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type
  TMyEvent = procedure(Sender: TObject; Param: string) of object;

  TBla = class
  private
    FMyEvent: TMyEvent;
  public
    property OnMyEvent: TMyEvent read FMyEvent write FMyEvent;
  end;

  TBle = class
  private
    FBla: TBla;
    procedure BlaMyEvent(Sender: TObject; Param: string);
  end;

...

procedure TBle. ...
begin
  FBla.OnMyEvent := BlaMyEvent;
end;                    

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unit MyUnit1;

interface

type
  TMyProc = procedure(_Param1, _Param2: integer; var _Param3: string);

var
  MyProc: TMyProc = nil;

implementation

procedure doMyProc(_Param1, _Param2: integer; var _Param3: string); inline;
begin
  if not Assigned(MyProc) then
    raise exception.Create('MyProc nicht zugewiesen');
  MyProc(_Param1, _Param2, _Param3);
end;

procedure WasAuchImmer;
var
  eins, zwei: ingeger;
  drei: integer;
begin
  eins := 1;
  zwei := 2;
  drei := 'drei';
  doMyProc(eins, zwei, drei);
end;

end;