<PackageReference Include="NETStandard.Library" Version="2.0.0-preview2-25401-01" />

System.Net.WebSockets.WebSocketState

public enum WebSocketState
namespace System.Net.WebSockets { public enum WebSocketState { Aborted = 6, Closed = 5, CloseReceived = 4, CloseSent = 3, Connecting = 1, None = 0, Open = 2 } }