Disable JsonSerializer.IsReflectionEnabledByDefault on `PublishTrimmed=true`

[eiriktsarpalis]

Have we decided we want this to be defaulted to off only when PublishAot=true? Or do we think we should go as far as turning it off by default when PublishTrimmed=true?

For the ASP.NET feature switch, we had it for both. But I'm a little concerned about making the switch for PublishTrimmed at the bottom of the stack. It is a breaking change. If we do this, we will need to update Maui and Blazor WASM SDKs to turn it back on.

We don't need to block this PR on the decision. For ASP.NET, we can just switch our SDK to change EnsureAspNetCoreJsonTrimmability => JsonSerializerIsReflectionEnabledByDefault once this is merged, and can react as changes happen.

Originally posted by @eerhardt in #83844 (comment)

Tagging subscribers to this area: @dotnet/area-system-text-json, @gregsdennis
See info in area-owners.md if you want to be subscribed.

Issue Details

---

Have we decided we want this to be defaulted to off only when PublishAot=true? Or do we think we should go as far as turning it off by default when PublishTrimmed=true?

For the ASP.NET feature switch, we had it for both. But I'm a little concerned about making the switch for PublishTrimmed at the bottom of the stack. It is a breaking change. If we do this, we will need to update Maui and Blazor WASM SDKs to turn it back on.

We don't need to block this PR on the decision. For ASP.NET, we can just switch our SDK to change EnsureAspNetCoreJsonTrimmability => JsonSerializerIsReflectionEnabledByDefault once this is merged, and can react as changes happen.

Originally posted by @eerhardt in #83844 (comment)

Author:	eiriktsarpalis
Assignees:	-
Labels:	area-System.Text.Json, untriaged
Milestone:	-

[eerhardt]

FYI @Redth @PureWeen @jonathanpeppers @SteveSandersonMS @javiercn @marek-safar @lewing @SamMonoRT @steveisok - (please also tag anyone else you'd think would be interested from a MAUI or Blazor WASM stand point).

We are considering disabling Reflection in System.Text.Json by default in a PublishTrimmed=true application. This will affect all app models that are using trimming today. If we disable it, we will need to update MAUI and Blazor WASM SDKs to turn the reflection feature switch back on in STJ. Tagging you as app model owners that may have feedback on this plan.

[Sergio0694]

I assume this would also affect WASDK (WinUI 3) apps?

[eerhardt]

I assume this would also affect WASDK (WinUI 3) apps?

Yes, if you set PublishTrimmed=true in those apps.

[Sergio0694]

Not sure if this point has already been raised (probably?), but are there concerns about code potentially breaking due to this change and surprising developers? Is the assumption that if you enable trimming you'd already get all trimming warnings anyway, and in order to fix those you would already have to switch to code paths that are guaranteed to be safe when this flag is disabled? As in, just so that people won't encounter unexpected issues that only come up in published builds and not in normal ones? 🤔

I guess what I'm trying to say is that for people wanting to publish their apps with trimming, my expectations would be that this flag should always be disabled to ensure they get a consistent behavior even for normal F5 builds. Is that just not doable and relying on trimming warnings alone is sufficient?