system-text-json-net11

# 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 — Make the requested `net11.0` validation possible These APIs only exist in the .NET 11 base class library. Before writing code: 1. Run `dotnet --list-sdks` and confirm an SDK that can target `net11.0` is present — an `11.x` SDK, or a later SDK with the `net11.0` targeting pack. 2. If the user explicitly asks you to run the sample and no suitable SDK is installed, use the official `dotnet-install` script to install the current .NET 11 SDK into a temporary or project-local directory. Prefer the GA channel build. Use a preview only when GA is not yet available or the user explicitly requested a preview. Do not require administrator access, change the machine-wide `PATH`, or replace an installed SDK. 3. Run the sample with that local `dotnet` executable. If download or execution is blocked, still provide the complete `net11.0` program and report that it was **not run**. Never substitute `net10.0`, a custom naming policy, or a different API and present that as validation of the .NET 11 feature. Use the channel, not a guessed version. Try the GA channel first: ```powershell $installScript = Join-Path $env:TEMP "dotnet-install-$([guid]::NewGuid()).ps1" try { Invoke-WebRequest -Uri 'https://dot.net/v1/dotnet-install.ps1' -OutFile $installScript & $installScript -Channel 11.0 -InstallDir .\.dotnet & .\.dotnet\dotnet.exe run --project <PATH_TO_NET11_PROJECT> } finally { Remove-Item -LiteralPath $installScript -Force -ErrorAction SilentlyContinue } ``` ```bash install_script="$(mktemp "${TMPDIR:-/tmp}/dotnet-install.XXXXXX")" trap 'rm -f "$install_script"' EXIT curl -fsSL https://dot.net/v1/dotnet-install.sh -o "$install_script" bash "$install_script" --channel 11.0 --install-dir ./.dotnet ./.dotnet/dotnet run --project <PATH_TO_NET11_PROJECT> ``` Before .NET 11 GA, retry the install with `-Quality preview` (PowerShell) or `--quality preview` (shell). ## 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"` | | PascalCase dictionary keys | `options.DictionaryKeyPolicy = JsonNamingPolicy.PascalCase;` | set only `PropertyNamingPolicy`; pre-transform the dictionary; define a custom policy | dictionary keys such as `pendingOrders` become `"PendingOrders"` | | 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: 1. Create or reuse a `JsonSerializerOptions` and set `PropertyNamingPolicy = JsonNamingPolicy.PascalCase`. 2. 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. ```csharp // 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} ``` ### Dictionary keys are a separate setting `PropertyNamingPolicy` does not transform `Dictionary<string, TValue>` keys. For that request, set `DictionaryKeyPolicy`: ```csharp var options = new JsonSerializerOptions { DictionaryKeyPolicy = JsonNamingPolicy.PascalCase }; var values = new Dictionary<string, int> { ["pendingOrders"] = 2, ["activeUsers"] = 5 }; Console.WriteLine(JsonSerializer.Serialize(values, options)); // {"PendingOrders":2,"ActiveUsers":5} ``` ## 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): 1. Call `options.GetTypeInfo<T>()` — it returns `JsonTypeInfo<T>` directly. 2. 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. ```csharp // 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: 1. Call `options.TryGetTypeInfo<T>(out var info)`. 2. 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. ```csharp // 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). When the installed SDK cannot target `net11.0` and execution was requested, install an SDK locally with the official script and invoke it by full path. The install script is non-administrative and does not persistently alter `PATH`. ### 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. ```csharp 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. ```csharp // 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: ```csharp // 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. - [ ] Dictionary-key requests set `DictionaryKeyPolicy`, not only `PropertyNamingPolicy`. - [ ] 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](https://learn.microsoft.com/dotnet/api/system.text.json.jsonnamingpolicy) — built-in naming policies including `PascalCase` - [JsonSerializerOptions.GetTypeInfo](https://learn.microsoft.com/dotnet/api/system.text.json.jsonserializeroptions.gettypeinfo) — typed and non-typed metadata access - [JsonTypeInfo\<T\>](https://learn.microsoft.com/dotnet/api/system.text.json.serialization.metadata.jsontypeinfo-1) — strongly-typed serialization metadata