In .NET Core, the "App Trim" feature helps reduce the size of the published application by removing unused code and assemblies. It analyzes the application's dependencies and trims out any unused parts, resulting in a smaller deployment package.

To enable App Trim in .NET Core, you need to perform the following steps:

1. Update your project file: Open your project file (.csproj) and ensure that the PublishTrimmed property is set to true. You can set this property inside a configuration like Debug or Release depending on your needs:

<PropertyGroup>
  <PublishTrimmed>true</PublishTrimmed>
</PropertyGroup>

dotnet publish -c Release -r <runtime-identifier>

Replace <runtime-identifier> with the appropriate runtime identifier for your target platform (e.g., win-x64, linux-x64, osx-x64).

3. Verify the trimmed output: After publishing, check the output folder for the trimmed application. You should notice a reduction in the size of the published files compared to an untrimmed build.

It's important to note that App Trim works best when your application uses the .NET SDK-style projects (<Project Sdk="Microsoft.NET.Sdk">). Additionally, App Trim requires .NET Core 3.0 or higher.

Keep in mind that while App Trim can significantly reduce the size of the published application, it may also remove some code that is used dynamically at runtime. Therefore, it's recommended to thoroughly test your application after enabling App Trim to ensure that all expected functionality is intact.

You can refer to the official documentation for more details on using App Trim in .NET Core: https://docs.microsoft.com/en-us/dotnet/core/deploying/app-trimming