Basic Asynchronous writer example.

AsyncWriter.cs

using System;
using System.IO;
using System.Threading;

namespace dotNetPlayGround
{
    class AsyncState
    {
        public string name;
        public Stream stream = null; 
    }

    class AsyncWriter
    {
        static void Main()
        {
            Stream sr = new FileStream("source.txt", FileMode.Open, FileAccess.Read);
            byte[] buffer = new byte[sr.Length];
            int bytesRead = sr.Read(buffer, 0, buffer.Length);
            Stream sw = new FileStream("temp.txt", FileMode.OpenOrCreate, FileAccess.Write);
            AsyncState state = new AsyncState();
            state.name = "tempt.txt";
            state.stream = sw;
            IAsyncResult writeResult = sw.BeginWrite(buffer, 0, buffer.Length, 
                (result) => {
                    AsyncState s = (AsyncState)result.AsyncState;
                    s.stream.EndWrite(result);
                    Console.WriteLine("Name:{0},Length:{1}", s.name, s.stream.Length);
                    Console.Write(state.name == s.name);
                    Console.Write(object.ReferenceEquals(state, s));
                 }
                 , state);
            while (!writeResult.IsCompleted) { }
            Console.ReadKey();
        }
    }
}