ObjectDisposedException: The CancellationTokenSource has been disposed

Question

I'm starting to develop with Xamarin.Forms using Xamarin Studio on my MacBook Pro. I built an application whose purpose is to query a PrestaShop Website, retrieve the Products and show them.

When deploying the application to Android, I had some problems with versions below Marshmallow, but I solved them, so I won't describe them here.

When deploying the application to iOS (Simulator), I'm still having a critical problem. The application runs, but when I click on the button to retrieve the data, it crashes giving me a System.ObjectDisposedException, whose message is "The CancellationTokenSource has been disposed". I'll paste here the relevant source code:

async void button1_Click(object sender, System.EventArgs e)
{
    try
    {
        HttpClientHandler hnd = new HttpClientHandler();
        hnd.Credentials = new NetworkCredential("[apikey]", "");
        string res;
        using (var client = new HttpClient(hnd))
        {
            var responseText = await client.GetStringAsync("[endpoint]/products");
            using (MemoryStream stream = new MemoryStream())
            using (StreamWriter writer = new StreamWriter(stream))
            {
                writer.Write(responseText.Replace("<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n", string.Empty));
                writer.Flush();
                stream.Position = 0;
                XDocument doc = XDocument.Load(stream);
                res = JsonConvert.SerializeXNode(doc, Formatting.Indented, true);
            }
            var data = JsonConvert.DeserializeObject<Dictionary<string, object>>(res);
            //Result.Text = data["products"].GetType().ToString() + Result.Text;
            Func<string, Task> creator_method = async (string id) =>
            {
                try
                {
                    var responseProd = await client.GetStringAsync($"[endpoint]/products/{id}"); // AT THIS ROW THE EXCEPTION IS RAISED!!!
                    using (MemoryStream stream = new MemoryStream())
                    using (StreamWriter writer = new StreamWriter(stream))
                    {
                        writer.Write(responseProd.Replace("<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n", string.Empty));
                        writer.Flush();
                        stream.Position = 0;
                        XDocument doc = XDocument.Load(stream);
                        res = JsonConvert.SerializeXNode(doc, Formatting.Indented, true);
                    }
                    var product_rawData = JsonConvert.DeserializeObject<Dictionary<string, object>>(res);
                    var productData = (JObject)product_rawData["product"];
                    try
                    {
                        views.Children.Add(new ProductXaml(productData["name"]["language"]["#cdata-section"].ToString(), float.Parse(productData["price"]["#cdata-section"].ToString().Replace('.', ',')), productData["id_default_image"]["@xlink:href"].ToString()));
                    }
                    catch (NullReferenceException nre)
                    {
                        views.Children.Add(new ProductXaml(productData["name"]["language"]["#cdata-section"].ToString(), float.Parse(productData["price"]["#cdata-section"].ToString().Replace('.', ',')), ""));
                    }
                }
                catch (Exception gex) { throw; }
            };
            foreach (var product in ((JObject)data["products"])["product"])
                try
                {
                    Device.BeginInvokeOnMainThread(async () => { await creator_method.Invoke(product["@id"].ToString()); });
                }
                catch (TaskCanceledException tce) { continue; }
                catch (WebException we) { continue;}
        }
    }
    catch (HttpRequestException ex)
    {
        Result.Text = ex.Message;
    }
}

How could I solve the problem?

Answer 1

"The CancellationTokenSource has been disposed"

It seems that your using (var client = new HttpClient())...'s CancellationToken has already been consumed by the first client.GetStringAsync() call.

How To Fix This
I would suggest putting your second call in its own using block, to avoid this.

Here's what your code should now look like, with a second using statement:

async void button1_Click(object sender, System.EventArgs e)
{
    try
    {
        HttpClientHandler hnd = new HttpClientHandler();
        hnd.Credentials = new NetworkCredential("[apikey]", "");
        string res;
        using (var client = new HttpClient(hnd))
        {
            var responseText = await client.GetStringAsync("[endpoint]/products");
            using (MemoryStream stream = new MemoryStream())
            using (StreamWriter writer = new StreamWriter(stream))
            {
                writer.Write(responseText.Replace("<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n", string.Empty));
                writer.Flush();
                stream.Position = 0;
                XDocument doc = XDocument.Load(stream);
                res = JsonConvert.SerializeXNode(doc, Formatting.Indented, true);
            }
            var data = JsonConvert.DeserializeObject<Dictionary<string, object>>(res);
            //Result.Text = data["products"].GetType().ToString() + Result.Text;
            Func<string, Task> creator_method = async (string id) =>
            {
                try
                {
                    using (var newClient = new HttpClient(hnd)) // Create a new client to avoid your other one's token being consumed
                    {
                        var responseProd = await newClient.GetStringAsync($"[endpoint]/products/{id}");
                        using (MemoryStream stream = new MemoryStream())
                        using (StreamWriter writer = new StreamWriter(stream))
                        {
                            writer.Write(responseProd.Replace("<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n", string.Empty));
                            writer.Flush();
                            stream.Position = 0;
                            XDocument doc = XDocument.Load(stream);
                            res = JsonConvert.SerializeXNode(doc, Formatting.Indented, true);
                        }
                        var product_rawData = JsonConvert.DeserializeObject<Dictionary<string, object>>(res);
                        var productData = (JObject)product_rawData["product"];
                        try
                        {
                            views.Children.Add(new ProductXaml(productData["name"]["language"]["#cdata-section"].ToString(), float.Parse(productData["price"]["#cdata-section"].ToString().Replace('.', ',')), productData["id_default_image"]["@xlink:href"].ToString()));
                        }
                        catch (NullReferenceException nre)
                        {
                            views.Children.Add(new ProductXaml(productData["name"]["language"]["#cdata-section"].ToString(), float.Parse(productData["price"]["#cdata-section"].ToString().Replace('.', ',')), ""));
                        }
                    }
                }
                catch (Exception gex) { throw; }
            };
            foreach (var product in ((JObject)data["products"])["product"])
                try
                {
                    Device.BeginInvokeOnMainThread(async () => { await creator_method.Invoke(product["@id"].ToString()); });
                }
                catch (TaskCanceledException tce) { continue; }
                catch (WebException we) { continue;}
        }
    }
    catch (HttpRequestException ex)
    {
        Result.Text = ex.Message;
    }
}

Hope this helps! :)