NullReferenceException when checking variable is Null

Question

I get this weird NullReferenceExcpetion, in a WPF application. I don't get this every time, even if I do same operation. Can anyone please explain the reason for this,

 public class AmazonUrl
 {
        public string Url { get; set; }
 }

 public partial class MainWindow : Window
 {

    public ObservableCollection<AmazonUrl> AmazonUrlList { get; set; }

    public MainWindow()
    {
        InitializeComponent();

        DataContext = this;
        AmazonUrlList = new ObservableCollection<AmazonUrl>();
    }

    public List<string> getURLList()
    {
        List<string> urlList = new List<string>();

        for(int i = 0; i < AmazonUrlList.Count; i++)
        {
            AmazonUrl url = AmazonUrlList[i];

            if (url == null)
                continue;

            String str = url.Url.ToString().Trim();

            if (str.Length > 0)
               urlList.Add(str);
       }

       return urlList;
   }

    private void openMenuItem_Click(object sender, RoutedEventArgs e)
    {
        List<string> urlList = getURLList();//This is where exception occur

        OpenFileDialog openFileDialog = new OpenFileDialog();
        openFileDialog.Filter = "SCR File (.scr)|*.scr|All Files|*.*";

        if (openFileDialog.ShowDialog() != true){ ... }
    }

Note: After adding these lines I made lots of changes to the application, and recompiled several times. So this is not a problem with Build or Compiling

Edit: My stack trace can find from here, https://pastebin.com/2vyH1qah

Stack trace would be useful - my money is on an overridden operator == causing the issue. — Defenestrator
– Defenestrator, Commented Oct 3, 2017 at 17:38
Is that definitely all that's in the AmazonUrl class? Are you able to reproduce this with a minimal reproducible example? — Jon Skeet
– Jon Skeet, Commented Oct 3, 2017 at 17:39
@maamaa we need the exception stack trace, not the call stack. It should be printed to the Output window. — Defenestrator
– Defenestrator, Commented Oct 3, 2017 at 17:40
@maamaa you can use a collection of strings as a source too, you just need to define the column header yourself in xaml — Scott Chamberlain
– Scott Chamberlain, Commented Oct 3, 2017 at 18:09

rw_ · Accepted Answer · 2017-10-03 20:16:27Z

1

I can't reproduce the error, it must be something with your collection. What does your collection look like? This should fix the problem either way--

public partial class MainWindow : Window
{
    private static object lockObj = new object(); //Add

...

public List<string> getURLList()
  {
     List<string> urlList = new List<string>();

     for (int i = 0; i < AmazonUrlList.Count; i++)
     {
        Amazon url = new AmazonUrl();  // Add
        url = AmazonUrlList[i];        // Update

...

private void openMenuItem_Click(object sender, RoutedEventArgs e)
  {
     lock (lockObj) // Add
     {
        List<string> urlList = getURLList(); //This is where exception occur
     }

answered Oct 3, 2017 at 20:16

rw_

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