JSON to C#

The options

Classes or records

Classes get { get; set; } properties; records get { get; init; }, so values are set once, when the JSON is read.

Collections

Arrays become List<T> or T[].

Attributes

When a property's C# name differs from its JSON name (first_name becomes FirstName), it gets [JsonPropertyName("first_name")] for System.Text.Json or [JsonProperty("first_name")] for Newtonsoft.Json, so it reads and writes the original names. Choose None if you configure a naming policy instead.

Namespace

A file-scoped namespace for the types, such as MyApp.Models.

Nullable reference types

On, the file starts with #nullable enable: references that can be null are marked ?, and the others start out empty (an empty string, collection or object), so none is left null. Off, it starts with #nullable disable.

Root type name

The name of the outermost type, or of each item when the JSON is an array.

How values become types

Nulls and missing properties

A property that is null in some objects, or missing from some, becomes nullable: int?, and with nullable reference types on, string?. One that is only ever null, or whose values are of different kinds, becomes object?.

Numbers

Whole numbers become int, or long past 32 bits. Past 64 bits they become Int128 with System.Text.Json and BigInteger with Newtonsoft, the type each one reads; past 128 bits, System.Text.Json reads them only as double, which rounds them. Numbers with a fraction or an exponent become decimal, or double when a value is beyond what a decimal holds.

Dates and GUIDs

ISO 8601 date-times with an offset (2026-10-18T12:00:00Z) become DateTimeOffset, and without one DateTime; a place where only some carry an offset becomes DateTimeOffset, with a note, and dates among date-times count as having none. Dates alone (2026-10-18) become DateOnly, and GUIDs Guid. A place is only typed that way when every value in it qualifies; otherwise it stays a string.

Names

Property and type names are PascalCase and valid C#, whatever the JSON key. Objects found under the same key with the same properties, of the same types, share one type; the items of an array are named in the singular (orders gives Order). A name that would shadow a common .NET type gets a suffix (string gives StringItem).

Requirements

The code needs C# 12 and .NET 8 or later, and Newtonsoft.Json 13.0.2 or later for DateOnly. It is tested by compiling it with the .NET 10 SDK and reading the sample JSON back with both serializers.

Questions

How do I read the JSON with these classes?

With System.Text.Json, JsonSerializer.Deserialize<Root>(json); with Newtonsoft.Json, JsonConvert.DeserializeObject<Root>(json). When the JSON is an array or a plain value, a comment at the top of the code says which type to read it as, such as List<Root>.

Why is a property object??

Its values were all null, or of different kinds, so there was nothing to tell its type from; an array that was always empty gives List<object?> for the same reason. The notes under the code list each such place; a sample with a real value there gives a real type.

Is my JSON uploaded?

No. It is parsed and converted in your browser, and the page's security policy blocks every network request from its scripts.