基本上，这是每当单击wpf按钮时执行的方法：

private void Dial(object sender, RoutedEventArgs e)
        {
            SendDataAsync(sender, e);
        }

private async Task SendDataAsync(object sender, RoutedEventArgs e)
        {   //this method does the actual dialing part
            var toSend = new Dictionary<string, string>
            {
                { "ClientID", ConfigurationManager.AppSettings["ClientID"] },
                { "PhoneNumber", numberToSend }
            };

            string jsonString = JsonSerializer.Serialize(toSend);

            

            var content = new StringContent(jsonString);

            content.Headers.Add("Content-type", "text/json");
            content.Headers.Add("Content-type", "application/json");

            var response = await client.PostAsync("https://webhook.site/4788266d-9caf-4747-a3d2-09777b233435", content);

            var responseString = await response.Content.ReadAsStringAsync();

        }

我已经尝试注解掉“content.headers.add”行，但错误仍然存在
EDIT：这是debug输出中显示的完整异常：抛出异常：System.Net.Http.dll中的“System.FormatException”