IOHandler is a TIdIOHandler property that represents the input / output mechanism used for connecting, disconnecting, sending, and receiving on the connection. 

TIdIOHandler, and descendant classes, provide an abstraction that allows use of Socket connections, streams, or any input / output data source where an IOHandler has been written to the interface for the framework. 

IOHandler is used in methods that perform low-level operations for the connection such as connecting, disconnecting, sending, and receiving. The properties, methods, and events in the IOHandler generally belong to the following usage categories: 

When accessing IOHandler in application code, always cast the IOHandler to the correct descendant class. This allows access to any methods or properties in the descendant that cannot be represented in the ancestor class at runtime. 

Use Socket to access an IOHandler known to descend from the TIdIOHandlerSocket class and allow access to the socket handle and socket options for the binding. 

IOHandler can be assigned using CreateIOHandler and the class reference needed for creating the new IOHandler instance. Use GIOHandlerClassDefault to access a class reference for creating the default IOHandler in Indy. 

An existing TIdIOHandler instance may also be assigned directly to the IOHandler property. When an object instance is assigned to the IOHandler property, the event handler properties in OnWorkBegin, OnWork, and OnWorkEnd are set to the internal event handler methods for the connection object. These event handlers may be re-assigned later in the users application. 

If the new IOHandler is derived from TIdIOHandlerSocket, the value in Socket is updated to reflect the IOHandler instance. In other words, both IOHandler and Socket may contain references to the new IOHandler. 

IOHandler is freed in Destroy when the ManagedIOHandler property contains True, or during the opRemove notification message for the component.

TIdIOHandler 

TIdIOHandlerSocket 

TIdIOHandlerStream 

CreateIOHandler 

ManagedIOHandler 

Destroy 

Connected 

Disconnect 

SendCmd 

WriteHeader 

WriteRFCStrings