Question

I use the following method in a piece of production code:

private void DownloadData(Uri uri)
{
    WebClient webClient = new WebClient();
    DownloadDataCompletedEventHandler eh = null;
    eh = delegate(object sender, DownloadDataCompletedEventArgs e)
        {
            webClient.DownloadDataCompleted -= eh;
            ((IDisposable) webClient).Dispose();
            OnDataDownloaded();
        };
    webClient.DownloadDataCompleted += eh;
    webClient.DownloadDataAsync(uri);
}

I am now worried that a hard to reproduce bug might be caused by the WebClient instance being garbage collected before the DownloadDataCompleted event is called: after exiting my DownloadData() method, there are no obvious references to the WebClient object, so that could plausibly happen.

So my question is: can this realistically happen? I can not reproduce the problem, so there might be some internal things happening that prevents the WebClient object from being garbage collected (e.g. the object might register itself with a global object somewhere while waiting for the response).

The code is running on .NET 2.0 if that makes any difference.

Answer 1

No, your object won’t be GC-ed until the callback completes. According to Does the Garbage Collector destroy temporarily unreferenced objects during async calls in .NET?, “the async API keeps a reference to your request (within the thread pool where async IO operations are lodged) and so it won’t be garbage collected until it completes.”

But, your code is also doing stuff it doesn’t need to: you don’t need to detach the event handler and don’t need to call Dispose on the webclient. (Dispose() is actually not implemented by WebClient– you can can see this in the .NET Framework reference source at http://referencesource.microsoft.com/netframework.aspx).

So you don’t actually need to refer to the webclient instance in your callback. In other words, the following code will work just as well, and avoid any potential issues (discussed above) of referencing external local variables from inside a delegate.

private void DownloadData(Uri uri)
{
    WebClient webClient = new WebClient();
    DownloadDataCompletedEventHandler eh = null;
    eh = delegate(object sender, DownloadDataCompletedEventArgs e)
    {
        OnDataDownloaded();
    };
    webClient.DownloadDataCompleted += eh;
    webClient.DownloadDataAsync(uri);
}

Anyway, you probably want to look elsewhere for the source of your bug. One place I’d look is at the results of the HTTP calls– you may be running out of memory, may be running into server errors, etc. You can look at e.Error to see if the calls are actually working.