public sealed class FileWriter : AsyncCodeActivity
{
    public FileWriter()
        : base() 
    { 
    }
    protected override IAsyncResult BeginExecute(AsyncCodeActivityContext context, AsyncCallback callback, object state)
    {
        string tempFileName = Path.GetTempFileName();
        Console.WriteLine("Writing to file: " + tempFileName);

        FileStream file = File.Open(tempFileName, FileMode.Create);

        context.UserState = file;

        byte[] bytes = UnicodeEncoding.Unicode.GetBytes("123456789");
        return file.BeginWrite(bytes, 0, bytes.Length, callback, state);
    }
    protected override void EndExecute(AsyncCodeActivityContext context, IAsyncResult result)
    {
        FileStream file = (FileStream)context.UserState;
        
        try
        {
            file.EndWrite(result);
            file.Flush();
        }
        finally
        {
            file.Close();
        }
    }
}