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The degree of .NET Framework support for backward and forward compatibility is version-specific. The .NET Framework supports both backward and forward compatibility for applications created using version 1.1 only. It does not support forward compatibility in applications created using version 2.0. In the context of the .NET Framework, backward compatibility means that an application created using an early version of the .NET Framework will run on a later version. Conversely, forward compatibility means that an application created using a later version of the .NET Framework will run on an earlier version.
The .NET Framework provides a high degree of support for backward compatibility. For example, most applications created using version 1.0 will run on version 1.1 and applications using version 1.1 will run on version 2.0. The .NET Framework also supports forward compatibility for version 1.1 only. However, for forward compatibility you might need to modify an application so that the application runs as expected. Applications created with version 2.0 will not run on earlier versions of the .NET Framework. For both backward and forward compatibility, a change to the .NET Framework that helps improve security, correctness, or functionality might also raise compatibility issues.
For more information about breaking changes that might affect your application, see Breaking Changes in the .NET Framework.