Breaking change for trimming in .NET 7 Preview 7

[agocke]

Breaking change notice

In .NET 7, trimming will trim all assemblies in the app by default. This breaking change only affects apps which are published with PublishTrimmed=true, and it only affects apps with trim warnings.

Description

This is a behavior change for console apps (i.e., plain .NET core apps that don't use WindowsDesktop, Android, iOS, WASM, or ASP.NET SDKs). The change is that trimming will now trim all the assemblies in the app by default, as opposed to the pre-.NET 7 behavior, which only trimmed assemblies that had opted-in with <IsTrimmable>true</IsTrimmable> in the library project file.

Effects

Apps which may have previously worked with PublishTrimmed may not in .NET 7. Only apps with trim warnings may be affected. If your app has no trim warnings, the change in behavior should not cause any adverse affects, and will likely decrease the app size.

Example

An app which uses Newtonsoft.Json or System.Text.Json without source generation to serialize and deserialize a type in the user project may function before the change because types in the user project were fully preserved. One or more trim warnings (warning codes ILxxxx) would have been present.

After the change, types in the user project will be trimmed and serialization may fail or produce unexpected results.

Fixes

The best fix is to resolve all trim warnings in your application. For your own libraries, that means resolving the warnings yourself (see https://docs.microsoft.com/en-us/dotnet/core/deploying/trimming/fixing-warnings for more information). For other libraries, you may contact the author to request that they resolve the warnings, or you may choose a different library which already supports trimming.

For instance, instead of Newtonsoft.Json you could choose System.Text.Json with source generation, which supports trimming.

Workaround

To go back to the previous behavior, use the TrimMode property. The pre-.NET 7 behavior is

<TrimMode>partial</TrimMode>

The .NET 7+ behavior is

<TrimMode>full</TrimMode>

[gewarren]

@agocke This section doesn't list full and partial as options for TrimMode. Does it need to be updated?

https://docs.microsoft.com/en-us/dotnet/core/deploying/trimming/trimming-options#trimming-granularity

[agocke]

It's in PR here: #30279