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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?
330
Call Dispose when you are finished using the CancellationTokenSource. The Dispose method leaves the CancellationTokenSource in an unusable state.
CancellationTokenSource - This is the object responsible for creating a cancellation token and sending a cancellation request to all copies of that token. CancellationToken - This is the structure used by listeners to monitor the token's current state.
The associated CancellationToken will be notified of the cancellation and will transition to a state where IsCancellationRequested returns true. Any callbacks or cancelable operations registered with the CancellationToken will be executed.
A CancellationTokenSource object, which provides a cancellation token through its Token property and sends a cancellation message by calling its Cancel or CancelAfter method. A CancellationToken object, which indicates whether cancellation is requested.
"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! :)
200
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