Invoking a Synchronous Query

A synchronous query is a query that maintains control over the process of your application for the duration of the query. A synchronous query requires a single interface call, and is therefore more simple than an asynchronous call. However, a synchronous query has the potential of locking up your application for large queries or queries over a network.

The following procedure describes how to issue a synchronous data query using PowerShell.

To issue a synchronous data query in PowerShell

• Describe your query to WMI using the WMI Get-WmiObject cmdlet and the -query parameter. The cmdlet returns either a single object, or a collection of objects, depending on how many objects fit the query.

  Get-WmiObject -query "SELECT * FROM Win32_logicalDisk WHERE DeviceID = 'C:'"
  

The following procedure describes how to issue a synchronous data query using C#.

To issue a synchronous data query in C# (Microsoft.Management.Infrastructure)

1. Describe your query to WMI using CimSession.QueryInstances. This method returns a collection of CimInstance objects.

   using Microsoft.Management.Infrastructure;
   ...
   string Namespace = @"root\cimv2";
   string diskDriveQuery = "SELECT * FROM Win32_LogicalDisk";
   CimSession mySession = CimSession.Create("localhost");
   IEnumerable<CimInstance> queryInstances = mySession.QueryInstances(Namespace, "WQL", diskDriveQuery);
   

2. Use standard C# language collection techniques to access each returned object.

   foreach (CimInstance drive in queryInstances)
   {
      Console.WriteLine(drive.CimInstanceProperties["DeviceID"]);
   }
   

The following procedure describes how to issue a synchronous data query using C#.

To issue a synchronous data query in C# (System.Management)

1. Create the query with a ManagementObjectSearcher object, and retrieve the information with a call to ManagementObjectSearcher.Get.

   This method returns a ManagementObjectCollection object.

   using System.Management;
   ...
   ManagementObjectSearcher mgmtObjSearcher = new ManagementObjectSearcher("SELECT * FROM Win32_LogicalDisk");
   ManagementObjectCollection objCol = mgmtObjSearcher.Get();
   

2. Use standard C# language collection techniques to access each returned object.

   foreach (ManagementObject drive in objCol)
   {
      Console.WriteLine(drive["DeviceID"]);
   }
   

The following procedure describes how to issue a synchronous data query using VBScript.

To issue a synchronous data query in VBScript

1. Describe your query to WMI using SWbemServices.ExecQuery. This method returns an SWbemObjectSet.

   GetObject("winmgmts:").ExecQuery _
           ("Select * from Win32_Service where State='Stopped'")
   

2. Use standard scripting language collection techniques to access each returned object.

   for each Service in _ 
       GetObject("winmgmts:").ExecQuery _
           ("Select * from Win32_Service where State='Stopped'")
       WScript.Echo "  "& Service.DisplayName & " [", Service.Name, "]"
   next
   

The following procedure describes how to issue a synchronous data query using C++.

To issue a synchronous query in C++

1. Describe your query to WMI through a call to IWbemServices::ExecQuery.

   The ExecQuery method takes a WQL search string as a parameter that describes your query. WMI performs the query and returns an IEnumWbemClassObject interface pointer. Through the IEnumWbemClassObject interface, you can access the classes or instances that make up the result set.

2. After you receive your query, you can enumerate your query with a call to IEnumWbemClassObject::Next. For more information, see Enumerating WMI.

   The following code example requires the following references and #include statements to compile correctly.

   #include <wbemidl.h>
   #include <iostream>
   using namespace std;
   

   The following code example describes how to query for the objects that represent the users and groups in WMI.

   void ExecQuerySync(IWbemServices *pSvc)
   {
       // Query for all users and groups.
   
       BSTR Language = SysAllocString(L"WQL");
       BSTR Query = SysAllocString(L"SELECT * FROM __Namespace");
   
       // Initialize the IEnumWbemClassObject pointer.
       IEnumWbemClassObject *pEnum = 0;
   
       // Issue the query.
       HRESULT hRes = pSvc->ExecQuery(
           Language,
           Query,
           WBEM_FLAG_FORWARD_ONLY,         // Flags
           0,                              // Context
           &pEnum
           );
   
       SysFreeString(Query);
       SysFreeString(Language);
   
       if (hRes != 0)
       {
           printf("Error\n");
           return;
       }
   
       ULONG uTotal = 0;
   
       // Retrieve the objects in the result set.
       for (;;)
       {
           IWbemClassObject *pObj = 0;
           ULONG uReturned = 0;
   
           hRes = pEnum->Next(
               0,                  // Time out
               1,                  // One object
               &pObj,
               &uReturned
               );
   
           uTotal += uReturned;
   
           if (uReturned == 0)
               break;
   
           // Use the object.
   
           // ...
   
           // Release it.
           // ===========
   
           pObj->Release();    // Release objects not owned.            
       }
   
       // All done.
       pEnum->Release();
   }