Question

I have a refresh button to update news in my WP7 application. When I double or triple click on the refresh button I am getting an error

"WebClient does not support concurrent I/O operations" .

I think thats because It is sending the request triple times and making it crash. Here is my Click code.

    private void NewsRefresh_Click(object sender, RoutedEventArgs e)
    {
        var vm = this.DataContext as MainPageViewModel;
        if (vm != null)
        {
            vm.UpdateNews();
        }
    }

How can I turn it as “if It is busy cancel the process”.

Answer 1

The ‘easy’ way (though not bullet proof):

private void NewsRefresh_Click(object sender, RoutedEventArgs e)
{
    try
    {
        NewsRefresh.Enabled = false;
        var vm = this.DataContext as MainPageViewModel;
        if (vm != null)
        {
            vm.UpdateNews();
        }
    }
    finally
    {
        NewsRefresh.Enabled = true;
    }
}

The more difficult approach would require more details on what exactly a MainPageViewModel is, and what UpdateNews() does. Basically you need to store a state value wherever you are storing the WebClient instance. Before using the WebClient you need check to see if you are already using it. The issue comes when multiple threads may operate on a single instance, or if you multiple operations (other than UpdateNews). When multiple threads are involved the easiest thing is to surround the usage of the WebClient with a Mutex.

Of course the other option is to not reuse the WebClient instance, rather create a new one for each new request.

UPDATE

Well, well, using DownloadStringAsync is certainly going to make things fun. The above code Disabling the UI will not work unless you move the re-enabling code. It would be easiest to go with my last suggestion and just create a new instance of WebClient. I’m not real fond of WebClient myself and prefer using WebRequest.Create.