The following API and behavior changes have the potential to break existing applications when upgrading them to 3.0.0. Changes that we expect to only impact database providers are documented under provider changes.  

This article explains how to update an existing ASP.NET Core 2.2 project to ASP.NET Core 3.0. 

So stay tuned for those details. A follow-on .NET Core 3.1 release -- which will include late bug fixes and more, and, who knows, maybe a stable Blazor WebAssembly -- is scheduled for November.  

Here are a couple of the most relevant improvements you may want to verify:

• Work on the EF Core in-memory provider was finished and most query features should now be working (the majority of it went into RC1)
• EF Core’s compilation performance was improved significantly for complex queries 

.NET Core 3.0 Release Candidate 1 (RC1) is now available. This release contains only a handful of bug fixes and closely represents what we expect to release for .NET Core 3.0. 

Today, we’re announcing .NET Core 3.0 Release Candidate 1. Just like with Preview 9, we’ve focused on polishing .NET Core 3.0 for a final release. We are now getting very, very close. We intend to release the final version on September 23 at .NET Conf. 

• .NET Core 2.2.7 and .NET Core SDK ( Download | Release Notes )
• .NET Core 2.1.13 and .NET Core SDK ( Download | Release Notes ) 

What follows is sort of a travel log of what I did and how the new CoreCLR functionality fares compared to writing C++ code, when processor intrinsics are involved. 

Maximising .NET Core API performance
Find bottlenecks and remove them! 

Besides all the other improvements in EF Core 3.0, preview 9 includes fixes for more than 100 issues that we resolved since Preview 8.