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Proxy Pattern provides a surrogate or placeholder for another object to control access to it.
E.g.

I defined a IGreeting interface

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

namespace ProxyPattern
{
    interface IGreeting
    {
        void SayHello();
    }
}

Greeting implements IGreeting

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

namespace ProxyPattern
{  

    class Greeting: IGreeting
    {
        private string hello;

        public Greeting(String hello)
        {
            this.hello = hello;
        }

        public void SayHello()
        {
            Console.WriteLine(hello);
        }

    }
}

GreetingProxy is used to access Greeting

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

namespace ProxyPattern
{
    class GreetingProxy
    {
        private Greeting greeting;

        public GreetingProxy(string hello)
        {
            greeting = new Greeting(hello);
        }

        public void SayHello()
        {
            // Can perform extra logic here
            greeting.SayHello();
        }

    }
}

To test it

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

namespace ProxyPattern
{
    class Program
    {
        static void Main(string[] args)
        {
            GreetingProxy proxy1 = new GreetingProxy("hello");
            GreetingProxy proxy2 = new GreetingProxy("bonjour");
            proxy1.SayHello();
            proxy2.SayHello();
            Console.ReadLine();
        }
    }
}