Overview

System.Text.Json creates a JSON contract for each .NET type that determines how it is serialized and deserialized.
The contract is created based on the shape of the type — its properties, fields and interfaces it implements.
Types are mapped to contracts either at run time (via reflection) or compile time (via source generation).
Documentation: https://learn.microsoft.com/en-us/dotnet/standard/serialization/system-text-json/custom-contracts

Customizing JSON Contracts

Availability: .NET 7

Modifiers

A modifier is an Action<JsonTypeInfo> or a static void method with a JsonTypeInfo parameter that gets the current state of the contract as an argument and makes modifications to it.

Modify the JsonTypeInfo.Get property to change serialization behavior
Modify the JsonTypeInfo.Set property to change deserialization behavior
Create a new property using JsonTypeInfo.CreateJsonPropertyInfo(Type, String) and add it to the JsonTypeInfo.Properties collection

Modifications

The following modifications can be made:

Modification	Applicable to this JsonTypeInfo.Kind	How
Customize a property’s value	`JsonTypeInfoKind.Object`	Modify the `Get` or `Set` delegate of the `JsonPropertyInfo` object
Add or remove properties	`JsonTypeInfoKind.Object`	Add or remove items from `JsonTypeInfo.Properties` list
Conditionally serialize a property	`JsonTypeInfoKind.Object`	Modify the `JsonPropertyInfo.ShouldSerialize` predicate
Customize number handling for a specific type	`JsonTypeInfoKind.None`	Modify the `JsonTypeInfo.NumberHandling` value

Process

To customize JSON contracts:

Create modifiers
Create a JsonSerializerOptions instance
In JsonSerializerOptions’s TypeInfoResolver property, set a new DefaultJsonTypeInfoResolver instance
In TypeInfoResolver’s Modifiers property, assign custom actions

Example:

JsonSerializerOptions options = new()
{
    TypeInfoResolver = new DefaultJsonTypeInfoResolver
    {
        Modifiers =
        {
            MyCustomModifier1,
            MyCustomModifier2
        }
    }
};