TypeCasting：下面两行代码之间有什么区别？ [英] TypeCasting : what is difference between below 2 lines of code?

问题描述

下面两行代码之间的区别是什么。两者都试图获取路径，一个正在工作，另一个正在抛出错误。我正在使用Delphi-7

what is the difference between below 2 lines of code. Both are trying to get the path and one is working and other is throwing error. i am working on Delphi-7

Path:= (((FFormOwner as TForm).Designer) as IDesigner).GetPrivateDirectory; --Working
Path:= IDesigner(TForm(FFormOwner).Designer).GetPrivateDirectory ;  --Error

下面是使用代码行获取路径的代码。

Below is the code which is using line of code to get the path.

constructor TsampleComponent.Create(AOwner: TComponent);
begin
  inherited Create(AOwner);
  FFormOwner:=TForm(Owner);
  if not (Owner is TForm) then
    repeat
      FFormOwner:=TForm(FFormOwner.Owner);
    until (FFormOwner is TForm) or (FFormOwner.Owner=nil);

  if (csDesigning in ComponentState) then
    Path:= (((FFormOwner as TForm).Designer) as IDesigner).GetPrivateDirectory
  else
    Path:=ExtractFilePath(Application.EXEName);
.
.

end;

推荐答案

IDesigner(TForm(FFormOwner).Designer)

此操作对 Designer执行简单的重新解释。由于 Designer 的类型为 IDesignerHook ，它与 IDesigner不同，因此失败。 code>。

This performs a simple reinterpretation cast of Designer. It will fail because Designer is of type IDesignerHook which is not the same as IDesigner.

(FFormOwner as TForm).Designer) as IDesigner

这将对 IDesigner 执行运行时查询，并通过调用<$ c $来解决c> QueryInterface 。这是从现有接口获取不同接口的正确方法。

This performs a runtime query for IDesigner and is resolved with a call to QueryInterface. This is the correct way to obtain a different interface from an existing one.

这篇关于TypeCasting：下面两行代码之间有什么区别？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！