为什么QueryInterface（）会失败，当接口确实实现并在Windows中有内置编组器？ [英] Why would QueryInterface() fail when the interface is surely implemented and has built-in marshaller in Windows?

问题描述

我有以下设置。有一个COM服务器安装到COM +（在单独的进程中运行），并具有此接口定义：

  uuid（InterfaceIdHere），nonextensible，oleautomation，hidden] 
 interface IMyInterface：IUnknown {
 HRESULT MyMethod（[in] IUnknown * param）; 
}; 
  

调用者像这样调用：

  HRESULT callComObject（IStream * stream）
 {
 return comObject-> MyMethod（stream）; 
} 
  

注意这里 IStream * 隐式地转换为 IUnknown * 。这是因为在IDL中声明一个类型 IStream * 的参数引起了一些我现在不记得的问题。无论如何，总是有效的 IStream * 代替 IUnknown * 。

COM服务器端有 MyMethod（）的实现：

  STDMETHODIMP CServer :: MyMethod（IUnknown * param）
 {
 if（param == 0）{
 return E_INVALIDARG; 
} 
 ATL :: CComQIPtr< IStream> stream（param）; 
 if（stream == 0）{
 return E_INVALIDARG; //控制权通过此处
} 
 //无论
} 
  IStream * 传入 callComObject（）  / code>在客户端被隐式转换为 IUnknown * ，后者被传递给COM编组器。编组 IUnknown * 在另一个进程中到达服务器，并且获得 IUnknown * ，然后有 QueryInterface（）从同一对象调用marshall  IStream * ，并且 QueryInterface（）失败。
 
 
 
这看起来很疯狂，因为编组 IStream * 应该在任何时候都工作 - 有一个marshaller 
 
 
 
为什么可能无法正常工作，如何找到原因？
解决方案
符合行为的一种可能情况如下：
 
 
 
 
 
在调用者和被调用者之间没有任何区别
 
 
接口指针有效
 
 
但是实现 IStream ，没有相应的 COM_INTERFACE_ENTRY 映射条目，并且不使接口可用，调用者可能已通过非COM方式获取指针，例如直接C ++转换
 
 
 
 
 
这很容易通过 QueryInterface 在呼叫前的呼叫方。 
 
 
 
此方案中的被调用者只需 reinterpret_cast 到 IStream 并且它很好地工作。
 
I have the following setup. There's a COM server that is installed into COM+ (to run in a separate process) and has this interface definition:
[object, uuid("InterfaceIdHere"), nonextensible, oleautomation, hidden]
interface IMyInterface : IUnknown {
   HRESULT MyMethod( [in] IUnknown* param );
};
The caller calls it like this:
HRESULT callComObject(IStream* stream)
{
    return comObject->MyMethod(stream);
}
Note that here IStream* is implicitly upcasted to IUnknown*. This is done because declaring a parameter of type IStream* in IDL caused some problems that I can't recall right now. Anyway it's always a valid IStream* that is passed in place of IUnknown*.

The COM server side has this implementation of MyMethod():
STDMETHODIMP CServer::MyMethod(IUnknown* param)
{
    if(param == 0) {
       return E_INVALIDARG;
    }   
    ATL::CComQIPtr<IStream> stream(param);
    if(stream == 0) {
       return E_INVALIDARG;// control passes HERE
    }
    // whatever
}
So I have IStream* passed into callComObject() on the client side which is implicitly upcasted to IUnknown* and the latter is passed to the COM marshaller. The marshalled IUnknown* reaches the server in another process and there IUnknown* is obtained and then there's a QueryInterface() call to marshall IStream* from the same object and that QueryInterface() fails.

This looks insane, because marshalling IStream* should just work at all times - there's a marshaller for this interface pre-installed in Windows.

Why could it possible not work and how do I find the reason?
解决方案
One of the possible scenarios that matches the behavior is the following: 


you don't have any marhshaling between caller and callee
the interface pointer is valid
however the object which implements IStream, does not have a corresponding COM_INTERFACE_ENTRY map entry and does not make the interface available, the caller might have obtained the pointer via non-COM way, e.g. direct C++ cast


This is easy to check by QueryInterface'ing the stream on the caller side before the call. 

The callee in this scenario can just reinterpret_cast to IStream and have it nicely working.

                        
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