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MessageEnumerator Class

Provides a forward-only cursor to enumerate through messages in a message queue.

For a list of all members of this type, see MessageEnumerator Members.

System.Object
   System.MarshalByRefObject
      System.Messaging.MessageEnumerator

[Visual Basic]
Public Class MessageEnumerator
   Inherits MarshalByRefObject
   Implements IEnumerator, IDisposable
[C#]
public class MessageEnumerator : MarshalByRefObject, IEnumerator,
   IDisposable
[C++]
public __gc class MessageEnumerator : public MarshalByRefObject,
   IEnumerator, IDisposable
[JScript]
public class MessageEnumerator extends MarshalByRefObject
   implements IEnumerator, IDisposable

Thread Safety

Any public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.

Remarks

Use MessageEnumerator for dynamic interaction with messages in a queue. Methods available through the MessageQueue class can return either a MessageEnumerator pointing to a dynamic list of messages in the queue, or an array that contains a copy at a given instant - a snapshot - of the queue at the time the specified method was called.

Unlike a static snapshot, an enumerator allows you to modify the collection. Using a MessageEnumerator, you can remove messages from the queue, and the change is immediately reflected in the queue.

An enumerator does not remove the messages from the queue when it queries the queue. It returns information about the message at the current cursor position, but it leaves the message in the queue.

A MessageEnumerator is a cursor, initialized to the head of a dynamic list. The list order is the same as the order of the messages in the queue, according to message priority. You can move the cursor to the first message in the queue by calling MoveNext. After the enumerator has been initialized, you can use MoveNext to step forward through the remaining messages. You can specify whether to wait for a message to become available by passing a timeout into the MoveNext method.

Because the enumerator is dynamic, a message that is appended beyond the cursor's current position (for example, due to low priority), can be accessed by the enumerator. A message that is inserted before the cursor's current position cannot be accessed. It is not possible to step backward with a MessageEnumerator. A cursor allows forward-only movement. The Reset method enables you to place the cursor back at the beginning of the queue.

Instances of MessageEnumerator for a given queue work independently. You can create two MessageEnumerator instances that apply to the same queue. The changes that one MessageEnumerator makes to the messages in the queue will be reflected immediately in a second enumerator if the second enumerator is positioned before the first. However, if two enumerators have the same position and one of them removes the message at that position, an exception is thrown if the other enumerator attempts to get the value of the Current property on the now-deleted message.

Note   If you create an instance of MessageQueue with MessageQueue.DenySharedReceive set to true, no other application can modify the messages in your enumerator while you have the connection to the queue.

Example

[Visual Basic, C#, C++] The following example gets a dynamic list of messages in a queue and counts all messages with the Priority property set to MessagePriority.Lowest.

[Visual Basic] 
Imports System
Imports 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 uses a cursor to step through the
        ' messages in a queue and counts the number of 
        ' Lowest priority messages.
        '**************************************************

        Public Shared Sub Main()

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

            ' Output the count of Lowest priority messages.
            myNewQueue.CountLowestPriority()

            Return

        End Sub 'Main


        '**************************************************
        ' Iterates through messages in a queue and examines
        ' their priority.
        '**************************************************

        Public Sub CountLowestPriority()

            ' Holds the count of Lowest priority messages.
            Dim numberItems As Int32 = 0

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

            ' Get a cursor into the messages in the queue.
            Dim myEnumerator As MessageEnumerator = _
                myQueue.GetMessageEnumerator()

            ' Specify that the messages's priority should be read.
            myQueue.MessageReadPropertyFilter.Priority = True

            ' Move to the next message and examine its priority.
            While myEnumerator.MoveNext()

                ' Increase the count if the priority is Lowest.
                If myEnumerator.Current.Priority = _
                    MessagePriority.Lowest Then
                    numberItems += 1
                End If

            End While

            ' Display final count.
            Console.WriteLine(("Lowest priority messages: " + _
                numberItems.ToString()))

            Return

        End Sub 'CountLowestPriority

    End Class 'MyNewQueue
End Namespace 'MyProject

[C#] 
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 uses a cursor to step through the
        // messages in a queue and counts the number of 
        // Lowest priority messages.
        //**************************************************

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

            // Output the count of Lowest priority messages.
            myNewQueue.CountLowestPriority();
                        
            return;
        }


        //**************************************************
        // Iterates through messages in a queue and examines
        // their priority.
        //**************************************************
        
        public void CountLowestPriority()
        {
            // Holds the count of Lowest priority messages.
            uint numberItems = 0;

            // Connect to a queue.
            MessageQueue myQueue = new MessageQueue(".\\myQueue");
    
            // Get a cursor into the messages in the queue.
            MessageEnumerator myEnumerator = 
                myQueue.GetMessageEnumerator();

            // Specify that the messages's priority should be read.
            myQueue.MessageReadPropertyFilter.Priority = true;

            // Move to the next message and examine its priority.
            while(myEnumerator.MoveNext())
            {
                // Increase the count if priority is Lowest.
                if(myEnumerator.Current.Priority == 
                    MessagePriority.Lowest)
                    
                    numberItems++;
            }

            // Display final count.
            Console.WriteLine("Lowest priority messages: " + 
                numberItems.ToString());
            
            return;
        }
    }
}

[C++] 
#using <mscorlib.dll>
#using <system.dll>
#using <system.messaging.dll>

using namespace System;
using namespace System::Messaging;

__gc class MyNewQueue 
{
public:
    void CountLowestPriority() 
    {
        // Holds the count of Lowest priority messages.
        UInt32 numberItems = 0;

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

        // Get a cursor into the messages in the queue.
        MessageEnumerator* myEnumerator = 
            myQueue->GetMessageEnumerator();

        // Specify that the messages's priority should be read.
        myQueue->MessageReadPropertyFilter->Priority = true;

        // Move to the next message and examine its priority.
        while(myEnumerator->MoveNext())
        {
            // Increase the count if priority is Lowest.
            if (myEnumerator->Current->Priority == 
                MessagePriority::Lowest)

                numberItems++;
        }

        // Display final count.
        Console::WriteLine(S"Lowest priority messages: {0}", __box(numberItems));

        return;
    }
};

int main() 
{
    // Create a new instance of the class.
    MyNewQueue* myNewQueue = new MyNewQueue();

    // Output the count of Lowest priority messages.
    myNewQueue->CountLowestPriority();

    return 0;
}

[JScript] No example is available for JScript. To view a Visual Basic, C#, or C++ example, click the Language Filter button Language Filter in the upper-left corner of the page.

Requirements

Namespace: System.Messaging

Platforms: Windows 98, Windows NT 4.0, Windows Millennium Edition, Windows 2000, Windows XP Home Edition, Windows XP Professional, Windows Server 2003 family

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

See Also

MessageEnumerator Members | System.Messaging Namespace | Message | MessageQueue.GetMessageEnumerator

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