JsonDerivedTypeGenerator 1.3.0

JsonDerivedTypeGenerator

JsonDerivedTypeGenerator is a C# source generator that automatically adds [JsonDerivedType] attributes to base classes marked with [JsonPolymorphic], enabling correct polymorphic serialization with System.Text.Json.

The generator supports:

• classes and interfaces
• public and internal access modifier
• deep inheritance
• generic base classes and interfaces
• excluding specific derived types via [JsonDerivedTypeIgnore]

Installation

You can add the generator to your project via NuGet:

  dotnet add package JsonDerivedTypeGenerator

The generator runs at compile time — you don’t need to invoke it manually.

Usage

1. Mark your base class as [JsonPolymorphic] and partial:

using System.Text.Json.Serialization;

[JsonPolymorphic]
public abstract partial class Animal
{
    public abstract void MakeNoise();
    public abstract string Kind { get; }
}

2. Implement your base class:

public class Dog : Animal
{
    public override void MakeNoise() => {}
    public override string Kind => "Dog";
}

public class Cat : Animal
{
    public override void MakeNoise() => {}
    public override string Kind => "Cat";
}

3. When you build the project, the generator will automatically add [JsonDerivedType] attributes to the base class:

[JsonDerivedType(typeof(Dog), "Dog")]
[JsonDerivedType(typeof(Cat), "Cat")]
public abstract partial class Animal
{
    public abstract void MakeNoise();
    public abstract string Kind { get; }
}

Generic base classes

Generic classes and interfaces are fully supported as polymorphic bases:

[JsonPolymorphic]
public abstract partial class Result<T> { }

public class IntSuccess : Result<int> { }
public class StringSuccess : Result<string> { }

The generator will emit:

[JsonDerivedType(typeof(IntSuccess), nameof(IntSuccess))]
[JsonDerivedType(typeof(StringSuccess), nameof(StringSuccess))]
public abstract partial class Result<T> { }

Multiple type parameters are also supported (Either<TLeft, TRight>, etc.).

Excluding derived types

Mark a derived type with [JsonDerivedTypeIgnore] to exclude it from the generated attributes:

using System.Text.Json.Serialization;

[JsonDerivedTypeIgnore]
public class UnknownAnimal : Animal
{
    public override void MakeNoise() => {}
    public override string Kind => "Unknown";
}

The generator will skip UnknownAnimal — no [JsonDerivedType(typeof(UnknownAnimal), ...)] will be emitted. Attempting to serialize an instance of an ignored type as the base type will throw NotSupportedException at runtime.