c# - 对象处置异常 : The CancellationTokenSource has been disposed

Question

我开始在 MacBook Pro 上使用 Xamarin Studio 进行 Xamarin.Forms 开发。我构建了一个应用程序，其目的是查询 PrestaShop 网站、检索产品并显示它们。

在将应用程序部署到Android时，我在Marshmallow以下的版本中遇到了一些问题，但我解决了它们，所以我不会在这里描述它们。

将应用程序部署到 iOS(模拟器)时，我仍然遇到一个严重问题。应用程序运行，但当我单击按钮检索数据时，它崩溃并给出 System.ObjectDisposeException，其消息是“CancellationTokenSource 已被处置”。我将相关的源代码粘贴在这里:

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;
    }
}

我该如何解决这个问题？

Answer 1

"The CancellationTokenSource has been disposed"

看来您的 using (var client = new HttpClient())... 的 CancellationToken 已被第一个 client.GetStringAsync 消耗() 调用。

如何解决此问题
我建议将第二个调用放在自己的 using block 中，以避免这种情况。

您的代码现在应如下所示，其中包含第二个 using 语句:

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;
    }
}

希望这有帮助! :)