System Text JSON Net11 Agent Skill

System.Text.Json — .NET 11

Three APIs were added to System.Text.Json in .NET 11. This skill tells you exactly

when to reach for each one, what to write, what not to write, and how to prove the

result runs. Do not describe these APIs to the user — apply them, then run the code and

show the output.

| API | Replaces the pre-.NET-11 workaround of... |

| --- | --- |

| JsonNamingPolicy.PascalCase (static property) | writing a custom JsonNamingPolicy subclass or hand-annotating every member with [JsonPropertyName] |

| JsonSerializerOptions.GetTypeInfo<T>() | calling non-generic GetTypeInfo(typeof(T)) and casting to JsonTypeInfo<T> |

| JsonSerializerOptions.TryGetTypeInfo<T>(out JsonTypeInfo<T>? info) | wrapping GetTypeInfo in try/catch to probe availability |

Step 0 — Confirm you can target .NET 11

These APIs only exist in the .NET 11 base class library. Before writing code:

• Run dotnet --list-sdks and confirm an SDK that can target net11.0 is present — an

11.x SDK, or any later SDK (12.x+) that has the net11.0 targeting pack installed.

• If no such SDK is available, stop: tell the user these APIs require targeting

net11.0 (on the .NET 11 SDK or later) and cannot compile on net10.0 or earlier. Do

not fall back to a custom implementation and pretend it is the new API.

Decision table — symptom → do this → never do this

Match the user's request to a row, apply the Do this cell verbatim, and confirm the

Verify column before you are done.

| User asks for… | Do this (on net11.0) | Never do this | Verify |

| --- | --- | --- | --- |

| PascalCase JSON property names | options.PropertyNamingPolicy = JsonNamingPolicy.PascalCase; | define class …: JsonNamingPolicy; add per-member [JsonPropertyName]; string-case the names yourself | output JSON keys are PascalCase — e.g. "Name", "Age" |

| Strongly-typed metadata JsonTypeInfo<T> | set TypeInfoResolver = new DefaultJsonTypeInfoResolver(), then JsonTypeInfo<T> ti = options.GetTypeInfo<T>(); | (JsonTypeInfo<T>)options.GetTypeInfo(typeof(T)) | variable is typed JsonTypeInfo<T>, no cast |

| Probe whether metadata is resolved | if (options.TryGetTypeInfo<T>(out var ti)) { … } else { … } | try { options.GetTypeInfo<T>(); } catch (…) { … } | no try/catch; both branches handled |

Rule 1 — PascalCase property names

When the user wants JSON output whose property names are PascalCase (Name, Age)

and asks for the built-in / framework-provided way:

• Create or reuse a JsonSerializerOptions and set

PropertyNamingPolicy = JsonNamingPolicy.PascalCase.

• Serialize with those options.

Do not write a JsonNamingPolicy subclass, do not add [JsonPropertyName("…")]

attributes to force casing, and do not upper-case the first letter of each name by

hand. JsonNamingPolicy.PascalCase is the single correct answer on .NET 11.

// Console project (reflection enabled by default). To run this as a file-based app
// (dotnet run app.cs), also set TypeInfoResolver = new DefaultJsonTypeInfoResolver()
// — see "Producing runnable output" below.
using System.Text.Json;

var options = new JsonSerializerOptions
{
    PropertyNamingPolicy = JsonNamingPolicy.PascalCase
};
string json = JsonSerializer.Serialize(new { name = "Jane", age = 30 }, options);
Console.WriteLine(json);
// {"Name":"Jane","Age":30}

Rule 2 — Strongly-typed JsonTypeInfo<T>

When the user wants type metadata back as JsonTypeInfo<T> (not the non-generic

JsonTypeInfo that needs a cast):

• Call options.GetTypeInfo<T>() — it returns JsonTypeInfo<T> directly.
• Assign it to a JsonTypeInfo<T> variable and use it (e.g. pass it to

JsonSerializer.Serialize/Deserialize).

Do not call the non-generic GetTypeInfo(Type) overload and cast the result.

> Requires a resolver. GetTypeInfo<T>() throws NotSupportedException

> (NoMetadataForType) unless the options have a TypeInfoResolver — set

> TypeInfoResolver = new DefaultJsonTypeInfoResolver() for reflection-based apps, or use

> a source-generated JsonSerializerContext for trimmed/AOT apps.

// File-based app (run: dotnet run app.cs). In a .csproj project, remove this line and
// set <TargetFramework>net11.0</TargetFramework> in the project file instead.
#:property TargetFramework=net11.0

using System.Text.Json;
using System.Text.Json.Serialization.Metadata;

var options = new JsonSerializerOptions
{
    TypeInfoResolver = new DefaultJsonTypeInfoResolver()
};

JsonTypeInfo<Person> typeInfo = options.GetTypeInfo<Person>();
Console.WriteLine(typeInfo.Type.Name); // Person

record Person(string Name, int Age);

Rule 3 — Probe metadata without throwing

When the user wants to check whether metadata for T is available and branch on it —

*without* an exception being thrown when it is not:

• Call options.TryGetTypeInfo<T>(out var info).
• Handle the true branch (metadata resolved, use info) and the false branch

(not resolved) explicitly.

Do not wrap GetTypeInfo<T>() in try/catch to detect the missing case — that is

exactly the anti-pattern this API removes. TryGetTypeInfo<T> returns false (instead of

throwing) when no resolver can produce metadata for T, which is precisely the case you

want to branch on.

// File-based app (run: dotnet run app.cs). In a .csproj project, remove this line and
// set <TargetFramework>net11.0</TargetFramework> in the project file instead.
#:property TargetFramework=net11.0

using System.Text.Json;
using System.Text.Json.Serialization.Metadata;

// Configured with a resolver → metadata is available.
var configured = new JsonSerializerOptions
{
    TypeInfoResolver = new DefaultJsonTypeInfoResolver()
};
if (configured.TryGetTypeInfo<Person>(out JsonTypeInfo<Person>? info) && info is not null)
{
    Console.WriteLine($"Resolved: {info.Type.Name}"); // Resolved: Person
}
else
{
    Console.WriteLine("Type info not available");
}

// No resolver → TryGetTypeInfo returns false instead of throwing.
var empty = new JsonSerializerOptions();
Console.WriteLine(empty.TryGetTypeInfo<Person>(out _)); // False

record Person(string Name, int Age);

Producing runnable output on net11.0

The task is not done until the program runs on net11.0 and prints its JSON. Prefer a

console project — reflection-based serialization works there out of the box. A

file-based app also works but has one important caveat (below).

Option A — console project (recommended)

Create a project whose .csproj contains <TargetFramework>net11.0</TargetFramework>,

put the code in Program.cs, then run dotnet run. Confirm the process exits with code 0

and prints the expected JSON.

using System.Text.Json;

var options = new JsonSerializerOptions { PropertyNamingPolicy = JsonNamingPolicy.PascalCase };
Console.WriteLine(JsonSerializer.Serialize(new { name = "Jane", age = 30 }, options));
// {"Name":"Jane","Age":30}

Option B — file-based app (quickest, one caveat)

Save as app.cs, then run dotnet run app.cs; the first directive pins the framework.

> Caveat — file-based apps disable System.Text.Json reflection. In a dotnet run app.cs

> file-based app, JsonSerializer.IsReflectionEnabledByDefault is false, so plain

> reflection serialization throws NotSupportedException (NoMetadataForType). Set an

> explicit TypeInfoResolver = new DefaultJsonTypeInfoResolver() on the options (as below),

> or use a source-generated JsonSerializerContext. A regular project does not need this.

// File-based app (run: dotnet run app.cs). In a .csproj project, remove this line and
// set <TargetFramework>net11.0</TargetFramework> in the project file instead.
#:property TargetFramework=net11.0

using System.Text.Json;
using System.Text.Json.Serialization.Metadata;

var options = new JsonSerializerOptions
{
    PropertyNamingPolicy = JsonNamingPolicy.PascalCase,
    TypeInfoResolver = new DefaultJsonTypeInfoResolver()
};
Console.WriteLine(JsonSerializer.Serialize(new { name = "Jane", age = 30 }, options));
// {"Name":"Jane","Age":30}

Worked example — serialize with typed metadata + PascalCase

The record below uses lowercase member names on purpose, so the PascalCase policy

visibly rewrites them in the output:

// File-based app (run: dotnet run app.cs). In a .csproj project, remove this line and
// set <TargetFramework>net11.0</TargetFramework> in the project file instead.
#:property TargetFramework=net11.0

using System.Text.Json;
using System.Text.Json.Serialization.Metadata;

var options = new JsonSerializerOptions
{
    PropertyNamingPolicy = JsonNamingPolicy.PascalCase,
    TypeInfoResolver = new DefaultJsonTypeInfoResolver()
};

JsonTypeInfo<Person> typeInfo = options.GetTypeInfo<Person>();
string json = JsonSerializer.Serialize(new Person("Jane", 30), typeInfo);
Console.WriteLine(json);
// {"Name":"Jane","Age":30}

record Person(string name, int age);

Validation checklist

Before reporting success, confirm every applicable box:

• [ ] The project or file-based app targets net11.0 (visible in the .csproj or the

#:property TargetFramework=net11.0 directive).

• [ ] PascalCase requests use JsonNamingPolicy.PascalCase — no custom JsonNamingPolicy

subclass and no per-member [JsonPropertyName] attributes just to change casing.

• [ ] Typed-metadata requests use the generic GetTypeInfo<T>() — no cast of a

non-generic JsonTypeInfo — and the options set a TypeInfoResolver (e.g.

DefaultJsonTypeInfoResolver) so the call doesn't throw NoMetadataForType.

• [ ] Probing requests use TryGetTypeInfo<T>(out …) — no try/catch around

GetTypeInfo.

• [ ] The program was actually run (dotnet run …), exited 0, and its printed JSON shows

the expected property names (e.g. "Name", "Age").

• [ ] If a file-based app (dotnet run app.cs) is used, every JsonSerializerOptions

sets a TypeInfoResolver — file-based apps disable reflection so plain serialization

throws NoMetadataForType without one.

Common pitfalls

| Pitfall | Fix |

| --- | --- |

| Hand-rolling a class … : JsonNamingPolicy for PascalCase | Delete it; set PropertyNamingPolicy = JsonNamingPolicy.PascalCase. |

| Adding [JsonPropertyName("Name")] to every member to force casing | Remove the attributes; the naming policy handles all members at once. |

| Casting (JsonTypeInfo<T>)options.GetTypeInfo(typeof(T)) | Call the generic options.GetTypeInfo<T>(); no cast needed. |

| try { options.GetTypeInfo<T>(); } catch (…) { … } to test availability | Replace with if (options.TryGetTypeInfo<T>(out var info)) { … }. |

| NotSupportedException / NoMetadataForType from GetTypeInfo<T>() | The options have no resolver. Set TypeInfoResolver = new DefaultJsonTypeInfoResolver() (reflection) or a source-generated JsonSerializerContext (trim/AOT). |

| NoMetadataForType even for a plain Serialize in a dotnet run app.cs file-based app | File-based apps disable STJ reflection. Add TypeInfoResolver = new DefaultJsonTypeInfoResolver(), or run it as a normal project instead. |

| Leaving the app on the SDK's default TFM | Pin net11.0 explicitly so the .NET 11 APIs resolve and the output shows the target. |

| Claiming success without running | Run dotnet run and paste the actual JSON output; the target is a working, executed program. |

More info

• JsonNamingPolicy class — built-in naming policies including PascalCase
• JsonSerializerOptions.GetTypeInfo — typed and non-typed metadata access
• JsonTypeInfo\<T\> — strongly-typed serialization metadata
• DefaultJsonTypeInfoResolver — reflection-based resolver required by GetTypeInfo/TryGetTypeInfo
• File-based apps — dotnet run app.cs and the #:property directive