<PackageReference Include="System.Reactive" Version="6.0.1-preview.1" />

System.Reactive.Subjects.Subject<T>

public sealed class Subject<T> : SubjectBase<T>
Represents an object that is both an observable sequence as well as an observer. Each notification is broadcasted to all subscribed observers.
namespace System.Reactive.Subjects { public sealed class Subject<[System.Runtime.CompilerServices.Nullable(2)] T> : SubjectBase<T> { public Subject(); } }