Question

related to my last question, I now have the following problem:

function TNodeFactory <T>.CreateNode (ID : Integer) : INodeInterface <T>;
var
  NodeClass : TClass;
begin
  NodeClass := FindRegisteredClass (ID);
  Result := NodeClass.Create;      
end;

This yields a compiler error:

E2010 Incompatible Types: 'INodeInterface<TNodeFactory<T>.T>' and 'TObject'

A cast does not work too.

What am I missing here?

EDIT: Current implementation

TNodeFactory <T> = class
private
  type
    TRegisteredNodeType = record
      ID : Integer;
      NodeClass : TClass;
    end;
private
  FNodeTypeList : TList <TRegisteredNodeType>
public
  procedure RegisterNodeType (ID : Integer; NodeClass : TClass);
  function  CreateNode (ID : Integer) : INodeInterface <T>;
end;

procedure TNodeFactory <T>.RegisterNodeType (ID : Integer; NodeClass : TClass);
var
  RegisteredType : TRegisteredNodeType;
begin
  RegisteredType.ID := ID;
  RegisteredType.NodeClass := NodeClass;
  FNodeTypeList.Add (RegisteredType);
end;

function TNodeFactory <T>.CreateNode (ID : Integer);
var
  RegisteredType : TRegisteredNodeType;
begin
  for RegisteredType in FNodeTypeList do
    if (RegisteredType.ID = ID) then
      Exit (RegisteredType.NodeClass.Create);
  raise EInvalidNodeType.Create ('No node type with ID ' + IntToStr (ID) + ' registered');
end;

(simplified and removed error checks)

Answer 1

The problem is NodeClass being a TClass. If you create NodeClass it will be a mere TObject, not a INodeInterface.

You could try

Result := TInterfacedObject(RegisteredType.NodeClass).Create as INodeInterface

or change RegisterNodeType to

type
  TInterfacedObjectClass = class of TInterfacedObject;   
...   
procedure RegisteredNodeType...(ID: Integer; NodeClass : TInterfacedObjectClass);

and return

Result := RegisteredType.NodeClass.Create as INodeInterface.