Newtonsoft.Json by James Newton-King

<PackageReference Include="Newtonsoft.Json" Version="10.0.3" />

 Required

public enum Required
Indicating whether a property is required.

The property must be defined in JSON but can be a null value.

Always = 2

The property must be defined in JSON and cannot be a null value.

Default = 0

The property is not required. The default state.

The property is not required but it cannot be a null value.