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MessageEnumerator Class

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

System.Object
  System.MarshalByRefObject
    System.Messaging.MessageEnumerator

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

public class MessageEnumerator : MarshalByRefObject, 
	IEnumerator, IDisposable

The MessageEnumerator type exposes the following members.

  NameDescription
Public propertyCurrentGets the current Message that this enumerator points to.
Public propertyCursorHandleGets the native Message Queuing cursor handle used to browse messages in the queue.
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  NameDescription
Public methodCloseFrees the resources associated with the enumerator.
Public methodCreateObjRefCreates an object that contains all the relevant information required to generate a proxy used to communicate with a remote object. (Inherited from MarshalByRefObject.)
Public methodDispose()Releases all resources used by the MessageEnumerator.
Protected methodDispose(Boolean)Releases the unmanaged resources used by the MessageEnumerator and optionally releases the managed resources.
Public methodEquals(Object)Determines whether the specified object is equal to the current object. (Inherited from Object.)
Protected methodFinalizeInfrastructure. Allows an object to try to free resources and perform other cleanup operations before it is reclaimed by garbage collection. (Inherited from Object.)
Public methodGetHashCodeServes as the default hash function. (Inherited from Object.)
Public methodGetLifetimeServiceRetrieves the current lifetime service object that controls the lifetime policy for this instance. (Inherited from MarshalByRefObject.)
Public methodGetTypeGets the Type of the current instance. (Inherited from Object.)
Public methodInitializeLifetimeServiceObtains a lifetime service object to control the lifetime policy for this instance. (Inherited from MarshalByRefObject.)
Protected methodMemberwiseClone()Creates a shallow copy of the current Object. (Inherited from Object.)
Protected methodMemberwiseClone(Boolean)Creates a shallow copy of the current MarshalByRefObject object. (Inherited from MarshalByRefObject.)
Public methodMoveNext()Advances the enumerator to the next message in the queue, if one is currently available.
Public methodMoveNext(TimeSpan)Advances the enumerator to the next message in the queue. If the enumerator is positioned at the end of the queue, MoveNext waits until a message is available or the given timeout expires.
Public methodRemoveCurrent()Removes the current message from a transactional or non-transactional queue and returns the message to the calling application. There is no timeout specified for a message to arrive in the queue.
Public methodRemoveCurrent(MessageQueueTransaction)Removes the current message from a transactional queue and returns the message to the calling application. There is no timeout specified for a message to arrive in the queue.
Public methodRemoveCurrent(MessageQueueTransactionType)Removes the current message from a queue and returns the message to the calling application. There is no timeout specified for a message to arrive in the queue.
Public methodRemoveCurrent(TimeSpan)Removes the current message from the queue and returns the message to the calling application. If there is a message to remove, the method returns it immediately. Otherwise, the method waits the specified timeout for a new message to arrive.
Public methodRemoveCurrent(TimeSpan, MessageQueueTransaction)Removes the current message from a transactional queue and returns the message to the calling application. If there is a message to remove, the method returns it immediately. Otherwise, the method waits the specified timeout for a new message to arrive.
Public methodRemoveCurrent(TimeSpan, MessageQueueTransactionType)Removes the current message from a queue and returns the message to the calling application. If there is a message to remove, the method returns it immediately. Otherwise, the method waits the specified timeout for a new message to arrive.
Public methodResetResets the current enumerator so it points to the head of the queue.
Public methodToStringReturns a string that represents the current object. (Inherited from Object.)
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  NameDescription
Explicit interface implemetationPrivate propertyIEnumerator.CurrentGets the current Message that this enumerator points to.
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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.

NoteNote

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.

The following example gets a dynamic list of messages in a queue and counts all messages with the Priority property set to MessagePriority.Lowest.

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;
		}
	}
}

.NET Framework

Supported in: 4.6, 4, 3.5, 3.0, 2.0, 1.1, 1.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

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