[Serialization] `JsonSerializerOptions` is not being respected

[erikvilima]

Current behavior

Even when explicitly setting JsonSerializerOptions with ReadCommentHandling = JsonCommentHandling.Skip, deserializing a JSON that contains comments throws an exception saying that / is an invalid start of a property name.

Expected behavior

The provided JsonSerializerOptions should respected and skip comments in the JSON when ReadCommentHandling is set to Skip.

How to reproduce it (as minimally and precisely as possible)

1. Create a minimal Uno app with Serialization
2. Use serializer.FromString<T>(json) with JsonSerializerOptions { ReadCommentHandling = JsonCommentHandling.Skip }.
3. Pass a JSON string or file that contains comments (e.g., // this is a comment).
4. Observe that an exception is thrown despite the ReadCommentHandling setting.

You can find a minimal repro project attached.

serialization.zip

Environment

"Uno.Sdk": "6.0.146"
"Uno.Sdk": "6.2.0-dev.16"

Affected platform(s): I have only tested Skia, WinAppSdk and WASM, but I assume it occurs everywhere.

• iOS
• macOS (AppKit)
• Mac Catalyst
• Android
• WebAssembly
• Windows
• Desktop
• Build tasks

Visual Studio:

• 2019 (version: )
• 2022 (version: 17.14.5 )
• Visual Studio Code (version: )
• Rider Windows (version: )
• Rider macOS (version: )

Relevant plugins:

• Resharper (version: )

Anything else we need to know?

[agneszitte]

Root-Cause Analysis

Summary

Custom JsonSerializerOptions registered via DI have never been respected by the serializer since November 2021 (PR #149). A fix attempt in January 2026 (PR #2996) introduced ConfigureJsonSerializationOptions() but still ignores user-provided options due to operator precedence.

---

Timeline

Date	Event	Details
Nov 11, 2021	Clean state (6285ea5)	No JsonSerializerOptions registered in DI at all
Nov 22, 2021	Bug introduced (490f5b0e5)	Hardcoded JsonSerializerOptions singleton added to DI
Nov 30, 2021	PR #149 merged	"chore: tidy up" by @nickrandolph — contains the offending commit
Jul 8, 2025	Issue #2845 filed	@erikvilima reports ReadCommentHandling = JsonCommentHandling.Skip is silently ignored
Jan 9, 2026	PR #2996 merged	Fix attempt by @jonpryor — adds GetJsonSerializationOptions() + ConfigureJsonSerializationOptions() — but introduces a new precedence bug

---

Before the Bug (commit 6285ea568, Nov 11 2021)

// ServiceCollectionExtensions.cs — NO JsonSerializerOptions registered
public static IServiceCollection AddSystemTextJsonSerialization(this IServiceCollection services)
{
    return services
        .AddSingleton<SystemTextJsonStreamSerializer>()
        .AddSingleton<ISerializer>(services => services.GetRequiredService<SystemTextJsonStreamSerializer>())
        .AddSingleton<IStreamSerializer>(services => services.GetRequiredService<SystemTextJsonStreamSerializer>());
}

// SystemTextJsonStreamSerializer.cs — accepts nullable options via DI
public SystemTextJsonStreamSerializer(JsonSerializerOptions? serializerOptions = null)
{
    _serializerOptions = serializerOptions; // null if no one registers one → uses System.Text.Json defaults
}

At this point, if a consumer registered their own JsonSerializerOptions singleton, it would be injected and used. ✅

---

After PR #149 (commit 490f5b0e5, Nov 22 2021) — Bug Introduced

// ServiceCollectionExtensions.cs — hardcoded options ALWAYS registered
public static IServiceCollection AddSystemTextJsonSerialization(this IServiceCollection services)
{
    return services
        .AddSingleton(sp => new JsonSerializerOptions        // ← always wins over user options
        {
            NumberHandling = JsonNumberHandling.AllowReadingFromString,
            AllowTrailingCommas = true
        })
        .AddSingleton<SystemTextJsonStreamSerializer>()
        .AddSingleton<ISerializer>(services => services.GetRequiredService<SystemTextJsonStreamSerializer>())
        .AddSingleton<IStreamSerializer>(services => services.GetRequiredService<SystemTextJsonStreamSerializer>());
}

The framework now always registers a JsonSerializerOptions singleton. Since DI resolves the first registration of a given type, any consumer-registered options are ignored. ❌

---

After PR #2996 (merged Jan 9, 2026) — Still Broken

// ServiceCollectionExtensions.cs
internal static JsonSerializerOptions GetJsonSerializationOptions(this IServiceProvider services)
    => services.GetService<IOptions<JsonSerializationOptions>>()?.Value?.SerializerOptions
        ?? JsonSerializationOptions.DefaultSerializerOptions;  // ← NEVER null

// SystemTextJsonSerializer.cs
public SystemTextJsonSerializer(IServiceProvider services, JsonSerializerOptions? serializerOptions = null)
{
    _serializerOptions = services.GetJsonSerializationOptions()  // ← always returns non-null
        ?? serializerOptions                                     // ← dead code
        ?? JsonSerializationOptions.DefaultSerializerOptions;
}

GetJsonSerializationOptions() returns DefaultSerializerOptions when no ConfigureJsonSerializationOptions callback is registered — so it is never null. The DI-injected serializerOptions parameter is effectively dead code. ❌

---

Suggested Fix

Invert the precedence so consumer-provided options win (suggested by @dr1rrb):

_serializerOptions = serializerOptions
    ?? services.GetJsonSerializationOptions()
    ?? JsonSerializationOptions.DefaultSerializerOptions;

This restores the intended behavior: if a consumer registers custom JsonSerializerOptions via DI, those options take priority. The framework-provided defaults only apply when no explicit options are configured.

---

Impact

Any consumer registering custom JsonSerializerOptions via DI (e.g., ReadCommentHandling = JsonCommentHandling.Skip) has those options silently ignored since the initial framework release containing PR #149 (late 2021).

---

Related PRs & Issues

Item	Description
PR #149	Original cause — introduced hardcoded JsonSerializerOptions singleton (Nov 2021)
PR #2996	Fix attempt that introduced GetJsonSerializationOptions() with broken precedence (Jan 2026)
PR #2970	Closed first attempt at fixing this
Issue #2908	Related serialization issue
Issue #3001	Related serialization issue

[agneszitte]

Latest repro

@erikvilima previously shared a repro sample in the description of this issue.

I updated the solution to use the latest Uno.Sdk 6.7.0-dev.31 and added Console.WriteLine logging in MainViewModel.LoadData<T>'s catch block.

The attached zip reproduces immediately on Skia Desktop for example: serialization-repro-6.7.0-dev.31.zip

[BUG CONFIRMED] Deserialization failed: JsonException: '/' is an invalid start of a property name.
Expected a '"'. Path: $ | LineNumber: 2 | BytePositionInLine: 1.

[jonpryor]

Interesting-to-me note: the global.json present within the repro uses Uno.Sdk 6.0.146 (2025-06-18). As @agneszitte noted, this change has been present since #149 in 2021…

[jonpryor]

Workaround/solution: Don't Do That™ (?).

PR #2996 added support for $(JsonSerializerIsReflectionEnabledByDefault)=false, which required lots of related changes in this area.

If App.xaml.cs does:

var builder = this.CreateBuilder(args)
	.Configure(host => host
#if DEBUG
		.UseEnvironment(Environments.Development)
#endif
		.ConfigureServices((context, services) =>
		{
			services.AddSingleton<MainViewModel>();
		})
		.UseSerialization([PersonContext.Default], services =>
		{
			services.ConfigureJsonSerializationOptions(options => options.SerializerOptions.ReadCommentHandling = JsonCommentHandling.Skip);
		})
	);

then Person.json can be parsed, comments and all.

Tested with Uno.Sdk 6.6.0-dev.232.

This diff against the repro should work:

diff --git a/global.json b/global.json
index 378615b..a4721cd 100644
--- a/global.json
+++ b/global.json
@@ -1,7 +1,7 @@
 {
   // To update the version of Uno please update the version of the Uno.Sdk here. See https://aka.platform.uno/upgrade-uno-packages for more information.
   "msbuild-sdks": {
-    "Uno.Sdk": "6.0.146"
+    "Uno.Sdk": "6.6.0-dev.232"
   },
   "sdk":{
     "allowPrerelease": false
diff --git a/serialization/App.xaml.cs b/serialization/App.xaml.cs
index adfe703..6e18e35 100644
--- a/serialization/App.xaml.cs
+++ b/serialization/App.xaml.cs
@@ -27,10 +27,9 @@ public partial class App : Application
 				{
 					services.AddSingleton<MainViewModel>();
 				})
-				.UseSerialization(services =>
+				.UseSerialization([PersonContext.Default], services =>
 				{
-					services.AddJsonTypeInfo(PersonContext.Default.Person);
-					services.AddSingleton(new JsonSerializerOptions { ReadCommentHandling = JsonCommentHandling.Skip });
+					services.ConfigureJsonSerializationOptions(options => options.SerializerOptions.ReadCommentHandling = JsonCommentHandling.Skip);
 				})
 			);
 		MainWindow = builder.Window;