Detailed graphic explanation of file download code implemented by WebClient in C#

This article mainly introduces the implementation of file downloading by WebClient in C#. The editor thinks it is quite good. Now I will share it with you and give it as a reference. Let’s follow the editor to take a look

In view of various complex network environments, the author decided to use different programming interfaces to try downloading to increase the usability of the program.

Here we only introduce the method of using WebClient, and subsequent articles will introduce other methods. The blog post mainly introduces the ideas and key codes, and the complete demo is attached at the end of the article.

Use a proxy to access the Internet

Employees of many companies access the Internet through proxies set by the company. Accessing the Internet through a proxy is mainly to facilitate the company to carry out various controls. Of course, it can also implement some special functions... However, this will cause some problems for our program to access the network.

In fact, the API in WebClient is already very smart. For example, the HttpWebRequest object we created has a Proxy attribute. That is, WebHttpRequest will use the found proxy by default. This is great and can handle a lot of situations. However, if this default proxy needs to verify the identity information of domain users, using WebHttpRequest to access the network may fail. At this point I look at the Proxy. Credentials property and find that it is null.

The system default Credentials can be obtained from the WebClient API, but it is not clear why the Proxy.Credentials property is not set to this value by default. We can just set it ourselves.

request.Proxy.Credentials = CredentialCache.DefaultCredentials;

But the actual network environment may be more complex, requiring users to specify the proxy for networking and also specify the Credentials required for networking. The writing is as follows:

myProxy = new WebProxy("proxyAddress"); 
myProxy.Credentials = new NetworkCredential(ProxyUserName, ProxyUserPasswd, DomainName);

Overcoming cache

Caching is everywhere. There will be caching on the server-side CDN, and there will also be caching on the client-side proxy layer. So a common problem is: even though the files on the server have been updated, some customers will still download the old files. Let's deal with the client's caching problem first.

The CachePolicy.Level property of HttpWebRequest is used to set the cache policy, but its default value is BypassCache. We just change it to Reload:

Copy code The code is as follows:

request.CachePolicy = new System .Net.Cache.RequestCachePolicy(System.Net.Cache.RequestCacheLevel.Reload);

The next step is the server-side cache problem.

Now everyone seems to be using CDN, but during use we often find problems with cache updates on the CDN side. I searched online and found no good solution, but there is a good workaround, which is to add a random string as a parameter in the request.

Random rdm = new Random();
string s = rdm.Next().ToString();
myUrl += "?" + s;

It should be noted that regarding caching, you must use a strategy that fits the current use case and cannot be one-size-fits-all.

More friendly download process

Use the scroll bar to display the download progress, display the real-time download speed, and allow the user to cancel the download:

The following is the core code for downloading. We divide it into calculating the download percentage and calculating the current download speed.

// 获得下载文件的长度
double contentLength = DownloadManager.GetContentLength(myHttpWebClient);
byte[] buffer = new byte[BufferSize];
long downloadedLength = 0;
long currentTimeSpanDataLength = 0;   
int currentDataLength;
while ((currentDataLength = stream.Read(buffer, 0, BufferSize)) > 0 && !this._cancelDownload)
{
 fileStream.Write(buffer, 0, currentDataLength);
 downloadedLength += (long)currentDataLength;
 currentTimeSpanDataLength += (long)currentDataLength;
 int intDownloadSpeed = 0;
 if (this._downloadStopWatch.ElapsedMilliseconds > 800)
 {
  double num5 = (double)currentTimeSpanDataLength / 1024.0;
  double num6 = (double)this._downloadStopWatch.ElapsedMilliseconds / 1000.0;
  double doubleDownloadSpeed = num5 / num6;
  intDownloadSpeed = (int)Math.Round(doubleDownloadSpeed, 0);
  this._downloadStopWatch.Reset();
  this._downloadStopWatch.Start();
  currentTimeSpanDataLength = 0;
 }

 double doubleDownloadPersent = 0.0;
 if (contentLength > 0.0)
 {
  doubleDownloadPersent = (double)downloadedLength / contentLength;
 }
}

Calculate the download percentage during the download process

First you need to request from http Get the length of the file to be downloaded. For details, please refer to the demo attached to this article.

double contentLength = DownloadManager.GetContentLength(myHttpWebClient);

Every time data is read from the file stream, we know how many bytes have been read (currentDataLength), and the cumulative total is the file that has been downloaded. length.

downloadedLength += (long)currentDataLength;

Then just do a division:

doubleDownloadPersent = (double)downloadedLength / contentLength;

Calculate real-time download speed

For the current download speed, we calculate the number of bytes downloaded in the past period of time. The time period can be obtained using StopWatch. The time period I selected requires greater than 800 milliseconds.

if (this._downloadStopWatch.ElapsedMilliseconds > 800)
{
 /***********************************/
 // 计算上一个时间段内的下载速度
 double num5 = (double)currentTimeSpanDataLength / 1024.0;
 double num6 = (double)this._downloadStopWatch.ElapsedMilliseconds / 1000.0;
 double doubleDownloadSpeed = num5 / num6;
 /***********************************/

 intDownloadSpeed = (int)Math.Round(doubleDownloadSpeed, 0);
 // 本次网速计算完成后重置时间计时器和数据计数器，开始下次的计算
 this._downloadStopWatch.Reset();
 this._downloadStopWatch.Start();
 currentTimeSpanDataLength = 0;
}

In fact, the length of the time period for each calculation of download speed is not fixed, but this does not affect the calculation results. I only need to ensure that the distance from the last calculation is Just over 800 milliseconds.

Allow users to cancel downloads

For a task that takes a long time to execute, not allowing users to cancel it is deeply hated! Especially when the internet speed is not very good. So we need to give users a choice: they can end the current journey happily (rather than painfully).

And all this is so simple for us!

The code is as follows:

while ((currentDataLength = stream.Read(buffer, 0, BufferSize)) > 0 && !this._cancelDownload){}

When reading data from the data stream, we check whether the user pressed the "Cancel" button, which is the this._cancelDownload variable here. If it is true, end the current download.

So far, we have solved the most common complaints from users. In fact, not much code has been added, and every knowledge point seems so subtle. But it obviously improves the user experience. This also brings us some inspiration. Completing the main functions may only be part of the work. Other tasks may not be so obvious and require us to constantly experience and discover...

Demo download address: WebClientDemo_jb51. rar

The above is the graphic and textual explanation of the WebClient implementation file download code in C#. For more related content, please pay attention to the PHP Chinese website (www.php.cn)!