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PublishedTrimmed projects fail reflection-based serialization

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Projects that enable the PublishTrimmed MSBuild property now automatically turn off the reflection-based defaults of System.Text.Json. In other words, setting PublishTrimmed to true automatically sets the JsonSerializerIsReflectionEnabledByDefault MSBuild property to false unless otherwise specified in project configuration.

Previous behavior

Prior to this change, projects that have the PublishTrimmed property enabled, that is, <PublishTrimmed>true</PublishTrimmed>, published a trimmed application. However, the reflection-based default serialization behavior wasn't necessarily disabled. Depending on what code got trimmed, the follow code might or might not succeed serialization, or might or might not output the correct serialization data.

JsonSerializer.Serialize(new { Value = 42 });

New behavior

Starting in .NET 8, projects that have the PublishTrimmed property enabled fail serialization outright. The code JsonSerializer.Serialize(new { Value = 42 }); throws the following exception:

System.InvalidOperationException: Reflection-based serialization has been disabled for this application.

Version introduced

.NET 8 Preview 7

Type of breaking change

This change is a behavioral change.

Reason for change

This change ensures that trimmed applications use appropriate defaults. It also guides users towards adopting best practices suitable for trimmed applications—that is, use the source generator, and avoid accidental dependency on the unsafe reflection-based components.

To ensure that serialization suceeds, we recommend that you migrate your trimmed applications to use the source generator.

However, if you must use reflection, you can revert to the original behavior by explicitly enabling the JsonSerializerIsReflectionEnabledByDefault property in your project file:

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

Affected APIs

N/A