If you need to have two Silverlight apps talking to each other, you would be using the LocalMessageSender and LocalMessageReceiver classes from the System.Windows.Messaging namespace. The problem with the LocalMessageSender is that it might not be able to send message at the very first try. So you will have to write code which retries to send the message for an x number of times. Rather than duplicating this code all over the place, I decided to create a class which will do this for you.

using System;
using System.Windows.Messaging;

namespace Common
{
    public class RetryMessageSender
    {
        private readonly int maxRetryCount;
        private LocalMessageSender messageSender;
        private int retryCount = 1;

        public event EventHandler<SendCompletedEventArgs> SendCompleted;

        public RetryMessageSender(LocalMessageSender messageSender)
            : this(messageSender, 100 /*default value of max retries*/)
        {
        }

        public RetryMessageSender(LocalMessageSender messageSender, int maxRetries)
        {
            this.messageSender = messageSender;
            this.maxRetryCount = maxRetries;
            messageSender.SendCompleted += new System.EventHandler<SendCompletedEventArgs>(messageSender_SendCompleted);
        }

        public void SendMessage(string msg)
        {
            messageSender.SendAsync(msg);
        }

        void messageSender_SendCompleted(object sender, SendCompletedEventArgs e)
        {
            if (e.Error != null)
            {
                if (retryCount <= maxRetryCount)
                {
                    retryCount++;
                    SendMessage(e.Message);
                }
                else
                {
                    SendCompleted(sender, e);
                }
            }
            else
            {
                retryCount = 0;
                SendCompleted(sender, e);
            }
        }
    }
}

Now you can use this class like this:

RetryMessageSender msgSender = new RetryMessageSender(messageSender, 10);
msgSender.SendCompleted += new System.EventHandler<SendCompletedEventArgs>(msgSender_SendCompleted);
msgSender.SendMessage("hello");

In the SendCompleted event handler, you can check whether the message was sent successfully within the specified number of retries (10 in this case).

static void msgSender_SendCompleted(object sender, SendCompletedEventArgs e)
{
    if (e.Error != null)
    {
         throw new System.Exception("Unable to send message even after 10 retries");
    }
}