[fpc-pascal]How to make a constructor failed?

[mili]

Hello, Every one

  I intend to create my own class such as the following:
>>>>
type
  TMyClass = class(TObject)
    public
      constructor Create(fn:String);
      destructor Free;
  end;//TMyClass

implementation

constructor TMyClass.Create(fn:String);
begin
  if not FileExists(fn) then begin
    // ---> Here I want to make the new object nil
  end;
end;//TMyClass.Create

demo:
  myclass:=TMyClass.Create('aaaa.txt');
  if myclass=nil then exit;// ---> Here I can check if the object is valid
<<<<<<<<

  Can you tell me what's the constructor should be like to implement this?

  Thank you,
  mili