I am trying to create a generic list class for use with tiOPF (an Object Persistence Framework for delphi @ www.tiopf.com). Specifically I am trying to take an existing generic class (TtiObjectList) and make a generic version that uses TtiObject descenants. 

I have limited scope for altering the base classes as they need to compile under D7 - D2009 and Free Pascal. I need to descend from TtiObjectList to keep the existing persistence mechanisms working.

```
// base class  
type  
  TtiObjectList = class(TtiObject)
...  
protected  
  function GetItems(i: integer): TtiObject; virtual;  
  procedure SetItems(i: integer; const AValue: TtiObject); virtual;  
...  
public  
  function Add(const AObject : TtiObject): integer; overload; virtual;  
...  
end;  

```

My class is defined as follows:

```
TtiGenericObjectList<T: TtiObject> = class(TtiObjectList)  
protected  
  function GetItems(i:integer): T; reintroduce;  
  procedure SetItems(i:integer; const Value: T); reintroduce;  
public  
  function Add(const AObject: T): integer; reintroduce;  
  property Items[i:integer]: T read GetItems write SetItems; default;  
end;

implementation

{ TtiGenericObjectList<T> }

function TtiGenericObjectList<T>.Add(const AObject: T): integer;  
var obj: TtiObject;  
begin  
  obj:= TtiObject(AObject); /// Invalid typecast
  result:= inherited Add(obj);  
end;  

// alternate add, also fails  
function TtiGenericObjectList<T>.Add(const AObject: T): integer;  
begin  
  result:= inherited Add(AObject); /// **There is no overloaded version**
 /// **of 'Add' that can be called with these arguments**  
end;  

function TtiGenericObjectList<T>.GetItems(i: integer): T;  
begin  
  result:= T(inherited GetItems(i)); /// **Invalid typecast  **
end;  

procedure TtiGenericObjectList<T>.SetItems(i: integer; const Value: T);  
begin  
  inherited SetItems(i, Value);  
end;  

```

The problem I have is that delphi is not seeing T as a TtiObject descendant. I am getting invalid typecast errors when I do something like:

```
function TtiGenericObjectList<T>.Add(const AObject: T): integer;  
var obj: TtiObject;  
begin  
  obj:= TtiObject(AObject); /// **Invalid typecast***
  result:= inherited Add(obj);  
end;  

```

If I don't do a type cast, then I get overload errors instead as shown in the listing above.

Any ideas where I am going wrong?

Sean