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

System.Reactive.IEventPatternSource<TEventArgs>

public interface IEventPatternSource<TEventArgs>
Represents a data stream signaling its elements by means of an event.
namespace System.Reactive { public interface IEventPatternSource<[System.Runtime.CompilerServices.Nullable(2)] TEventArgs> { event EventHandler<TEventArgs> OnNext; } }