<PackageReference Include="System.Reactive" Version="6.0.0-preview.16" />

System.Reactive.IEventPatternSource<TSender, TEventArgs>

public interface IEventPatternSource<TSender, TEventArgs>
Represents a data stream signaling its elements by means of an event.
event TypedEventHandler<TSender, TEventArgs> OnNext

Event signaling the next element in the data stream.