MessageQueue.Peek Method

Reference

Definition

Namespace:: System.Messaging

Assembly:: System.Messaging.dll

Important

Some information relates to prerelease product that may be substantially modified before it’s released. Microsoft makes no warranties, express or implied, with respect to the information provided here.

Returns a copy of the first message in the queue without removing the message from the queue.

Overloads

Peek()	Returns without removing (peeks) the first message in the queue referenced by this MessageQueue. The Peek() method is synchronous, so it blocks the current thread until a message becomes available.
Peek(TimeSpan)	Returns without removing (peeks) the first message in the queue referenced by this MessageQueue. The Peek() method is synchronous, so it blocks the current thread until a message becomes available or the specified time-out occurs.
Peek(TimeSpan, Cursor, PeekAction)	Returns without removing (peeks) the current or next message in the queue, using the specified cursor. The Peek() method is synchronous, so it blocks the current thread until a message becomes available or the specified time-out occurs.

Peek()

Returns without removing (peeks) the first message in the queue referenced by this MessageQueue. The Peek() method is synchronous, so it blocks the current thread until a message becomes available.

public:
 System::Messaging::Message ^ Peek();

public System.Messaging.Message Peek ();

member this.Peek : unit -> System.Messaging.Message

Public Function Peek () As Message

Returns

Message

The Message that represents the first message in the queue.

Exceptions

MessageQueueException

An error occurred when accessing a Message Queuing method.

Examples

The following examples use the Peek method on a queue.

In the first example, the application waits until a message becomes available in the queue. Note that the first example does not access the message that arrives; it merely pauses processing until a message arrives. If a message already exists in the queue, it will return immediately.

In the second example, a message that contains an application-defined Order class is sent to the queue, and then peeked from the queue.

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

using namespace System;
using namespace System::Messaging;

// This class represents an object the following example 
// sends to a queue and receives from a queue.
ref class Order
{
public:
   int orderId;
   DateTime orderTime;
};


/// <summary>
/// Provides a container class for the example.
/// </summary>
ref class MyNewQueue
{
public:

   //*************************************************
   // Posts a notification when a message arrives in 
   // the queue S"monitoredQueue". Does not retrieve any 
   // message information when peeking the message.
   //*************************************************
   void NotifyArrived()
   {
      // Connect to a queue.
      MessageQueue^ myQueue = gcnew MessageQueue( ".\\monitoredQueue" );

      // Specify to retrieve no message information.
      myQueue->MessageReadPropertyFilter->ClearAll();

      // Wait for a message to arrive. 
      Message^ emptyMessage = myQueue->Peek();

      // Post a notification when a message arrives.
      Console::WriteLine( "A message has arrived in the queue." );
      return;
   }


   //*************************************************
   // Sends an Order to a queue.
   //*************************************************
   void SendMessage()
   {
      // Create a new order and set values.
      Order^ sentOrder = gcnew Order;
      sentOrder->orderId = 3;
      sentOrder->orderTime = DateTime::Now;

      // Connect to a queue on the local computer.
      MessageQueue^ myQueue = gcnew MessageQueue( ".\\myQueue" );

      // Send the Order to the queue.
      myQueue->Send( sentOrder );
      return;
   }

   //*************************************************
   // Peeks a message containing an Order.
   //*************************************************
   void PeekFirstMessage()
   {
      // Connect to a queue.
      MessageQueue^ myQueue = gcnew MessageQueue( ".\\myQueue" );

      // Set the formatter to indicate the body contains an Order.
      array<Type^>^p = gcnew array<Type^>(1);
      p[ 0 ] = Order::typeid;
      myQueue->Formatter = gcnew XmlMessageFormatter( p );
      try
      {
         // Peek and format the message. 
         Message^ myMessage = myQueue->Peek();
         Order^ myOrder = static_cast<Order^>(myMessage->Body);

         // Display message information.
         Console::WriteLine( "Order ID: {0}", myOrder->orderId );
         Console::WriteLine( "Sent: {0}", myOrder->orderTime );
      }
      catch ( MessageQueueException^ ) 
      {
         // Handle Message Queuing exceptions.
      }
      // Handle invalid serialization format.
      catch ( InvalidOperationException^ e ) 
      {
         Console::WriteLine( e->Message );
      }

      // Catch other exceptions as necessary.
      return;
   }
};

//*************************************************
// Provides an entry point into the application.
//         
// This example posts a notification that a message
// has arrived in a queue. It sends a message 
// containing an other to a separate queue, and then
// peeks the first message in the queue.
//*************************************************
int main()
{
   // Create a new instance of the class.
   MyNewQueue^ myNewQueue = gcnew MyNewQueue;

   // Wait for a message to arrive in the queue.
   myNewQueue->NotifyArrived();

   // Send a message to a queue.
   myNewQueue->SendMessage();

   // Peek the first message in the queue.
   myNewQueue->PeekFirstMessage();
   return 0;
}

using System;
using System.Messaging;

namespace MyProject
{

    // This class represents an object the following example
    // sends to a queue and receives from a queue.
    public class Order
    {
        public int orderId;
        public DateTime orderTime;
    };	

    /// <summary>
    /// Provides a container class for the example.
    /// </summary>
    public class MyNewQueue
    {

        //**************************************************
        // Provides an entry point into the application.
        //		
        // This example posts a notification that a message
        // has arrived in a queue. It sends a message
        // containing an other to a separate queue, and then
        // peeks the first message in the queue.
        //**************************************************

        public static void Main()
        {
            // Create a new instance of the class.
            MyNewQueue myNewQueue = new MyNewQueue();

            // Wait for a message to arrive in the queue.
            myNewQueue.NotifyArrived();

            // Send a message to a queue.
            myNewQueue.SendMessage();	

            // Peek the first message in the queue.
            myNewQueue.PeekFirstMessage();
                        
            return;
        }

        //**************************************************
        // Posts a notification when a message arrives in
        // the queue "monitoredQueue". Does not retrieve any
        // message information when peeking the message.
        //**************************************************
        
        public void NotifyArrived()
        {

            // Connect to a queue.
            MessageQueue myQueue = new
                MessageQueue(".\\monitoredQueue");
    
            // Specify to retrieve no message information.
            myQueue.MessageReadPropertyFilter.ClearAll();

            // Wait for a message to arrive.
            Message emptyMessage = myQueue.Peek();

            // Post a notification when a message arrives.
            Console.WriteLine("A message has arrived in the queue.");

            return;
        }

        //**************************************************
        // Sends an Order to a queue.
        //**************************************************
        
        public void SendMessage()
        {
            
            // Create a new order and set values.
            Order sentOrder = new Order();
            sentOrder.orderId = 3;
            sentOrder.orderTime = DateTime.Now;

            // Connect to a queue on the local computer.
            MessageQueue myQueue = new MessageQueue(".\\myQueue");

            // Send the Order to the queue.
            myQueue.Send(sentOrder);

            return;
        }

        //**************************************************
        // Peeks a message containing an Order.
        //**************************************************
        
        public void PeekFirstMessage()
        {
            // Connect to a queue.
            MessageQueue myQueue = new MessageQueue(".\\myQueue");
    
            // Set the formatter to indicate the body contains an Order.
            myQueue.Formatter = new XmlMessageFormatter(new Type[]
                {typeof(MyProject.Order)});
            
            try
            {
                // Peek and format the message.
                Message myMessage =	myQueue.Peek();
                Order myOrder = (Order)myMessage.Body;

                // Display message information.
                Console.WriteLine("Order ID: " +
                    myOrder.orderId.ToString());
                Console.WriteLine("Sent: " +
                    myOrder.orderTime.ToString());
            }
            
            catch (MessageQueueException)
            {
                // Handle Message Queuing exceptions.
            }

            // Handle invalid serialization format.
            catch (InvalidOperationException e)
            {
                Console.WriteLine(e.Message);
            }
            
            // Catch other exceptions as necessary.

            return;
        }
    }
}

Imports System.Messaging



    ' This class represents an object the following example 
    ' sends to a queue and peeks from a queue.
    Public Class Order
        Public orderId As Integer
        Public orderTime As DateTime
    End Class


   
    Public Class MyNewQueue


        
        ' Provides an entry point into the application.
        '		 
        ' This example posts a notification that a message
        ' has arrived in a queue. It sends a message 
        ' containing an other to a separate queue, and then
        ' peeks the first message in the queue.
        

        Public Shared Sub Main()

            ' Create a new instance of the class.
            Dim myNewQueue As New MyNewQueue()

            ' Wait for a message to arrive in the queue.
            myNewQueue.NotifyArrived()

            ' Send a message to a queue.
            myNewQueue.SendMessage()

            ' Peek the first message in the queue.
            myNewQueue.PeekFirstMessage()

            Return

        End Sub


        
        ' Posts a notification when a message arrives in 
        ' the queue "monitoredQueue". Does not retrieve any 
        ' message information when peeking the message.
        
        Public Sub NotifyArrived()

            ' Connect to a queue.
            Dim myQueue As New MessageQueue(".\monitoredQueue")

            ' Specify to retrieve no message information.
            myQueue.MessageReadPropertyFilter.ClearAll()

            ' Wait for a message to arrive. 
            Dim emptyMessage As Message = myQueue.Peek()

            ' Post a notification when a message arrives.
            Console.WriteLine("A message has arrived in the queue.")

            Return

        End Sub


        
        ' Sends an Order to a queue.
        

        Public Sub SendMessage()

            ' Create a new order and set values.
            Dim sentOrder As New Order()
            sentOrder.orderId = 3
            sentOrder.orderTime = DateTime.Now

            ' Connect to a queue on the local computer.
            Dim myQueue As New MessageQueue(".\myQueue")

            ' Send the Order to the queue.
            myQueue.Send(sentOrder)

            Return

        End Sub


        
        ' Peeks a message containing an Order.
        

        Public Sub PeekFirstMessage()

            ' Connect to a queue.
            Dim myQueue As New MessageQueue(".\myQueue")

            ' Set the formatter to indicate body contains an Order.
            myQueue.Formatter = New XmlMessageFormatter(New Type() _
                {GetType(Order)})

            Try

                ' Peek and format the message. 
                Dim myMessage As Message = myQueue.Peek()
                Dim myOrder As Order = CType(myMessage.Body, Order)

                ' Display message information.
                Console.WriteLine(("Order ID: " + _
                    myOrder.orderId.ToString()))
                Console.WriteLine(("Sent: " + _
                    myOrder.orderTime.ToString()))

            Catch m as MessageQueueException
                ' Handle Message Queuing exceptions.


            Catch e As InvalidOperationException
                ' Handle invalid serialization format.
                Console.WriteLine(e.Message)

                ' Catch other exceptions as necessary.

            End Try

            Return

        End Sub

End Class

Remarks

Use this overload to peek a queue, or to wait until a message exists in the queue.

The Peek method reads, but does not remove, the first message from the queue. Therefore, repeated calls to Peek return the same message, unless a higher priority message arrives in the queue. The Receive method, on the other hand, both reads and removes the first message from the queue. Repeated calls to Receive, therefore, return different messages.

Message Queuing orders messages in the queue according to priority and arrival time. A newer message is placed before an older one only if it is of a higher priority.

Use Peek when it is acceptable for the current thread to be blocked while it waits for a message to arrive in the queue. Because this overload does not specify a time-out, the application might wait indefinitely. If you need the application processing to continue without waiting, use the asynchronous BeginPeek method. Alternatively, you can specify a time-out for a message to arrive in the queue by using the overload of Peek that specifies a time-out.

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

Applies to

Peek(TimeSpan)

Returns without removing (peeks) the first message in the queue referenced by this MessageQueue. The Peek() method is synchronous, so it blocks the current thread until a message becomes available or the specified time-out occurs.

public:
 System::Messaging::Message ^ Peek(TimeSpan timeout);

public System.Messaging.Message Peek (TimeSpan timeout);

member this.Peek : TimeSpan -> System.Messaging.Message

Public Function Peek (timeout As TimeSpan) As Message

Parameters

timeout: TimeSpan

A TimeSpan that indicates the maximum time to wait for the queue to contain a message.

Returns

Message

The Message that represents the first message in the queue.

Exceptions

ArgumentException

The value specified for the timeout parameter is not valid, possibly timeout is less than Zero or greater than InfiniteTimeout.

MessageQueueException

An error occurred when accessing a Message Queuing method.

Examples

The following code example uses the Peek method with a time-out of zero to check whether the queue is empty.

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

using namespace System;
using namespace System::Messaging;
ref class MyNewQueue
{
public:

   //*************************************************
   // Determines whether a queue is empty. The Peek()
   // method throws an exception if there is no message
   // in the queue. This method handles that exception 
   // by returning true to the calling method.
   //*************************************************
   bool IsQueueEmpty()
   {
      bool isQueueEmpty = false;
      
      // Connect to a queue.
      MessageQueue^ myQueue = gcnew MessageQueue( ".\\myQueue" );
      try
      {
         
         // Set Peek to return immediately.
         myQueue->Peek( TimeSpan(0) );
         
         // If an IOTime->Item[Out] was* not thrown, there is a message 
         // in the queue.
         isQueueEmpty = false;
      }
      catch ( MessageQueueException^ e ) 
      {
         if ( e->MessageQueueErrorCode == MessageQueueErrorCode::IOTimeout )
         {
            
            // No message was in the queue.
            isQueueEmpty = true;
         }

         
         // Handle other sources of MessageQueueException.
      }

      
      // Handle other exceptions as necessary.
      // Return true if there are no messages in the queue.
      return isQueueEmpty;
   }

};


//*************************************************
// Provides an entry point into the application.
//         
// This example determines whether a queue is empty.
//*************************************************
int main()
{
   
   // Create a new instance of the class.
   MyNewQueue^ myNewQueue = gcnew MyNewQueue;
   
   // Determine whether a queue is empty.
   bool isQueueEmpty = myNewQueue->IsQueueEmpty();
   return 0;
}

using System;
using System.Messaging;

namespace MyProject
{
    /// <summary>
    /// Provides a container class for the example.
    /// </summary>
    public class MyNewQueue
    {

        //**************************************************
        // Provides an entry point into the application.
        //		
        // This example determines whether a queue is empty.
        //**************************************************

        public static void Main()
        {
            // Create a new instance of the class.
            MyNewQueue myNewQueue = new MyNewQueue();

            // Determine whether a queue is empty.
            bool isQueueEmpty = myNewQueue.IsQueueEmpty();
                        
            return;
        }

        //**************************************************
        // Determines whether a queue is empty. The Peek()
        // method throws an exception if there is no message
        // in the queue. This method handles that exception
        // by returning true to the calling method.
        //**************************************************
        
        public bool IsQueueEmpty()
        {
            bool isQueueEmpty = false;

            // Connect to a queue.
            MessageQueue myQueue = new MessageQueue(".\\myQueue");

            try
            {
                // Set Peek to return immediately.
                myQueue.Peek(new TimeSpan(0));

                // If an IOTimeout was not thrown, there is a message
                // in the queue.
                isQueueEmpty = false;
            }

            catch(MessageQueueException e)
            {
                if (e.MessageQueueErrorCode ==
                    MessageQueueErrorCode.IOTimeout)
                {
                    // No message was in the queue.
                    isQueueEmpty = true;
                }

                // Handle other sources of MessageQueueException.
            }

            // Handle other exceptions as necessary.

            // Return true if there are no messages in the queue.
            return isQueueEmpty;
        }
    }
}

Imports System.Messaging



   
    Public Class MyNewQueue


        '
        ' Provides an entry point into the application.
        '		 
        ' This example determines whether a queue is empty.
        '

        Public Shared Sub Main()

            ' Create a new instance of the class.
            Dim myNewQueue As New MyNewQueue()

            ' Determine whether a queue is empty.
            Dim IsQueueEmpty As Boolean = myNewQueue.IsQueueEmpty()
        if IsQueueEMpty=True Then Console.WriteLine("Empty")
            

            Return

        End Sub


        '
        ' Determines whether a queue is empty. The Peek()
        ' method throws an exception if there is no message
        ' in the queue. This method handles that exception 
        ' by returning true to the calling method.
        '

        Public Function IsQueueEmpty() As Boolean

            'Dim QueueEmpty As Boolean = False

            ' Connect to a queue.
            Dim myQueue As New MessageQueue(".\myQueue")

            Try

                ' Set Peek to return immediately.
                myQueue.Peek(New TimeSpan(0))

                ' If an IOTimeout was not thrown, there is a message 
                ' in the queue.
                'queueEmpty = False

            Catch e As MessageQueueException

                If e.MessageQueueErrorCode = _
                    MessageQueueErrorCode.IOTimeout Then

                    ' No message was in the queue.
                    IsQueueEmpty = True

                End If

                ' Handle other sources of MessageQueueException as necessary.

                ' Handle other exceptions as necessary.

            End Try

            ' Return true if there are no messages in the queue.
            'Return queueEmpty
        IsQueueEmpty = False

        End Function 'IsQueueEmpty 

End Class

Remarks

Use this overload to peek a queue, or to wait a specified period of time until a message exists in the queue. The method returns immediately if a message already exists in the queue.

Message Queuing orders messages in the queue according to priority and arrival time. A newer message is placed before an older one only if it is of a higher priority.

Use Peek when it is acceptable for the current thread to be blocked while it waits for a message to arrive in the queue. The thread will be blocked up to the specified period of time, or indefinitely if you indicated InfiniteTimeout. If you need the application processing to continue without waiting, use the asynchronous BeginPeek method.

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

Applies to

Peek(TimeSpan, Cursor, PeekAction)

Returns without removing (peeks) the current or next message in the queue, using the specified cursor. The Peek() method is synchronous, so it blocks the current thread until a message becomes available or the specified time-out occurs.

public:
 System::Messaging::Message ^ Peek(TimeSpan timeout, System::Messaging::Cursor ^ cursor, System::Messaging::PeekAction action);

public System.Messaging.Message Peek (TimeSpan timeout, System.Messaging.Cursor cursor, System.Messaging.PeekAction action);

member this.Peek : TimeSpan * System.Messaging.Cursor * System.Messaging.PeekAction -> System.Messaging.Message

Public Function Peek (timeout As TimeSpan, cursor As Cursor, action As PeekAction) As Message

Parameters

timeout: TimeSpan

A TimeSpan that indicates the maximum time to wait for the queue to contain a message.

cursor: Cursor

A Cursor that maintains a specific position in the message queue.

action: PeekAction

One of the PeekAction values. Indicates whether to peek at the current message in the queue, or the next message.

Returns

Message

A Message that represents a message in the queue.

Exceptions

ArgumentOutOfRangeException

A value other than PeekAction.Current or PeekAction.Next was specified for the action parameter.

ArgumentNullException

The cursor parameter is null.

ArgumentException

The value specified for the timeout parameter is not valid. Possibly timeout is less than Zero or greater than InfiniteTimeout.

MessageQueueException

An error occurred when accessing a Message Queuing method.

Remarks

Use this overload to peek a queue, or to wait a specified period of time until a message exists in the queue. The method returns immediately if a message already exists in the queue.

The Peek method reads, but does not remove, a message from the queue. The Receive method, on the other hand, both reads and removes a message from the queue.

Use Peek when it is acceptable for the current thread to be blocked while it waits for a message to arrive in the queue. The thread is blocked up to the specified period of time, or indefinitely if you indicated InfiniteTimeout. If you need the application processing to continue without waiting, use the asynchronous BeginPeek method.

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

Applies to

Thread Safety

The method is not thread safe.

Delen via

MessageQueue.Peek Method

Definition

Overloads

Peek()

Returns

Exceptions

Examples

Remarks

See also

Applies to

Peek(TimeSpan)

Parameters

Returns

Exceptions

Examples

Remarks

See also

Applies to

Peek(TimeSpan, Cursor, PeekAction)

Parameters

Returns

Exceptions

Remarks

See also

Applies to

Thread Safety

Aanvullende resources