<PackageReference Include="System.Reactive" Version="6.0.2" />

IEventPatternSource<TEventArgs>

public interface IEventPatternSource<TEventArgs>
Represents a data stream signaling its elements by means of an event.
using System.Runtime.CompilerServices; namespace System.Reactive { [System.Runtime.CompilerServices.NullableContext(1)] public interface IEventPatternSource<[System.Runtime.CompilerServices.Nullable(2)] TEventArgs> { event EventHandler<TEventArgs> OnNext; } }