<PackageReference Include="System.Reactive" Version="6.0.0-preview.9" />

System.Reactive.Subjects.AsyncSubject<T>

public sealed class AsyncSubject<T> : SubjectBase<T>, INotifyCompletion
Represents the result of an asynchronous operation. The last value before the OnCompleted notification, or the error received through OnError, is sent to all subscribed observers.
namespace System.Reactive.Subjects { public sealed class AsyncSubject<[System.Runtime.CompilerServices.Nullable(2)] T> : SubjectBase<T>, INotifyCompletion { public bool IsCompleted { get; } public AsyncSubject(); public AsyncSubject<T> GetAwaiter(); public void OnCompleted(Action continuation); public T GetResult(); } }