How to download image from SharePoint with WebClient by image URL in C#?

Question

In my project, i need to download the image and do some checking, then delete and re-upload it. However, i got an error when i trying to download the image.

    foreach (SPListItem item in docLib.Items)//SPList
    {
        SPFile file = item.File;
        if (file != null)
        {
            string fileURL = file.Url;  //libraryname/foldername/filename.extension
            string fileName = file.Name;//filename.extension
            string folderURL = fileURL.Substring(0, fileURL.Length - (fileName.Length + 1));//libraryname/foldername
            string fileExtension = fileName.Substring(fileName.Length - 5);// .extension, assume it may contain 4 chars. (e.g. .tiff)

            Console.WriteLine("file url: " + fileURL);
            Console.WriteLine("folder url: " + folderURL);


            if (too.checkExtension(fileExtension))
            {
                Console.WriteLine("True");// is image type

                    WebClient wc = new WebClient();
                    byte[] bytes = wc.DownloadData(siteurl + fileURL);
                    MemoryStream ms = new MemoryStream(bytes);
                    Image myImage = Image.FromStream(ms);


                    otherClass.Checking(myImage); // my job
                    item.Delete();// delete item directly 

                    web.Files.Add(string.Format("{0}/{1}", fileURL, fileName), bytes, false);// upload it to original place


            }
        }
    }

The error is about:

unhandled Exception: System.Net.WebException: The remote server returned an error: <401> Unauthorized. at System.Net.WebClient.DownloadDataInternal

So, how to fix it? Moreover, can anyone tell me the feasibility of my delete and upload part.

Answer 1

You need to use the CSOM (Client Object Model) To Download your image

Here you use WebClient with this, you loose your sharePoint context, and so your are not connected to sharePoint when want to download image by using : byte[] bytes = wc.DownloadData(siteurl + fileURL);
Try to use methode of the SPFile object, like OpenBinaryStream to get the Byte of your file in a Stream.

Answer 2

No need for CSOM.

WebClient, by default, does not pass on credentials. You will need to configure it to either use specific credentials or the credentials of the current user using wc.UseDefaultCredentials (see: https://stackoverflow.com/questions/12212116/how-to-get-httpclient-to-pass-credentials-along-with-the-request for an explanation).