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Trimming may not be used with .NET Standard or .NET Framework

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Projects that set <PublishTrimmed>true</PublishTrimmed>, <IsTrimmable>true</IsTrimmable> or <EnableTrimAnalyzer>true</EnableTrimAnalyzer> with a TargetFramework that is any version of .NET Standard or .NET Framework produce a warning or error because trimming is unsupported for these target frameworks.

Previous behavior

Previously, when used in a .NET Standard or .NET Framework project, these settings behaved as follows:

  • <PublishTrimmed>true</PublishTrimmed> had no effect.
  • <IsTrimmable>true</IsTrimmable> embedded an assembly-level attribute [assembly: AssemblyMetadata("IsTrimmable", "true")] into the output assembly. That attribute opted the assembly into trimming when consumed in a trimmed app (even an app that uses <TrimMode>partial</TrimMode>).
  • <EnableTrimAnalyzer>true</EnableTrimAnalyzer> enabled trim analysis for the library, using the .NET Standard or .NET Framework reference assemblies corresponding to the library's TargetFramework even though these reference assemblies aren't annotated for trimming.

New behavior

Starting in the .NET 8 SDK, in a project targeting .NET Standard or .NET Framework:

  • <PublishTrimmed> produces an error indicating that this setting is unsupported for the target framework.
  • <IsTrimmable> and <EnableTrimAnalyzer> produce a warning indicating that <IsTrimmable> is unsupported for the target framework. These settings otherwise have no effect on the build output.

Version introduced

.NET 8 RC 1

Type of breaking change

This change is a behavioral change.

Reason for change

Allowing the use of <PublishTrimmed> in a .NET Standard or .NET Framework project might have given the false impression that this setting was trimming the output, when in fact it had no effect.

Allowing the use of <IsTrimmable> or <EnableTrimAnalyzer> in a .NET Standard or .NET Framework project meant that it was easy for library authors to opt into trimming, without being alerted about trim incompatibilities. Because the .NET Standard and .NET Framework reference assemblies aren't annotated for trimming, there were no warnings about uses of framework APIs that are incompatible with trimming.

Avoid setting <PublishTrimmed> in projects that target .NET Standard or .NET Framework. Also avoid setting <PublishAot> which implies the former setting.

Avoid setting <IsTrimmable> or <EnableTrimAnalyzer> in libraries that target .NET Standard or .NET Framework. Also avoid setting <IsAotCompatible>, which implies the former settings. Instead, multi-target the library to include the latest TargetFramework, and enable <IsTrimmable> only for the supported target frameworks. Setting <IsTrimmable> will run the latest version of the trim analyzer using trim-compatibility annotations from the latest version of the framework.

For example, these settings multi-target to include net8.0, and set <IsTrimmable> only for this target framework. This logic uses IsTargetFrameworkCompatible so that it will apply to any frameworks compatible with net6.0, when trimming was first officially supported. This way, the condition doesn't need to be updated when adding new target frameworks.

<PropertyGroup>
  <TargetFrameworks>netstandard2.1;net8.0</TargetFrameworks>
  <IsTrimmable Condition="$([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)', 'net6.0'))">true</IsTrimmable>
</PropertyGroup>

See also