public class Downloader
{
    public string FileName { get; private set; }
    public string Url { get; private set; }
    public int Buffer { get { return 512; } }

    public Downloader(string fileName, string url)
    {
        FileName = fileName;
        Url = url;
    }

    private WebResponse GetResponse(string method, long start = 0)
    {
        var request = HttpWebRequest.Create(Url) as HttpWebRequest;
        request.Method = method;
        if (start > 0)
        {
            request.AddRange(Convert.ToInt32(start));
        }
        return request.GetResponse();
    }

    private void Read(WebResponse response, Stream stream)
    {
        using (var ns = response.GetResponseStream())
        {
            byte[] nbytes = new byte[Buffer];
            int size = 0;
            size = ns.Read(nbytes, 0, Buffer);
            while (size > 0)
            {
                stream.Write(nbytes, 0, size);
                size = ns.Read(nbytes, 0, Buffer);
            }
        }

    }

    public void Start()
    {
        bool exist = File.Exists(FileName);
        long start = 0;

        using (var stream = exist ? File.OpenWrite(FileName) : new FileStream(FileName, FileMode.Create))
        {
            if (exist)
            {
                start = stream.Length;
                stream.Seek(start, SeekOrigin.Current);
            }

            if (this.Length > start)
            {
                WebResponse response = null;

                try
                {
                    response = GetResponse("GET", start);
                    Read(response, stream);
                }
                finally
                {
                    if (response != null)
                    {
                        response.Close();
                    }
                }
                    
            }
        }
    }

    private long? _length;
    public long Length 
    {
        get
        {
            if (!_length.HasValue)
            {
                WebResponse response = null;

                try
                {
                    response = GetResponse("HEAD");
                    _length = response.ContentLength;

                }
                finally
                {
                    if (response != null)
                    {
                        response.Close();
                    }
                }
            }

            return _length.Value;
        }
    }
}