WebClient does not support concurrent I/O operations

Question

How can I get this error from with in the DownloadStringCompleted Event? Doesn\'t that mean, it\'s finished? Is there another event I can fire this from?

I get this

Answer 1

Instead of using WebClient use HttpClient to do parallel HTTP calls. Below code shows how to download files.

        HttpClient httpClient = new HttpClient();
        var documentList=_documentManager.GetAllDocuments();
        documentList.AsParallel().ForAll(doc =>
        {

            var responseResult= httpClient.GetAsync(doc.FileURLPath);
            using (var memStream = responseResult.Result.Content.ReadAsStreamAsync().Result)
            {
                using (var fileStream =File.Create($"{filePath}\\{doc.FileName}"))
                {
                    memStream.CopyTo(fileStream);
                }

            }
        });

Answer 2

The solution, I found is to use multiple WebClient objects, so to modify your pseudocode example; try

var client = new WebClient("URL 1");
client.CompletedEvent += CompletedEvent;
client.downloadasync();

void CompletedEvent(){
Dosomestuff;
var client2 = new WebClient();
client2.downloadasync(); 
}

Answer 3

The WebClient only supports a single operations, it cannot download multiple files. You haven't shown your code, but my guess is that you are somehow firing a new request before the old is completed. My bet is that WebClient.IsBusy is true when you attempt to perform another fetch.

See the following thread:

wb.DownloadFileAsync throw "WebClient does not support concurrent I/O operations." exception

Answer 4

The only answer is to create a new webclient within the scope of the Completed Event. You can't set it to new since webclient is readonly. Creating a new client is the only solution. This allows the old client to complete in the background. This does have slight memory implications since you are creating a new instance instead of reusing an old. But the garbage collector should keep it clean if your scope is setup right.