Example: Getting WMI Data from the Local Computer Asynchronously

You can use the procedure and code examples in this topic to create a complete WMI client application that performs COM initialization, connects to WMI on the local computer, gets data asynchronously, and then cleans up. This example gets the name of the operating system on the local computer and displays it.

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 steps 6 and seven are where the operating system name is retrieved asynchronously.

Aa390424.wedge(en-us,VS.85).gifTo get WMI data from the local computer asynchronously

  1. Initialize COM parameters with a call to CoInitializeEx.

    For 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. For more information about how 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::ExecQueryAsync method to receive the data asynchronously. Whenever you receive data asynchronously, you must provide an implementation of IWbemObjectSink. This example provides the implementation in the QuerySink class. The implementation code and header file code for this class are provided following the main example. The IWbemServices::ExecQueryAsync method calls the QuerySink::Indicate method whenever the data is received.

    For more information about how to create a WMI request, see Manipulating Class and Instance Information and Calling a Method.

  7. Wait for the data to be retrieved asynchronously. Use the IWbemServices::CancelAsyncCall method to manually stop the asynchronous call.

The following code example gets WMI data from the local computer asynchronously.


#include "querysink.h"


int main(int argc, 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 authentication
                                 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: --------------------------------------------------
    // Use the IWbemServices pointer to make requests of WMI ----

    // For example, get the name of the operating system.
    // The IWbemService::ExecQueryAsync method will call
    // the QuerySink::Indicate method when it receives a result
    // and the QuerySink::Indicate method will display the OS name
    QuerySink* pResponseSink = new QuerySink();
    pResponseSink->AddRef();
    hres = pSvc->ExecQueryAsync(bstr_t("WQL"), 
                                bstr_t("SELECT * FROM Win32_OperatingSystem"),
                                WBEM_FLAG_BIDIRECTIONAL, 
                                NULL,
                                pResponseSink);
    
    if (FAILED(hres))
    {
        cout << "Query for operating system name failed."
            << " Error code = 0x" 
            << hex << hres << endl;
        pSvc->Release();
        pLoc->Release();
        pResponseSink->Release();
        CoUninitialize();
        return 1;               // Program has failed.
    }

    // Step 7: -------------------------------------------------
    // Wait to get the data from the query in step 6 -----------
    Sleep(500);
    pSvc->CancelAsyncCall(pResponseSink);

    // Cleanup
    // ========
    pSvc->Release();
    pLoc->Release();
    CoUninitialize();

    return 0;   // Program successfully completed.
 
}

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



// QuerySink.h
#ifndef QUERYSINK_H
#define QUERYSINK_H

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

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

class QuerySink : public IWbemObjectSink
{
    LONG m_lRef;
    bool bDone;
 CRITICAL_SECTION threadLock; // for thread safety

public:
    QuerySink() { m_lRef = 0; bDone = false; 
     InitializeCriticalSection(&threadLock); }
    ~QuerySink() { bDone = true;
        DeleteCriticalSection(&threadLock); }

    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
            );

 bool IsDone();
};

#endif    // end of QuerySink.h

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


// QuerySink.cpp
#include "querysink.h"

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

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

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

    for (int i = 0; i < lObjectCount; i++)
    {
        VARIANT varName;
        hres = apObjArray[i]->Get(_bstr_t(L"Name"),
            0, &varName, 0, 0);

        if (FAILED(hres))
        {
            cout << "Failed to get the data from the query"
                << " Error code = 0x"
                << hex << hres << endl;
            return WBEM_E_FAILED;       // Program has failed.
        }

        printf("Name: %ls\n", V_BSTR(&varName));
    }

    return WBEM_S_NO_ERROR;
}

HRESULT QuerySink::SetStatus(
            /* [in] */ LONG lFlags,
            /* [in] */ HRESULT hResult,
            /* [in] */ BSTR strParam,
            /* [in] */ IWbemClassObject __RPC_FAR *pObjParam
        )
{
 if(lFlags == WBEM_STATUS_COMPLETE)
 {
  printf("Call complete.\n");

  EnterCriticalSection(&threadLock);
  bDone = true;
  LeaveCriticalSection(&threadLock);
 }
 else if(lFlags == WBEM_STATUS_PROGRESS)
 {
  printf("Call in progress.\n");
 }
    
    return WBEM_S_NO_ERROR;
}


bool QuerySink::IsDone()
{
    bool done = true;

 EnterCriticalSection(&threadLock);
 done = bDone;
 LeaveCriticalSection(&threadLock);

    return done;
}    // end of QuerySink.cpp

 

 

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