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Example: Receiving Event Notifications Through WMI

You can use the procedure and code example in this topic to complete WMI client application that performs COM initialization, connects to WMI on the local computer, receives an event notification, and then cleans up. The example notifies the user of an event when a new process is created. The events are received asynchronously.

The following procedure is used to execute the WMI application. Steps 1 through 5 contain all of the steps required to set up and connect to WMI, and step 6 is where the event notifications are received.

Aa390425.wedge(en-us,VS.85).gifTo receive a event notification through WMI

  1. Initialize COM parameters with a call to CoInitializeEx.

    Fore more information, see Initializing COM for a WMI Application.

  2. Initialize COM process security by calling CoInitializeSecurity.

    For more information, see Setting the Default Process Security Level Using C++.

  3. Obtain the initial locator to WMI by calling CoCreateInstance.

    For more information, see Creating a Connection to a WMI Namespace.

  4. Obtain a pointer to IWbemServices for the root\cimv2 namespace on the local computer by calling IWbemLocator::ConnectServer. To connect to a remote computer, see Example: Getting WMI Data from a Remote Computer.

    For more information, see Creating a Connection to a WMI Namespace.

  5. Set IWbemServices proxy security so the WMI service can impersonate the client by calling CoSetProxyBlanket.

    For more information, see Setting the Security Levels on a WMI Connection.

  6. Use the IWbemServices pointer to make requests to WMI. This example uses the IWbemServices::ExecNotificationQueryAsync method to receive asynchronous events. When you receive asynchronous events, you must provide an implementation of IWbemObjectSink. This example provides the implementation in the EventSink class. The implementation code and header file code for this class are provided below the main example. The IWbemServices::ExecNotificationQueryAsync method calls the EventSink::Indicate method whenever an event is received. For this example the EventSink::Indicate method is called whenever a process is created. To test this example, run the code and start a process such as Notepad.exe. This triggers an event notification.

    For more information about making requests of WMI, see Manipulating Class and Instance Information and Calling a Method.

The following example code receives event notifications through WMI.



#include "eventsink.h"

int main(int iArgCnt, char ** argv)
{
    HRESULT hres;

    // Step 1: --------------------------------------------------
    // Initialize COM. ------------------------------------------

    hres =  CoInitializeEx(0, COINIT_MULTITHREADED); 
    if (FAILED(hres))
    {
        cout << "Failed to initialize COM library. Error code = 0x" 
             << hex << hres << endl;
        return 1;                  // Program has failed.
    }

    // Step 2: --------------------------------------------------
    // Set general COM security levels --------------------------

    hres =  CoInitializeSecurity(
        NULL, 
        -1,                          // COM negotiates service
        NULL,                        // Authentication services
        NULL,                        // Reserved
        RPC_C_AUTHN_LEVEL_DEFAULT,   // Default authentication 
        RPC_C_IMP_LEVEL_IMPERSONATE, // Default Impersonation  
        NULL,                        // Authentication info
        EOAC_NONE,                   // Additional capabilities 
        NULL                         // Reserved
        );

                      
    if (FAILED(hres))
    {
        cout << "Failed to initialize security. Error code = 0x" 
             << hex << hres << endl;
        CoUninitialize();
        return 1;                      // Program has failed.
    }
    
    // Step 3: ---------------------------------------------------
    // Obtain the initial locator to WMI -------------------------

    IWbemLocator *pLoc = NULL;

    hres = CoCreateInstance(
        CLSID_WbemLocator,             
        0, 
        CLSCTX_INPROC_SERVER, 
        IID_IWbemLocator, (LPVOID *) &pLoc);
 
    if (FAILED(hres))
    {
        cout << "Failed to create IWbemLocator object. "
             << "Err code = 0x"
             << hex << hres << endl;
        CoUninitialize();
        return 1;                 // Program has failed.
    }

    // Step 4: ---------------------------------------------------
    // Connect to WMI through the IWbemLocator::ConnectServer method

    IWbemServices *pSvc = NULL;
 
    // Connect to the local root\cimv2 namespace
    // and obtain pointer pSvc to make IWbemServices calls.
    hres = pLoc->ConnectServer(
        _bstr_t(L"ROOT\\CIMV2"), 
        NULL,
        NULL, 
        0, 
        NULL, 
        0, 
        0, 
        &pSvc
    );
     
    if (FAILED(hres))
    {
        cout << "Could not connect. Error code = 0x" 
             << hex << hres << endl;
        pLoc->Release();     
        CoUninitialize();
        return 1;                // Program has failed.
    }

    cout << "Connected to ROOT\\CIMV2 WMI namespace" << endl;


    // Step 5: --------------------------------------------------
    // Set security levels on the proxy -------------------------

    hres = CoSetProxyBlanket(
        pSvc,                        // Indicates the proxy to set
        RPC_C_AUTHN_WINNT,           // RPC_C_AUTHN_xxx 
        RPC_C_AUTHZ_NONE,            // RPC_C_AUTHZ_xxx 
        NULL,                        // Server principal name 
        RPC_C_AUTHN_LEVEL_CALL,      // RPC_C_AUTHN_LEVEL_xxx 
        RPC_C_IMP_LEVEL_IMPERSONATE, // RPC_C_IMP_LEVEL_xxx
        NULL,                        // client identity
        EOAC_NONE                    // proxy capabilities 
    );

    if (FAILED(hres))
    {
        cout << "Could not set proxy blanket. Error code = 0x" 
             << hex << hres << endl;
        pSvc->Release();
        pLoc->Release();     
        CoUninitialize();
        return 1;               // Program has failed.
    }

    // Step 6: -------------------------------------------------
    // Receive event notifications -----------------------------

    // Use an unsecured apartment for security
    IUnsecuredApartment* pUnsecApp = NULL;

    hres = CoCreateInstance(CLSID_UnsecuredApartment, NULL, 
        CLSCTX_LOCAL_SERVER, IID_IUnsecuredApartment, 
        (void**)&pUnsecApp);
 
    EventSink* pSink = new EventSink;
    pSink->AddRef();

    IUnknown* pStubUnk = NULL; 
    pUnsecApp->CreateObjectStub(pSink, &pStubUnk);

    IWbemObjectSink* pStubSink = NULL;
    pStubUnk->QueryInterface(IID_IWbemObjectSink,
        (void **) &pStubSink);

    // The ExecNotificationQueryAsync method will call
    // The EventQuery::Indicate method when an event occurs
    hres = pSvc->ExecNotificationQueryAsync(
        _bstr_t("WQL"), 
        _bstr_t("SELECT * " 
            "FROM __InstanceCreationEvent WITHIN 1 "
            "WHERE TargetInstance ISA 'Win32_Process'"), 
        WBEM_FLAG_SEND_STATUS, 
        NULL, 
        pStubSink);

    // Check for errors.
    if (FAILED(hres))
    {
        printf("ExecNotificationQueryAsync failed "
            "with = 0x%X\n", hres);
        pSvc->Release();
        pLoc->Release();
        pUnsecApp->Release();
        pStubUnk->Release();
        pSink->Release();
        pStubSink->Release();
        CoUninitialize();    
        return 1;
    }

    // Wait for the event
    Sleep(10000);
         
    hres = pSvc->CancelAsyncCall(pStubSink);

    // Cleanup
    // ========

    pSvc->Release();
    pLoc->Release();
    pUnsecApp->Release();
    pStubUnk->Release();
    pSink->Release();
    pStubSink->Release();
    CoUninitialize();

    return 0;   // Program successfully completed.
 
}

The following header file is used for the class EventSink. The EventSink class is used in the previous code example.


// EventSink.h
#ifndef EVENTSINK_H
#define EVENTSINK_H

#define _WIN32_DCOM
#include <iostream>
using namespace std;
#include <comdef.h>
#include <Wbemidl.h>

# pragma comment(lib, "wbemuuid.lib")

class EventSink : public IWbemObjectSink
{
    LONG m_lRef;
    bool bDone;

public:
    EventSink() { m_lRef = 0; }
   ~EventSink() { bDone = true; }

    virtual ULONG STDMETHODCALLTYPE AddRef();
    virtual ULONG STDMETHODCALLTYPE Release();        
    virtual HRESULT 
        STDMETHODCALLTYPE QueryInterface(REFIID riid, void** ppv);

    virtual HRESULT STDMETHODCALLTYPE Indicate( 
            LONG lObjectCount,
            IWbemClassObject __RPC_FAR *__RPC_FAR *apObjArray
            );
        
    virtual HRESULT STDMETHODCALLTYPE SetStatus( 
            /* [in] */ LONG lFlags,
            /* [in] */ HRESULT hResult,
            /* [in] */ BSTR strParam,
            /* [in] */ IWbemClassObject __RPC_FAR *pObjParam
            );
};

#endif    // end of EventSink.h

The following example code is an implementation of the EventSink class.


// EventSink.cpp
#include "eventsink.h"

ULONG EventSink::AddRef()
{
    return InterlockedIncrement(&m_lRef);
}

ULONG EventSink::Release()
{
    LONG lRef = InterlockedDecrement(&m_lRef);
    if(lRef == 0)
        delete this;
    return lRef;
}

HRESULT EventSink::QueryInterface(REFIID riid, void** ppv)
{
    if (riid == IID_IUnknown || riid == IID_IWbemObjectSink)
    {
        *ppv = (IWbemObjectSink *) this;
        AddRef();
        return WBEM_S_NO_ERROR;
    }
    else return E_NOINTERFACE;
}


HRESULT EventSink::Indicate(long lObjectCount,
    IWbemClassObject **apObjArray)
{
 HRESULT hres = S_OK;

    for (int i = 0; i < lObjectCount; i++)
    {
        printf("Event occurred\n");
    }

    return WBEM_S_NO_ERROR;
}

HRESULT EventSink::SetStatus(
            /* [in] */ LONG lFlags,
            /* [in] */ HRESULT hResult,
            /* [in] */ BSTR strParam,
            /* [in] */ IWbemClassObject __RPC_FAR *pObjParam
        )
{
    if(lFlags == WBEM_STATUS_COMPLETE)
    {
        printf("Call complete. hResult = 0x%X\n", hResult);
    }
    else if(lFlags == WBEM_STATUS_PROGRESS)
    {
        printf("Call in progress.\n");
    }

    return WBEM_S_NO_ERROR;
}    // end of EventSink.cpp


 

 

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