Casting object to interface type with no TInterfacedObject as base class

Question

I need a class implementing interface with no reference counting. I did the following: ``` IMyInterface = interface(IInterface) ['{B84904DF-9E8A-46E0-98E4-498BF03C2819}'] procedure InterfaceMethod; end; TMyClass = class(TObject, IMyInterface) protected function _AddRef: Integer;stdcall; function _Release: Integer;stdcall; function QueryInterface(const IID: TGUID; out Obj): HResult;stdcall; public procedure InterfaceMethod; end; procedure TMyClass.InterfaceMethod; begin ShowMessage('The Method'); end; function TMyClass.QueryInterface(const IID: TGUID; out Obj): HResult; begin if GetInterface(IID, Obj) then Result := 0 else Result := E_NOINTERFACE; end; function TMyClass._AddRef: Integer; begin Result := -1; end; function TMyClass._Release: Integer; begin Result := -1; end; ``` Lack of reference counting works fine. But my concern is that I cannot cast `TMyClass` to `IMyInterface` using `as` operator: ``` var MyI: IMyInterface; begin MyI := TMyClass.Create as IMyInterface; ``` I am give…

Accepted Answer

The reason you cannot use `as` in your code is that your class does not explicitly list `IInterface` in its list of supported interfaces. Even though your interface derives from `IInterface`, unless you actually list that interface, your class does not support it.

So, the trivial fix is to declare your class like this:

```
TMyClass = class(TObject, IInterface, IMyInterface)
```

The reason that your class needs to implement `IInterface` is that is what the compiler is relying on in order to implement the `as` cast.

The other point I would like to make is that you should, in general, avoid using interface inheritance. By and large it serves little purpose. One of the benefits of using interfaces is that you are free from the single inheritance constraint that comes with implementation inheritance.

But in any case, all Delphi interfaces automatically inherit from `IInterface` so in your case there's no point specifying that. I would declare your interface like this:

```
IMyInterface = interface
  ['{B84904DF-9E8A-46E0-98E4-498BF03C2819}'] 
  procedure InterfaceMethod;
end;
```

More broadly you should endeavour not to use inheritance with your interfaces. By taking that approach you will encourage less coupling and that leads to greater flexibility.

Casting object to interface type with no TInterfacedObject as base class

Solution

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