MessageQueue::BeginPeek Method ()

.NET Framework (current version)
 

Initiates an asynchronous peek operation that has no time-out. The operation is not complete until a message becomes available in the queue.

Namespace:   System.Messaging
Assembly:  System.Messaging (in System.Messaging.dll)

C#
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F#
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public:
IAsyncResult^ BeginPeek()

Return Value

Type: System::IAsyncResult^

The IAsyncResult that identifies the posted asynchronous request.

Exception Condition
MessageQueueException

An error occurred when accessing a Message Queuing method.

.NET Framework
Available since 1.1

The method is not thread safe.

In asynchronous processing, you use BeginPeek to raise the PeekCompleted event when a message becomes available in the queue.

PeekCompleted is also raised if a message already exists in the queue.

To use BeginPeek, create an event handler that processes the results of the asynchronous operation, and associate it with your event delegate. BeginPeek initiates an asynchronous peek operation; the MessageQueue is notified, through the raising of the PeekCompleted event, when a message arrives in the queue. The MessageQueue can then access the message by calling EndPeek(IAsyncResult^) or by retrieving the result using the PeekCompletedEventArgs.

The BeginPeek method returns immediately, but the asynchronous operation is not completed until the event handler is called.

Because BeginPeek is asynchronous, you can call it to peek the queue without blocking the current thread of execution. To synchronously peek the queue, use the Peek method.

Once an asynchronous operation completes, you can call BeginPeek or BeginReceive again in the event handler to keep receiving notifications.

The IAsyncResult that BeginPeek returns identifies the asynchronous operation that the method started. You can use this IAsyncResult throughout the lifetime of the operation, although you generally do not use it until EndPeek(IAsyncResult^) is called. However, if you start several asynchronous operations, you can place their IAsyncResult values in an array and specify whether to wait for all operations or any operation to complete. In this case, you use the AsyncWaitHandle property of the IAsyncResult to identify the completed operation.

If CanRead is false, the completion event is raised, but an exception will be thrown when calling EndPeek(IAsyncResult^).

The following table shows whether this method is available in various Workgroup modes.

Workgroup mode

Available

Local computer

Yes

Local computer and direct format name

Yes

Remote computer

No

Remote computer and direct format name

Yes

The following code example creates an event handler named MyPeekCompleted, attaches it to the PeekCompleted event handler delegate, and calls BeginPeek to initiate an asynchronous peek operation on the queue that is located at the path ".\myQueue". When a PeekCompleted event is raised, the example peeks the message and writes its body to the screen. The example then calls BeginPeek again to initiate a new asynchronous peek operation.

C#
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#using <system.dll>
#using <system.messaging.dll>

using namespace System;
using namespace System::Messaging;

// This example performs asynchronous peek operation
// processing.
//*************************************************
ref class MyNewQueue
{
public:

   // Provides an event handler for the PeekCompleted
   // event.
   static void MyPeekCompleted( Object^ source, PeekCompletedEventArgs^ asyncResult )
   {
      // Connect to the queue.
      MessageQueue^ mq = dynamic_cast<MessageQueue^>(source);

      // End the asynchronous peek operation.
      Message^ m = mq->EndPeek( asyncResult->AsyncResult );

      // Display message information on the screen.
      Console::WriteLine( "Message: {0}", static_cast<String^>(m->Body) );

      // Restart the asynchronous peek operation.
      mq->BeginPeek();
      return;
   }
};

// Provides an entry point into the application.
//         
int main()
{
   // Create an instance of MessageQueue. Set its formatter.
   MessageQueue^ myQueue = gcnew MessageQueue( ".\\myQueue" );
   array<Type^>^p = gcnew array<Type^>(1);
   p[ 0 ] = String::typeid;
   myQueue->Formatter = gcnew XmlMessageFormatter( p );

   // Add an event handler for the PeekCompleted event.
   myQueue->PeekCompleted += gcnew PeekCompletedEventHandler( MyNewQueue::MyPeekCompleted );

   // Begin the asynchronous peek operation.
   myQueue->BeginPeek();

   // Do other work on the current thread.
   return 0;
}
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