[macOS] Cannot use PublishTrimmed=true

[tipa]

Steps to Reproduce

1. dotnet new macos
2. Add <PublishTrimmed>true</PublishTrimmed> to test.csproj
3. dotnet publish

Expected Behavior

App builds and gets trimmed without errors

Actual Behavior

/usr/local/share/dotnet/sdk/8.0.100-rc.1.23463.5/Sdks/Microsoft.NET.Sdk/targets/Microsoft.NET.RuntimeIdentifierInference.targets(212,5): error NETSDK1191: A runtime identifier for the property 'SelfContained' couldn't be inferred. Specify a rid explicitly. [/Users/me/GitHub/test/test.csproj]

Environment

macos 13.3.8825-net8-rc1/8.0.100-rc.1 SDK 8.0.100-rc.1

Example Project (If Possible)

test.zip

Comments

Until today, I was under the impression, that using TrimMode=full did trim out every unused code from my application. However, I was surprised that my app did not throw any error when using System.Json.Text in .NET8 without source generators. I expected it to throw an System.InvalidOperationException as documented (and I also saw that exception in my .NET8 Android project).
As a side-note / additional question: I also added PublishTrimmed=true to my iOS app, built and uploaded it to TestFlight. It also does not throw the mentioned/expected System.InvalidOperationException. Does the PublishTrimmed property even have any effect in iOS/macOS projects?

[rolfbjarne]

I can reproduce this, and a fix is in progress. Hopefully we'll be able to get it in the initial .NET 8 release.

Note that PublishTrimmed=true is redundant on macOS projects, because that's the default, so you can safely remove it.

However, I was surprised that my app did not throw any error when using System.Json.Text in .NET8 without source generators. I expected it to throw an System.InvalidOperationException as documented (and I also saw that exception in my .NET8 Android project).

This is because we've changed the default for JsonSerializerIsReflectionEnabledByDefault to true, to avoid breaking people.

If you do this in your project:

<PropertyGroup>
    <JsonSerializerIsReflectionEnabledByDefault>false</JsonSerializerIsReflectionEnabledByDefault>
</PropertyGroup>

you should get the InvalidOperationException.

[tipa]

Thanks. I always though that settings TrimMode=full would also set PublishTrimmed=true implicitly on iOS as well. Is this not the case? If not, what is the difference and does setting it to true explicitly result in any additional size reductions?
On .NET Android, PublishTrimmed is set to true automatically in Release mode under certain conditions (and that causes JsonSerializerIsReflectionEnabledByDefault to be set to false.): https://github.com/xamarin/xamarin-android/blob/58a81eb148acb2d2f5d3df91629bba3a3129e773/src/Xamarin.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.DefaultProperties.targets#L71-L72

Also, does settings JsonSerializerIsReflectionEnabledByDefault to false result in any app size reduction?

Tbh, I would prefer if the default JsonSerializerIsReflectionEnabledByDefault=false from would not be overwritten. It still can be overwritten in the MAUI SDK if it's needed there. It gives a nice error message that should help everyone who runs into it: System.InvalidOperationException: Reflection-based serialization has been disabled for this application. Either use the source generator APIs or explicitly configure the 'JsonSerializerOptions.TypeInfoResolver' property.
It is a bit confusing when there are different defaults for each build property on each platform :)

[rolfbjarne]

We always set JsonSerializerIsReflectionEnabledByDefault=true:

https://github.com/xamarin/xamarin-macios/blob/1a05b370ef3905cc06c8c385fdaa2e920afd89e5/dotnet/targets/Xamarin.Shared.Sdk.props#L96C1-L97

Also, does settings JsonSerializerIsReflectionEnabledByDefault to false result in any app size reduction?

No idea, my guess is that it could go either way, so testing it on your app is the only way to know for sure.

Thanks. I always though that settings TrimMode=full would also set PublishTrimmed=true implicitly on iOS as well.

Note quite: we've hijacked the linker for our own purposes (in addition to what the linker usually does), and that means it's always executed (in fact we set PublishTrimmed=true, and if you try to tell us otherwise by setting PublishTrimmed=false yourself, you'll get a build error).

Then you can adjust exactly what's linked by setting the TrimMode property, or the old-style MtouchLink property (TrimMode takes precedence if you set both). You can even set MtouchLink=None, and the linker will still run, but not actually trim any assemblies (it'll only do what we hijacked it to do).

does setting it to true explicitly result in any additional size reductions?

Setting PublishTrimmed=true doesn't change anything.

Setting TrimMode=full will change the output, and likely save some space.