TIdIOHandler - IOHandler created with a registered class type.

MakeIOHandler is a class function used to create a new instance of an IOHandler using the class reference specified in ABaseType. 

MakeIOHandler checks for existing registered IOHandlers in GIOHandlerClassList. When an IOHandler class reference is located that descends from ABaseType, it is used to create the new object instance used as the return value for the function. If a class reference cannot be located that descends from the specified type, an exception will be raised. 

MakeIOHandler is an extensibility mechanism that allows TIdIOHandler to create instances of a more specialized descendant class. 

Use SetDefaultClass to update GIOHandlerClassDefault with class reference for the IOHandler class. 

Use RegisterIOHandler to ensure that the class type is included in the list of registered IOHandlers.

GIOHandlerClassList 

GIOHandlerClassDefault 

TIdIOHandler.SetDefaultClass 

TIdIOHandler.RegisterIOHandler 

RSIOHandlerTypeNotInstalled