It is possible to let one interface be a descendent from another interface:
IParentInterface = interface
['{0F78D56E-85A6-4024-98D7-720D7C7B9573}']
procedure Foo;
end;
IChildInterface = interface(IParentInterface)
['{1AB2EB85-6843-462E-8CE4-32ECC065011E}']
procedure Bar;
end;
IChildInterface will have two methods: foo and bar. Any class implementing this interface will therefore need to implement both interfaces:
TImplementor = class(TInterfacedObject, IChildInterface) public procedure Foo; procedure Bar; end; procedure TImplementor.Foo; begin end; procedure TImplementor.Bar; begin end;
Note that when a class declares a child interface, it can be assigned to a variable with the child interface. Given the above declarations, the following will compile:
var Child: IChildInterface; begin Child := TImplementor.Create;
But this does not imply that it automatically also is assignment compatible with a variable with the type of parent interface. The following will not compile:
var Parent: IParentInterface; begin Parent := TImplementor.Create;
To make this compile, it is necessary to declare the class as:
TImplementor = class(TInterfacedObject, IParentInterface, IChildInterface) public procedure Foo; procedure Bar; end;
The reason for this is that although the class actually implements the methods of IParentInterface, the compiler checks only actually declared interfaces when checking assignment compatibility: all declared interfaces are put in a table and only the contents of this table is checked.
The same check is performed at runtime: the compiler generates a table of all interfaces a class declares, and this table is checked at runtime. That means that although the following will compile if only IChildInterface is declared:
ParentImplementorInstance := (TImplementor.Create as IParentInterface);
it will still fail with a run-time error:
home:~> ./ti An unhandled exception occurred at $0000000000411A27: EInvalidCast: Invalid type cast $0000000000411A27