How to programmatically download a large file in C

问题:

I need to programmatically download a large file before processing it. What's the best way to do that? As the file is large, I want to specific time to wait so that I can forcefully exit.

I know of WebClient.DownloadFile(). But there does not seem a way to specific an amount of time to wait so as to forcefully exit.

try
{
    WebClient client = new WebClient();
    Uri uri = new Uri(inputFileUrl);
    client.DownloadFile(uri, outputFile);
}
catch (Exception ex)
{
    throw;
}

Another way is to use a command line utility (wget) to download the file and fire the command using ProcessStartInfo and use Process' WaitForExit(int ms) to forcefully exit.

ProcessStartInfo startInfo = new ProcessStartInfo();
//set startInfo object

try
{
    using (Process exeProcess = Process.Start(startInfo))
    {
        //wait for time specified
        exeProcess.WaitForExit(1000 * 60 * 60);//wait till 1m

        //check if process has exited
        if (!exeProcess.HasExited)
        {
            //kill process and throw ex
            exeProcess.Kill();
            throw new ApplicationException("Downloading timed out");
        }
    }
}
catch (Exception ex)
{
    throw;
}

Is there a better way? Please help. Thanks.

回答1:

Use a WebRequest and get the response stream. Then read from the reponse Stream blocks of bytes, and write each block to the destination file. This way you can control when to stop if the download takes too long, as you get control between chunks and you can decide if the download has timed out based on a clock:       

        DateTime startTime = DateTime.UtcNow;
        WebRequest request = WebRequest.Create("http://www.example.com/largefile");
        WebResponse response = request.GetResponse();
        using (Stream responseStream = response.GetResponseStream()) {
            using (Stream fileStream = File.OpenWrite(@"c:\temp\largefile")) { 
                byte[] buffer = new byte[4096];
                int bytesRead = responseStream.Read(buffer, 0, 4096);
                while (bytesRead > 0) {       
                    fileStream.Write(buffer, 0, bytesRead);
                    DateTime nowTime = DateTime.UtcNow;
                    if ((nowTime - startTime).TotalMinutes > 5) {
                        throw new ApplicationException(
                            "Download timed out");
                    }
                    bytesRead = responseStream.Read(buffer, 0, 4096);
                }
            }
        }

回答2:

How about using DownloadFileAsync in the WebClient class. The cool thing about going this route is that you can cancel the operation by calling CancelAsync if it takes too long. Basically, call this method, and if a specified amount of time elapses, call Cancel.

回答3:

Asked here: C#: Downloading a URL with timeout

Simplest solution:

public string GetRequest(Uri uri, int timeoutMilliseconds)
{
    var request = System.Net.WebRequest.Create(uri);
    request.Timeout = timeoutMilliseconds;
    using (var response = request.GetResponse())
    using (var stream = response.GetResponseStream())
    using (var reader = new System.IO.StreamReader(stream))
    {
        return reader.ReadToEnd();
    }
}

Better (more flexible) solution is this answer to the same question, in the form of a WebClientWithTimeout helper class.

回答4:

You can use DownloadFileAsync as @BFree said and then try with the following WebClient's events

protected virtual void OnDownloadProgressChanged(DownloadProgressChangedEventArgs e);
protected virtual void OnDownloadFileCompleted(AsyncCompletedEventArgs e);

Then you can know the Progress Percentage

e.ProgressPercentage

Hope this helps