public static void downloadFile(string sourceURL, string destinationPath)
{
    long fileSize = 0;
    int bufferSize = 1024;
    bufferSize *= 1000;
    long existLen = 0;
    
    System.IO.FileStream saveFileStream;
    if (System.IO.File.Exists(destinationPath))
    {
        System.IO.FileInfo destinationFileInfo = new System.IO.FileInfo(destinationPath);
        existLen = destinationFileInfo.Length;
    }

    if (existLen > 0)
        saveFileStream = new System.IO.FileStream(destinationPath,
                                                  System.IO.FileMode.Append,
                                                  System.IO.FileAccess.Write,
                                                  System.IO.FileShare.ReadWrite);
    else
        saveFileStream = new System.IO.FileStream(destinationPath,
                                                  System.IO.FileMode.Create,
                                                  System.IO.FileAccess.Write,
                                                  System.IO.FileShare.ReadWrite);
 
    System.Net.HttpWebRequest httpReq;
    System.Net.HttpWebResponse httpRes;
    httpReq = (System.Net.HttpWebRequest) System.Net.HttpWebRequest.Create(sourceURL);
    httpReq.AddRange((int) existLen);
    System.IO.Stream resStream;
    httpRes = (System.Net.HttpWebResponse) httpReq.GetResponse();
    resStream = httpRes.GetResponseStream();
 
    fileSize = httpRes.ContentLength;
 
    int byteSize;
    byte[] downBuffer = new byte[bufferSize];
 
    while ((byteSize = resStream.Read(downBuffer, 0, downBuffer.Length)) > 0)
    {
        saveFileStream.Write(downBuffer, 0, byteSize);
    }
}