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Making Calls to WMI

  • Artikel

Providers can call methods implemented by WMI from within their method implementations. However, there are special considerations when a provider calls the WMI implementation of an IWbemServices method from within its own implementation of the same method. These considerations are important regardless of whether the provider calls the synchronous or asynchronous version of the method.

Each IWbemServices method that a provider can implement has a pCtx parameter, a pointer to an IWbemContext interface implementation. When WMI calls the provider, WMI passes a valid pointer in this parameter. A provider must always pass this same pointer in any calls to WMI that they make while servicing requests. Neglecting to set pCtx appropriately can cause WMI to start an infinite loop.

The following code example shows the correct way to call the WMI implementation of GetObject from within an implementation of GetObjectAsync.

STDMETHODIMP CClassProv::GetObjectAsync (BSTR ObjectPath,
    long lFlags, IWbemContext *pCtx,
    IWbemObjectSink *pHandler)
{
  IWbemClassObject *pclObj = NULL;
  IWbemServices* m_pNamespace;
  HRESULT hr = m_pNamespace->GetObject(
      _bstr_t(L"AClass"), 0, pCtx, &pclObj, 
      NULL );
  pclObj->Release();
  return pHandler->SetStatus(0, hr, NULL, NULL);
}

The C++ code example in this topic requires the following references and #include statements to compile correctly.

#define _WIN32_DCOM
#include <iostream>
using namespace std;
#include <comdef.h>
#include <Wbemidl.h>
#pragma comment(lib, "wbemuuid.lib")

Instance, class, and property providers must not issue any calls to WMI requesting the modification of data while servicing a read request. The only providers that are exceptions to this rule are push providers. A push provider is a class provider that stores data in the WMI repository and relies on WMI to handle requests from clients. While servicing a read request, a push provider can update the WMI repository, but must set the lFlags parameter to WBEM_FLAG_OWNER_UPDATE in the appropriate IWbemServices call.

Event providers must not make any class changes while servicing a call. They also cannot issue any event-related calls, such as modifying an event filter.

Developing a WMI Provider

Setting Namepace Security Descriptors

Securing Your Provider