1.1, it requires that version 1.1 be present on a target computer in order to run.
Applications built using version 1.0 will continue to run with either runtime version.
Using Visual Studio .NET 2003 or later, it is still possible to create applications that target runtime version 1.0 using the Supported Runtimes property. For more information, see Working With Multiple Versions of the .NET Framework.