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

Unit

public struct Unit : IEquatable<Unit>
Represents a type with a single value. This type is often used to denote the successful completion of a void-returning method (C#) or a Sub procedure (Visual Basic).
using System.Runtime.CompilerServices; using System.Runtime.InteropServices; namespace System.Reactive { [Serializable] [StructLayout(LayoutKind.Sequential, Size = 1)] public readonly struct Unit : IEquatable<Unit> { public static Unit Default => default(Unit); public bool Equals(Unit other) { return true; } [System.Runtime.CompilerServices.NullableContext(2)] public override bool Equals(object obj) { return obj is Unit; } public override int GetHashCode() { return 0; } [System.Runtime.CompilerServices.NullableContext(1)] public override string ToString() { return "()"; } public static bool operator ==(Unit first, Unit second) { return true; } public static bool operator !=(Unit first, Unit second) { return false; } } }