C-C++ Code Example: Application-Specific Filters

 

Updated: July 19, 2016

Applies To: Windows 10, Windows 7, Windows 8, Windows 8.1, Windows Server 2008, Windows Server 2008 R2, Windows Server 2012, Windows Server 2012 R2, Windows Server Technical Preview, Windows Vista

This example provides an application-defined function that peeks at the PROPID_M_APPSPECIFIC property of each message in the queue and removes all the messages that have a specific application-specific information that is provided by the caller.

System_CAPS_ICON_note.jpg Note

In this example, the cursor used to navigate the queue behaves differently if a message is, or is not, removed from the queue. If the message is removed from the queue, Message Queuing moves the cursor to the next message. If the message is not removed from the queue, the application must move the cursor.

To filter messages by application-specific information

  1. Define the maximum number of queue properties to be specified and the queue property counter.

  2. Define the MQMSGPROPS structure.

  3. Specify PROPID_M_APPSPECIFIC.

  4. Initialize the MQMSGPROPS structure.

  5. Call MQOpenQueue to open the queue with receive access. Receive access allows the application to peek at or remove the messages in the queue.

  6. Call MQCreateCursor to create the cursor for navigating the queue. The returned cursor handle is used in the calls to read messages.

  7. Call MQReceiveMessage with peek access to peek at the first message in the queue. This call also initializes the cursor so that the cursor points at the first message in the queue.

  8. In a loop structure, continue to call MQReceiveMessage to peek at and remove the messages in the queue.

System_CAPS_ICON_note.jpg Note

The receive action of each call varies depending on if the previous message was or was not removed from the queue.

If the current message has the correct application specific information, set dwRecAction to MQ_ACTION_RECEIVE, call MQReceiveMessage to remove the message, and then reset dwRecAction to MQ_ACTION_PEEK_CURRENT to process the next message. This will remove the current message and Message Queuing moves the cursor to the next message.

If the message does not have the correct application specific information, set dwRecAction to MQ_ACTION_PEEK_NEXT to move the cursor and process the next message. In this case, the application must move the cursor to the next message.

  1. Call MQCloseQueue and MQCloseCursor to free the resources.

The following code example can be run on all versions of Message Queuing.

HRESULT FilterAppSpecific(  
                          LPCWSTR wszQueueFormatName,  
                          DWORD dwAppSpecific  
                          )  
{  
  
  // Validate the input string.  
  if (wszQueueFormatName == NULL)  
  {  
    return MQ_ERROR_INVALID_PARAMETER;  
  }  
  
  // Define the required parameters for the filter.  
  const int NUMBEROFPROPERTIES = 5;          // Number of properties  
  DWORD cPropId = 0;                         // Property counter  
  
  // Define an MQMSGPROPS structure.  
  MQMSGPROPS msgprops;  
  MSGPROPID aMsgPropId[NUMBEROFPROPERTIES];  
  PROPVARIANT aMsgPropVar[NUMBEROFPROPERTIES];  
  HRESULT aMsgStatus[NUMBEROFPROPERTIES];  
  
  HANDLE hQueue = NULL;                      // Queue handle  
  HANDLE hCursor = NULL;                     // Cursor handle  
  HRESULT hr = MQ_OK;                        // Return code  
  DWORD dwRecAction;                         // Receive action mode  
  
  // Specify the message properties to be retrieved  
  aMsgPropId[cPropId] = PROPID_M_APPSPECIFIC;      // Property ID  
  aMsgPropVar[cPropId].vt = VT_UI4;                // Type indicator  
  cPropId++;  
  
  // Initialize the MQMSGPROPS structure.  
  msgprops.cProp = cPropId;                  // Number of message properties  
  msgprops.aPropID = aMsgPropId;             // IDs of message properties  
  msgprops.aPropVar = aMsgPropVar;           // Values of message properties  
  msgprops.aStatus  = aMsgStatus;            // Error reports  
  
  // Open the queue to filter messages.  
  hr = MQOpenQueue(  
                   wszQueueFormatName,       // Format name of queue  
                   MQ_RECEIVE_ACCESS,        // Access mode  
                   MQ_DENY_RECEIVE_SHARE,    // Share mode  
                   &hQueue                   // OUT: Queue handle  
                   );  
  if (FAILED(hr))  
  {  
     return hr;  
  }  
  
  // Create the cursor used to navigate through the queue.  
  hr = MQCreateCursor(  
                      hQueue,                // Queue handle  
                      &hCursor               // OUT: Cursor handle  
                      );  
  if (FAILED(hr))  
  {  
    MQCloseQueue(hQueue);  
    return hr;  
  }  
  
  // Peek at the first message in the queue.  
  hr = MQReceiveMessage(  
                        hQueue,                  // Queue handle  
                        0,                       // Max time (msec)  
                        MQ_ACTION_PEEK_CURRENT,  // Receive action  
                        &msgprops,               // Message property structure  
                        NULL,                    // No OVERLAPPED structure  
                        NULL,                    // No callback function  
                        hCursor,                 // Cursor handle  
                        MQ_NO_TRANSACTION        // Not in a transaction  
                        );  
  if (FAILED(hr))  
  {  
    MQCloseCursor(hCursor);  
    MQCloseQueue(hQueue);  
    return hr;  
  }  
  
  // Filter messages from the queue.  
  do  
  {  
    if (msgprops.aPropVar[0].ulVal == dwAppSpecific)  
    {  
      dwRecAction = MQ_ACTION_RECEIVE;         // Receive action  
      hr = MQReceiveMessage(  
                            hQueue,            // Queue handle  
                            0,                 // Max time (msec)  
                            dwRecAction,       // Receive action  
                            &msgprops,         // Message property structure  
                            NULL,              // No OVERLAPPED structure  
                            NULL,              // No callback function  
                            hCursor,           // Cursor handle  
                            MQ_NO_TRANSACTION  // Not in a transaction  
                            );  
      if (FAILED(hr))  
      {  
        break;  
      }  
  
      //  
      // Add code to process message here.  
      //  
  
      // Peek at the current message. The cursor was moved when the  
      // message with the correct correlation identifier was   
      // removed.  
      dwRecAction = MQ_ACTION_PEEK_CURRENT;      // Reset to peek at the current cursor position.  
    }  
    else  
    {  
      dwRecAction = MQ_ACTION_PEEK_NEXT;             // Reset receive action mode.  
    }  
  
    // Peek at next message  
    hr = MQReceiveMessage(  
                          hQueue,             // Queue handle  
                          0,                  // Max time (msec)  
                          dwRecAction,        // Receive action  
                          &msgprops,          // Message property structure  
                          NULL,               // No OVERLAPPED structure  
                          NULL,               // No callback function.  
                          hCursor,            // Cursor handle  
                          MQ_NO_TRANSACTION   // Not in a transaction  
                          );  
    if (FAILED(hr))  
    {  
      break;  
    }  
  } while (SUCCEEDED(hr));  
  
  // Close cursor and queue.  
  hr = MQCloseCursor(hCursor);  
  if (FAILED(hr))  
  {  
    MQCloseQueue(hQueue);  
    return hr;  
  }  
  hr = MQCloseQueue(hQueue);  
  return hr;  
}  

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