从 .NET/C# 中的站点下载图像

Question

我正在尝试从该站点下载图像。当图像可用时，我使用的代码工作正常。如果图像不可用，则会产生问题。如何验证图像的可用性？

代码：

方法一：

WebRequest requestPic = WebRequest.Create(imageUrl);

WebResponse responsePic = requestPic.GetResponse();

Image webImage = Image.FromStream(responsePic.GetResponseStream()); // Error

webImage.Save("D:\\Images\\Book\\" + fileName + ".jpg");

方法二：

WebClient client = new WebClient();
Stream stream = client.OpenRead(imageUrl);

bitmap = new Bitmap(stream); // Error : Parameter is not valid.
stream.Flush();
stream.Close();
client.dispose();

if (bitmap != null)
{
    bitmap.Save("D:\\Images\\" + fileName + ".jpg");
}

编辑：

Stream 有以下语句：

      Length  '((System.Net.ConnectStream)(str)).Length' threw an exception of type  'System.NotSupportedException'    long {System.NotSupportedException}
    Position  '((System.Net.ConnectStream)(str)).Position' threw an exception of type 'System.NotSupportedException'    long {System.NotSupportedException}
 ReadTimeout  300000    int
WriteTimeout  300000    int

Answer 1

无需涉及任何图片类，直接调用WebClient.DownloadFile即可：

string localFilename = @"c:\localpath\tofile.jpg";
using(WebClient client = new WebClient())
{
    client.DownloadFile("http://www.example.com/image.jpg", localFilename);
}

---

更新
由于您需要检查文件是否存在并下载文件，因此最好在同一个请求中执行此操作。所以这里有一个方法可以做到这一点：

private static void DownloadRemoteImageFile(string uri, string fileName)
{
    HttpWebRequest request = (HttpWebRequest)WebRequest.Create(uri);
    HttpWebResponse response = (HttpWebResponse)request.GetResponse();

    // Check that the remote file was found. The ContentType
    // check is performed since a request for a non-existent
    // image file might be redirected to a 404-page, which would
    // yield the StatusCode "OK", even though the image was not
    // found.
    if ((response.StatusCode == HttpStatusCode.OK || 
        response.StatusCode == HttpStatusCode.Moved || 
        response.StatusCode == HttpStatusCode.Redirect) &&
        response.ContentType.StartsWith("image",StringComparison.OrdinalIgnoreCase))
    {

        // if the remote file was found, download oit
        using (Stream inputStream = response.GetResponseStream())
        using (Stream outputStream = File.OpenWrite(fileName))
        {
            byte[] buffer = new byte[4096];
            int bytesRead;
            do
            {
                bytesRead = inputStream.Read(buffer, 0, buffer.Length);
                outputStream.Write(buffer, 0, bytesRead);
            } while (bytesRead != 0);
        }
    }
}

简而言之，它发出文件请求，验证响应代码是OK、Moved 或Redirect 之一并且ContentType 是图像.如果这些条件为真，则下载文件。

Answer 2

我在一个项目中使用了 Fredrik 的上述代码，稍作修改，我想分享一下：

private static bool DownloadRemoteImageFile(string uri, string fileName)
{
    HttpWebRequest request = (HttpWebRequest)WebRequest.Create(uri);
    HttpWebResponse response;
    try
    {
        response = (HttpWebResponse)request.GetResponse();
    }
    catch (Exception)
    {
        return false;
    }

    // Check that the remote file was found. The ContentType
    // check is performed since a request for a non-existent
    // image file might be redirected to a 404-page, which would
    // yield the StatusCode "OK", even though the image was not
    // found.
    if ((response.StatusCode == HttpStatusCode.OK ||
        response.StatusCode == HttpStatusCode.Moved ||
        response.StatusCode == HttpStatusCode.Redirect) &&
        response.ContentType.StartsWith("image", StringComparison.OrdinalIgnoreCase))
    {

        // if the remote file was found, download it
        using (Stream inputStream = response.GetResponseStream())
        using (Stream outputStream = File.OpenWrite(fileName))
        {
            byte[] buffer = new byte[4096];
            int bytesRead;
            do
            {
                bytesRead = inputStream.Read(buffer, 0, buffer.Length);
                outputStream.Write(buffer, 0, bytesRead);
            } while (bytesRead != 0);
        }
        return true;
    }
    else
        return false;
}

主要变化是：

• 对 GetResponse() 使用 try/catch，因为当远程文件返回 404 时我遇到了异常
• 返回一个布尔值

Answer 3

        private static void DownloadRemoteImageFile(string uri, string fileName)
        {
            HttpWebRequest request = (HttpWebRequest)WebRequest.Create(uri);
            HttpWebResponse response = (HttpWebResponse)request.GetResponse();

            if ((response.StatusCode == HttpStatusCode.OK ||
                response.StatusCode == HttpStatusCode.Moved ||
                response.StatusCode == HttpStatusCode.Redirect) &&
                response.ContentType.StartsWith("image", StringComparison.OrdinalIgnoreCase)) 
            {
                using (Stream inputStream = response.GetResponseStream())
                using (Stream outputStream = File.OpenWrite(fileName))
                {
                    byte[] buffer = new byte[4096];
                    int bytesRead;
                    do
                    {
                        bytesRead = inputStream.Read(buffer, 0, buffer.Length);
                        outputStream.Write(buffer, 0, bytesRead);
                    } while (bytesRead != 0);
                }
            }
        }

Answer 4

也可以使用 DownloadData 方法

    private byte[] GetImage(string iconPath)
    {
        using (WebClient client = new WebClient())
        {
            byte[] pic = client.DownloadData(iconPath);
            //string checkPath = Environment.GetFolderPath(Environment.SpecialFolder.MyDocuments) +@"\1.png";
            //File.WriteAllBytes(checkPath, pic);
            return pic;
        }
    }

Answer 5

从服务器或网站下载图像并将其存储在本地的最佳做法。

WebClient client=new Webclient();
client.DownloadFile("WebSite URL","C:\\....image.jpg");
client.Dispose();

Answer 6

您可以使用此代码

using (WebClient client = new WebClient()) {
                    Stream stream = client.OpenRead(imgUrl);
                    if (stream != null) {
                        Bitmap bitmap = new Bitmap(stream);
                        ImageFormat imageFormat = ImageFormat.Jpeg;
                        if (bitmap.RawFormat.Equals(ImageFormat.Png)) {
                            imageFormat = ImageFormat.Png;
                        }
                        else if (bitmap.RawFormat.Equals(ImageFormat.Bmp)) {
                            imageFormat = ImageFormat.Bmp;
                        }
                        else if (bitmap.RawFormat.Equals(ImageFormat.Gif)) {
                            imageFormat = ImageFormat.Gif;
                        }
                        else if (bitmap.RawFormat.Equals(ImageFormat.Tiff)) {
                            imageFormat = ImageFormat.Tiff;
                        }

                        bitmap.Save(fileName, imageFormat);
                        stream.Flush();
                        stream.Close();
                        client.Dispose();
                    }
                }

项目地址：github