<PackageReference Include="System.Reactive" Version="6.1.0-preview.9" />

System.Reactive.IEventPatternSource<TSender, TEventArgs>

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