从OleVariant获取IStream [英] Getting an IStream from an OleVariant

问题描述

我正在使用Delphi与WinHTTP一起执行一个HTTP请求，从互联网上下载一些文件，我可以做这个请求，但是我不知道如何从OleVariant获取从 ResponseStream 。我花了很多时间去搜索，但是我不知道该怎么做。这是我试过的：

  var 
 req：IWinHTTPRequest; 
 instream：IStream; 
 begin 
 req：= CoWinHTTPRequest.Create; 
 
 req.Open（'GET'，'http://google.com'，false）; 
 req.Send（''）; 
 
如果req.Status<> 200然后
 begin 
 ShowMessage（'failure'＃10 + req.StatusText）; 
 
 FreeAndNil（req）; 
 
 Application.Terminate; 
结束
 
 instream：= req.ResponseStream as IStream; 
 
 ShowMessage（'success'）; 
 
 FreeAndNil（instream）; 
 FreeAndNil（req）; 
 
 end; 
  

但是我收到错误 [DCC Error] main.pas（45） ：E2015操作符不适用于此操作数类型（第45行是 instream：= req.ResponseStream as IStream; ）。

如何从OleVariant中剔除IStream？

解决方案

尝试这个

  instream：= IUnknown（req.ResponseStream）as IStream; 
  

编辑1
您不得在界面上调用FreeAndNil 。 FreeAndNil只能传递一个对象实例。否则会导致异常。由于界面是引用计数，所以你可以简单地让它们超出范围，它们将被清理。所以，你需要删除：

  FreeAndNil（intream）; 
 FreeAndNil（req）; 
  

编辑2：尝试解释发生了什么

如果您认为这不准确或可以更好地解释，请随时进行修改或补充。

req.ResponseStream 是一个 OleVariant 。 as 关键字正在调用 QueryInterface ，而不是由 OleVariant 。

OleVariant 内置类型转换从 OleVariant 到 IUnknown ，所以你需要先将 OleVariant 转换为 IUnknown ，然后使用作为运算符来执行 QueryInterface ，以获得 IStream 界面。

您不能直接将 OleVariant 一个 IStream ，因为没有从 OleVariant 到 IStream 。

I am using Delphi along with WinHTTP to do an HTTP request to download some files from the internet, and I can do the request but I don't know how to get the IStream from the OleVariant that is returned from ResponseStream. I have spent a lot of time googling but I can't figure out how to do it. Here is what I have tried:

var
  req: IWinHTTPRequest;
  instream: IStream;
begin
  req := CoWinHTTPRequest.Create;

  req.Open('GET', 'http://google.com', false);
  req.Send('');

  if req.Status <> 200 then
  begin
    ShowMessage('failure'#10 + req.StatusText);

    FreeAndNil(req);

    Application.Terminate;
  end;

  instream := req.ResponseStream as IStream;

  ShowMessage('success');

  FreeAndNil(instream);
  FreeAndNil(req);

end;

But I get the error [DCC Error] main.pas(45): E2015 Operator not applicable to this operand type (line 45 is instream := req.ResponseStream as IStream;).

How do I scare the IStream out of an OleVariant?

解决方案

Try this

instream := IUnknown(req.ResponseStream) as IStream;

Edit 1 You must not call FreeAndNil on an interface. FreeAndNil can only be passed an object instance. Failure to do so results in an exception. Since interfaces are reference counted anyway you can simply let them go out of scope and they will be cleaned up. So, you need to remove:

  FreeAndNil(instream);
  FreeAndNil(req);

Edit2: A try to explain what is going on

Please feel free to edit or complement if you think this is not accurate or if it can be explained better.

req.ResponseStream is an OleVariant. The as keyword is doing a call to QueryInterface and that is not implemented by OleVariant.

OleVariant has a built in type conversion from OleVariant to IUnknown so you need to first cast the OleVariant to IUnknown and then use the as operator to do a QueryInterface in order to get the IStream interface.

You can not cast a OleVariant directly to a IStream because there is no built in type conversion from OleVariant to IStream.

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