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COM: Typecasting between TObject and Interfaces

I have a manager object which holds a list of Interfaces to objects which want notifying of a particular event.

My manager object stores these Interfaces by typecasting them as Objects and dropping them into a TList.

When it comes to retrieving the interfaces, i cant typecast it back to an ICapPlugIn interface.

So, how can I typecast it, or if I cant, how do I store a list of Interfaces ?

Thx
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Carpathia
Asked:
Carpathia
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1 Solution
 
MadshiCommented:
If you store the interfaces as TObject, the reference count is not incremented. That means, if the TList reference is the only reference to the interface left, then the interface is being deallocated and if you then later typecast ICapPlugIn(TList[0]) this instance is already released. You can do one of two things:

(1) Use a little trick to handle the reference count correctly:

Don't do this:

  YourList.Add(TObject(AnInterface));

But do this:

var obj : TObject;
begin
  ICapPlugIn(obj) := AnInterface;
  YourList.Add(obj);

Now you can use it like this:

  ICapPlugIn(YourList[index]).AMethod;

To release it do this:

var obj : TObject;
begin
  obj := YourList[index];
  YourList.Delete(index);
  ICapPlugIn(obj) := nil;

(2) Use a "real" interface list. I'm not sure, I think Delphi already has something like that. But perhaps D5 only. I'm not sure. If all else fails, you can copy the TList code from the VCL sources and change TObject into IUnknown.

Regards, Madshi.
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vinnairCommented:
After retrieving the object as TObject, you cannot directly typecast it as the interface. You will have to use the following piece of code. The function below takes the index of the element in the TList and then returns the ICapPlugIn interface corresponding to it. Suppose MyList is your TList.

function convert(index: integer): ICapPlugIn;
var
  myobj: TObject;
  myintf: ICapPlugIn;
begin
  myobj := TObject(MyList[index]);
  try
    myobj.GetInterface(ICapPlugIn,myintf);
  except
    begin
      Showmessage('Interface not supported by class');
      exit;
    end;
  end;
  Result := myintf;
end;

The Getinterface function is used to retrieve interface for an object.

This should solve your problem. If not you can get back to me.
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MadshiCommented:
Hi vinnair, if Carpathia has typecasted the original interface to TObject, we don't have a valid TObject pointer!! We simply have a typecasted ICapPlugIn pointer. That means you can't call TObject(ICapPlugInPointer).GetInterface.
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vinnairCommented:
Hi Madshi,
Since ICapPlugIn is an interface, it will be implemented by some object, which will always be a descendent of TObject. Remember all objects are derived from TObject.

If the object Implementing ICapPlugIn was not a descendent of TObject then the typecasting from an ICapPlugIn interface to a TObject would not have been allowed in the first place. No dummy typecasting is allowed. Even if a compile time error is not raised, a run time error will surely be raised.

Any comments?
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MadshiCommented:
Hi vinnair,

you are not allowed to typecast an interface into an object and then use it as an object. That simply doesn't work. You would get a runtime error. You're right, interfaces are implemented by objects, but if you have e.g. an interface ICapPlugIn, this interface can be implemented by TObject1 or by TObject2. How should Delphi know just from the interface pointer, where the implementing object is and which type it has? This logic is simply not supported in interfaces and COM objects. An interface pointer only gives you the addresses of the interface methods. The address of the implementing object is not stored in the interface.
Typecasting is not nice, of course. But it works alright, if you don't use the typecasted pointer as a TObject reference.

Look, you can do this:

var i1 : integer;
    p1 : pointer;
begin
  i1 := 777;
  p1 := pointer(i1);
  i1 := integer(p1);

No problem with this code - as long as you don't try to use "p1^". The same is true for interfaces and objects. You can typecast them, but you MUST NOT use the typecasted type as a reference.
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CarpathiaAuthor Commented:
My problems are solved. I started converting borlands TList to a TInterfaceList, only to find that one already exists in the same unit :)

I've given Madshi the points, for teaching me about the reference count trick, and suggesting modifying TList.



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CarpathiaAuthor Commented:
Problem was solved using Classes.TInterfaceList.
Thanks
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