OnBeforeCommandHandler is a TIdCmdTCPServerBeforeCommandHandlerEvent property that represents the event handler signalled prior to execution of a command handler for a client connection. 

The client connection context generating the command and/or data is passed as an argument to the event handler. 

OnBeforeCommandHandler is assigned to the corresponding event handler in TIdCommandHandlers during initialization of the component. OnBeforeCommandHandler is triggered in the TIdCommandHandlers.HandleCommand method before checking for a valid command handler in the collection. 

Use OnAfterCommandHandler to access the client connection context after checking for a valid command handler in the collection.

TIdCmdTCPServerBeforeCommandHandlerEvent 

TIdCommandHandlers.OnBeforeCommandHandler 

TIdCommandHandlers.HandleCommand 

TIdCommandHandler