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Decorator Pattern attaches additional responsibilities to an object dynamically and provides a flexible alternative to subclassing for extending functionality.

E.g.

BaseMessage is defined

using System;
using System.Collections.Generic;
using System.Text;

namespace DecoratorPattern
{
    abstract class BaseMessage
    {
        private string sender;
        private string recipient;

        public string Sender
        {
            get
            {
                return this.sender;
            }
            set
            {
                this.sender = value;
            }
        }

        public string Recipient {
            get
            {
                return this.recipient;
            }
            set
            {
                this.recipient = value;
            }
        }

        public abstract void Send();

    }
}

Both SMS and MMS inherites BaseMessage

using System;
using System.Collections.Generic;
using System.Text;

namespace DecoratorPattern
{
    class SMS: BaseMessage
    {
        private string message;

        public SMS(string sender, string recipient, string message)
        {
            this.Sender = sender;
            this.Recipient = recipient;
            this.Message = message;
        }

        public string Message
        {
            get
            {
                return this.message;
            }
            set
            {
                this.message = value;
            }
        }

        public override void Send()
        {
            Console.WriteLine("Send to SMSC.");
        }
    }
}

using System;
using System.Collections.Generic;
using System.Text;

namespace DecoratorPattern
{
    class MMS:BaseMessage
    {
        private byte[] image;

        public MMS(string sender, string recipient, byte[] image)
        {
            this.Sender = sender;
            this.Recipient = recipient;
            this.Image = image;
        }

        public byte[] Image
        {
            get
            {
                return this.image;
            }
            set
            {
                this.image = value;
            }
        }

        public override void Send()
        {
            Console.WriteLine("Send to MMSC.");
        }
    }
}

To add additional behaviors to the class, define Decorator class which extends BaseMessage also.

using System;
using System.Collections.Generic;
using System.Text;

namespace DecoratorPattern
{
    class Decorator: BaseMessage
    {
        protected BaseMessage message;

        public Decorator(BaseMessage message)
        {
            this.message = message;
        }

        public override void Send()
        {
            message.Send();
        }
    }
}

The additional processing is defined in MessageProcessor which extends Decorator

using System;
using System.Collections.Generic;
using System.Text;

namespace DecoratorPattern
{
    class MessageProcessor:Decorator
    {
        private string serviceProvider;

        public MessageProcessor(BaseMessage message)
            : base(message)
        {
        }

        public string ServiceProvider
        {
            get
            {
                return this.serviceProvider;
            }
            set
            {
                this.serviceProvider = value;
            }
        }

        public void PreProcess()
        {
            Console.WriteLine("Preprocessing message.");

        }

        public void PostProcess()
        {
            Console.WriteLine("Post processing message.");
        }

        public override void Send()
        {
            PreProcess();
            base.Send();
            PostProcess();
        }
    }
}

To test it

using System;
using System.Collections.Generic;
using System.Text;

namespace DecoratorPattern
{
    class Program
    {
        static void Main(string[] args)
        {
            // Create SMS
            SMS sms = new SMS("123", "456", "test message");
            sms.Send();

            // Create MMS
            MMS mms = new MMS("123", "456", new byte[] { 1, 2 });
            mms.Send();

            // Set service provider, add pre and post post processing
            MessageProcessor messageProcessor
                 = new MessageProcessor(sms);
            messageProcessor.ServiceProvider = "telco1";
            messageProcessor.Send();

            // Wait for user
            Console.Read();
        }
    }
}

The output

Send to SMSC.
Send to MMSC.
Preprocessing message.
Send to SMSC.
Post processing message.