I am working on a simple test project where if you type into an entry any stock ticker symbol it produces a WebClient and link that returns a string of that stock price.
I know the exact URL that I need to acquire the data from, and if the user inputs an existing ticker symbol it works perfectly fine. However, I have a run-time error if the user makes up a ticker symbol, as the program tries to pull data from a non-existent web page.
On run of the application, it goes directly into break mode and returning the error System.NullReferenceException: 'Object reference not set to an instance of an object.'
.
How can I check if the URL is valid before making the request or at least be able to catch the error before putting the application into break mode?
This is the c# code:
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Net;
using Xamarin.Forms;
using Xamarin.Forms.Xaml;
namespace Stock_WatchList
{
[XamlCompilation(XamlCompilationOptions.Compile)]
public partial class sybmolSelector : ContentPage
{
public string price { get; set; }
public sybmolSelector()
{
InitializeComponent();
BindingContext = this;
}
private void Entry_PropertyChanged(object sender, System.ComponentModel.PropertyChangedEventArgs e)
{
var nameValue = entry.Text.ToString();
var link = "https://sandbox.iexapis.com/stable/stock/" + nameValue + "/price?token=Tsk_57bebc1d051340dbaad8656ab0027e90";
var client1 = new WebClient();
string b = client1.DownloadString(link);
price = "$" + b;
Symbol.Text = nameValue.ToString();
Price.Text = price;
}
}
}
and the Xamarin code:
<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
x:Class="Stock_WatchList.sybmolSelector"
BackgroundColor="Black"
NavigationPage.IconColor="HotPink">
<ContentPage.Content>
<StackLayout>
<Label TextColor="HotPink" BackgroundColor="Black" Text="------------------- Enter A Symbol Below -------------------" FontSize="19" LineBreakMode="TailTruncation"></Label>
<Entry PlaceholderColor="HotPink" PropertyChanged="Entry_PropertyChanged" FontSize="19" x:Name="entry" TextColor="HotPink" BackgroundColor="#111111"></Entry>
<Label x:Name="Symbol" Margin="5" Text="" HorizontalOptions="Start" TextColor="HotPink" FontSize="40"/>
<Label x:Name="Price" Margin="5,0,5,5" Text="" HorizontalOptions="Start" TextColor="HotPink" FontSize="50"/>
</StackLayout>
</ContentPage.Content>
</ContentPage>
Don't make a call if Entry.Text is null or empty. When your form gets loaded PropertyChanged will get called. At that moment Text is still null. It certainly doesn't have a default in your markdown.
private void Entry_PropertyChanged(object sender, System.ComponentModel.PropertyChangedEventArgs e)
{
// guard for null
if (entry == null) return;
// no need to call the api if we don't have a value
if (String.IsNullOrEmpty(entry.Text)) return;
var link = "https://sandbox.iexapis.com/stable/stock/" + entry.Text + "/price?token=sometoken";
using(var client1 = new WebClient()) // WebClient does implement (useless) IDisposable
{
price = "$" + client1.DownloadString(link);
}
// maybe these are null as well, who knows
if (Symbol != null) Symbol.Text = entry.Text;
if (Price != null) Price.Text = price;
}
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