Visual Studio 16.4 Preview 2 may uninstall .NET Core 3.0 when installed with the .NET Core 3.1 SDK. We recommend you re-install or repair the [.NET Core 3.0 SDK](https://dotnet.microsoft.com/download/dotnet-core/3.0) if this happens.
### .NET Core installation on macOS Catalina (10.15)
macOS Catalina introduced new security features which affect the .NET Core installers. If you run the installer after downloading using a web browser, a warning dialog will open stating the package `"...can't be opened because Apple cannot check it for malicious software."`, and the installation will be stopped.
To override this behavior, go to Settings : Security & Privacy and choose "Open Anyway" next to the dotnet installer text.
We are working to update our installers for the new Catalina requirements. These will be released during an upcoming monthly update.
* **The 3.1.0 Hosting Bundle for Windows installs a .NET Core Runtime incorrectly branded "3.1.0 Preview 3".**
This is a cosmetic bug: the hosting bundle is expected to function normally.
This will be fixed with 3.1.1. More information at [dotnet/core#3962](https://github.com/dotnet/core/issues/3962).
The Hosting Bundle briefly shows that it is installing "Microsoft .NET Core Runtime - 3.1.0 Preview 3". That string is also visible in Add/Remove Programs after the install completes. This is **not** the old November 2019 3.1.0 Preview 3 Runtime. The Hosting Bundle is installing a more recent build of 3.1.0 that has incorrect branding.
* **If the 3.1.0 SDK and Hosting Bundle were previously installed on Windows, installing the 3.1.0 .NET Core Runtime directly may remove important files such as `hostpolicy.dll`.**
To fix this, repair the .NET Core Runtime installation. (Execute the installer again and click Repair, or find the .NET Core Runtime 3.1.0 installation in Add/Remove Programs, select Modify, then click Repair.)
This will be fixed with 3.1.1. More information at [dotnet/core#3962](https://github.com/dotnet/core/issues/3962) and [dotnet/runtime#703](https://github.com/dotnet/runtime/issues/703).
Missing this file can cause errors like the following, for example when running `dotnet --info`:
> A fatal error was encountered. The library 'hostpolicy.dll' required to execute the application was not found in 'C:\Program Files\dotnet\shared\Microsoft.NETCore.App\3.1.0'.
Due to an package authoring error, the `Microsoft.AspNetCore.Mvc.Razor.RuntimeCompilation` is missing build targets that are required to for runtime compilation to work.
Users of this package will see errors along the lines of `"System.InvalidOperationException: Cannot find reference assembly '...' file for package '...'*`
To resolve this issue, add the following contents to the your project file:
