ASP.NET Core 3 and the underlying .NET Core Framework runtime indeed provide some major enhancements and performance improvements, but there are still some specific scenarios where those new application patterns do not apply and where the full .NET Framework will be the best and sometimes even the only choice.

Migrating your whole existing applications to ASP.NET Core right from the start might be difficult or even impossible to do. You should think about how to transform your applications progressively to lower the risk of failure or over complication and give yourself time to really understand the new patterns and paradigms.

You could start for instance by only using ASP.NET Core 3 for all new developments, then see how to migrate your legacy code later and sometimes even leave it be since there will be no real benefits for migrating it over. If you are really interested in the migration topic, please consider the Appendix since we have a full chapter dedicated to this important topic.

ASP.NET Core and .NET Core Framework get more and more framework and client library support each day. Microsoft, tool and framework vendors, and the different developer communities work hard to provide a large set of functionalities for allowing feature-rich and high-performing web applications. Everybody wants to work on this promising technology that could sustainably shape the future.

The possibility to use .NET Core and .NET Framework libraries together at the same time when using .NET Standard 2.0 extends the possibilities even more and gives developers a temporary solution until every important feature and every major framework will be available in .NET Core.

To recap what has been discussed in this section, you should use ASP.NET Core 3 for your server applications if the following is true:

• You have cross-platform needs.
• You are specifically targeting microservices.
• You want to use Docker containers.
• You need high-performance and highly scalable applications.
• The presented technical restrictions do not apply to your application requirements.