if Assigned(FXMLDoc) then FreeAndNil(FXMLDoc)出错

Question

if Assigned(FXMLDoc) then FreeAndNil(FXMLDoc);
出现 Einvalidpointer with message "invalid pointeer operation" 异常
这是为什么?

Answer 1

var
  FXMLDoc: TXMLDocument;
begin
  FXMLDoc := TXMLDocument.create(application);
  FXMLDoc.Free 
end;
不会出错

Answer 2

TControlForm = class(TForm)
private
  FXMLdoc: TXMLDocument;
protected
  procedure FormInitialize; virtual;
public
  destructor Destroy; override;
end;TControlForm.FormInitialize
begin
  FXMLdoc: TXMLDocument.Create(Application);
endTControlForm.Destroy
begin
  FXMLdoc.Free;
end;TBillControlForm = class(TControlform)
end;
 
var
  billControlForm: TBillControlForm procedure Test;
begin 
  billControlForm := TBillControlForm.Create(Applcation)
  billControlForm.FormInitialize;
  billControlForm.Free;
end;
运行到   FXMLdoc.Free; 就出错
如果改成
TControlForm.Destroy
var
  aXMLdoc: TXMLDocument;
begin
  aXMLdoc := TXMLDocument.Create(Application);
  aFXMLdoc.Free; 则不出错
end;

Answer 3

if Assigned(FXMLDoc) then FreeAndNil(FXMLDoc);Einvalidpointer with message "invalid pointeer operation" 异常因为"Assigned"函数不是准确的确定,所以当FXMLDoc指向的对象已被释放,而FXMLDoc却依然存在,当用
FreeAndNil去释放时就出错了.....