Can't catch NullReferenceException while explicitly defined for that - C# Discussion Boards

Re: Can't catch NullReferenceException while explicitly defined for that

1-Oct-21 23:03

Hello fellow members. Im trying to understand why my code is not catching NullReferenceException and i cant figure it out.

I'm using this code to run async function

private async Task SearchMostLessViewsAsync(DateTime dateFrom, DateTime dateTo, int amountOfNumbersSelected, int frequencyOption, int fromDrawNumber, int toDrawNumber)
        {
            Microsoft.AppCenter.Analytics.Analytics.TrackEvent($"{typeof(FragmentDrawsNumbersFrequency).Name}.{nameof(SearchMostLessViewsAsync)}",
                new Dictionary<string, string>()
                    {
                        {nameof(dateFrom), dateFrom.ToString()},
                        {nameof(dateTo), dateTo.ToString()},
                        {nameof(amountOfNumbersSelected), amountOfNumbersSelected.ToString()},
                        {nameof(frequencyOption), frequencyOption.ToString()},
                        {nameof(fromDrawNumber), fromDrawNumber.ToString()},
                        {nameof(toDrawNumber), toDrawNumber.ToString()},
                    }
                );
            ((MainActivity)Activity).DisplayLoadingMessage(true, GetString(Resource.String.Common_SearchTitle),
                GetString(Resource.String.Common_SearchMessage));

            //ApplicationState.ChangeCancellationTokenSource(new CancellationTokenSource());

            var task = Task.Run(async () =>
            {
                var textResult = await SearchLeast(dateFrom, dateTo, amountOfNumbersSelected, frequencyOption, fromDrawNumber, toDrawNumber).ConfigureAwait(false);

                if (!string.IsNullOrEmpty(textResult))
                {
                    UpdateHistoryList(textResult);
                    DatabaseFunctions.SaveHistoryList(HistoryList, Settings.DrawsNumbersFrequencyHistoryListViewKey);

                    ((MainActivity)Activity).ShowSearchResults(textResult);
                }
            }, ApplicationState.GetCancellationToken()); // Pass same token to Task.Run.

            try
            {
                await task.ConfigureAwait(false);
            }
            catch (TaskCanceledException tce)
            {
                Console.WriteLine($"{nameof(TaskCanceledException)} thrown with message: {tce.Message}");
            }
            catch (System.ObjectDisposedException ode)
            {
                Console.WriteLine($"{nameof(System.ObjectDisposedException)} thrown with message: {ode.Message}");
            }
            catch (System.OperationCanceledException e)
            {
                Console.WriteLine($"{nameof(System.OperationCanceledException)} thrown with message: {e.Message}");
            }
            catch (NullReferenceException nre)
            {
                Console.WriteLine($"{nameof(NullReferenceException)} thrown with message: {nre.Message}");
            }
            finally
            {
                ApplicationState.DisposeCancellationTokenSource();
                ((MainActivity)Activity).DisplayLoadingMessage(false);
            }

            ((MainActivity)Activity).DisplayLoadingMessage(false);
        }

But sometimes i get this exception

FragmentDrawsNumbersFrequency.SearchMostLessViewsAsync (System.DateTime dateFrom, System.DateTime dateTo, System.Int32 amountOfNumbersSelected, System.Int32 frequencyOption, System.Int32 fromDrawNumber, System.Int32 toDrawNumber)
FragmentDrawsNumbersFrequency.ShowMostLessViews ()
FragmentDrawsNumbersFrequency.<OnCreateView>b__50_8 (System.Object sender, System.EventArgs e)
AsyncMethodBuilderCore+<>c.<ThrowAsync>b__7_0 (System.Object state)
SyncContext+<>c__DisplayClass2_0.<Post>b__0 ()
Thread+RunnableImplementor.Run ()
IRunnableInvoker.n_Run (System.IntPtr jnienv, System.IntPtr native__this)
(wrapper dynamic-method) Android.Runtime.DynamicMethodNameCounter.61(intptr,intptr)

I'm not sure why. Shouldn't the try catch wrap catch nullreference exceptions? Something tells me i miss something else here

modified 2-Oct-21 8:56am.

Pete O'Hanlon1-Oct-21 23:38

1-Oct-21 23:38

The exception is referring to this method: SearchMostLessViewsAsync. That's not anywhere in the code you're showing here.

Re: Can't catch NullReferenceException while explicitly defined for that

OriginalGriff2-Oct-21 0:33

OriginalGriff

2-Oct-21 0:33

As Pete has said, the error isn't in the code you show, nor is the method that throws the error directly called in any of that code.

Run your program in the debugger and when it fails, it will show you the line it found the problem on. You can then start looking at the various parts of it to see what value is null and start looking back through your code to find out why. So put a breakpoint at the beginning of the method containing the error line, and run your program from the start again. This time, the debugger will stop before the error, and let you examine what is going on by stepping through the code looking at your values.

But we can't do that - we don't have your code, we don't know how to use it if we did have it, we don't have your data. So try it - and see how much information you can find out!

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modified 2-Oct-21 8:14am.

Re: Can't catch NullReferenceException while explicitly defined for that

Pete O'Hanlon2-Oct-21 1:32

2-Oct-21 1:32

Richard? Are you saying I'm a complete and utter Richard? Poke tongue | ;-P

Re: Can't catch NullReferenceException while explicitly defined for that

OriginalGriff2-Oct-21 2:14

OriginalGriff

2-Oct-21 2:14

No I'm saying I'm a complete and utter pillock ... Laugh | :laugh:

Sorry ... fixed. Blush | :O

Re: Can't catch NullReferenceException while explicitly defined for that

Exoskeletor2-Oct-21 3:01

Re: Can't catch NullReferenceException while explicitly defined for that

2-Oct-21 3:01

what means to be a Richard? Smile | :)

i didnt get the joke

Pete O'Hanlon2-Oct-21 3:51

2-Oct-21 3:51

Griff called me Richard. He's edited the post to use my name now.

Re: Can't catch NullReferenceException while explicitly defined for that

OriginalGriff2-Oct-21 3:55

OriginalGriff

2-Oct-21 3:55

I made a mistake, and referred to Pete as Richard, is all. No joke, just a mistake on my part.

Re: Can't catch NullReferenceException while explicitly defined for that

Exoskeletor2-Oct-21 3:58

Re: Can't catch NullReferenceException while explicitly defined for that

2-Oct-21 3:58

oh i though it had a deeper meaning i didn't get Smile | :)

Exoskeletor2-Oct-21 2:58

Re: Can't catch NullReferenceException while explicitly defined for that

2-Oct-21 2:58

oh yes you are right, sorry i have used the wrong part of the code. i edit the question again.
I cant reproduce the bug in my device, it is happening in around 2% of the devices that is being used, this code is from an app that is available from google play already

Pete O'Hanlon3-Oct-21 20:22

3-Oct-21 20:22

You're making the assumption that the method that is throwing the null reference is inside your task. What's interesting is that you have failed to apply any forms of input guards to SearchMostLessViewsAsync and you're assuming you actually have values coming in. It wouldn't surprise me if the null reference you are seeing occurred in this call:

Microsoft.AppCenter.Analytics.Analytics.TrackEvent($"{typeof(FragmentDrawsNumbersFrequency).Name}.{nameof(SearchMostLessViewsAsync)}",
    new Dictionary<string, string>()
        {
            {nameof(dateFrom), dateFrom.ToString()},
            {nameof(dateTo), dateTo.ToString()},
            {nameof(amountOfNumbersSelected), amountOfNumbersSelected.ToString()},
            {nameof(frequencyOption), frequencyOption.ToString()},
            {nameof(fromDrawNumber), fromDrawNumber.ToString()},
            {nameof(toDrawNumber), toDrawNumber.ToString()},
        }
    );

If any of those is null, you'll get a null reference because you're calling ToString() on it. The bottom line is that you shouldn't make assumptions about where exceptions are occurring; look at the whole part of the code.

Re: Can't catch NullReferenceException while explicitly defined for that

Exoskeletor4-Oct-21 0:21

how read file from Open with in C#

4-Oct-21 0:21

hello, those values cannot ever be null from the way im setting them app and also from the crash reports they are not null, they have a value, the crash comes later. some times this code doesnt crash. some times it does. even in the devices that crash it doesnt crash always.

From what i understand my biggest problem is that i dont try catch inside the task and i loose exceptions because of that. I have added a try catch inside the task and now i see that even in my device thaat never crash sometimes i get an error about ssl.

The correct way is to check all those values if they are null in the beggining of the function even if by the way are initialized always have value?

Member 1522978130-Sep-21 6:24

Member 15229781

30-Sep-21 6:24

im Programing one Program(editor)

my question is how i can open one file txt in open with in right click and my program read the file

can you help please???

I searched for this question a lot but I could not find anything. Please help me find the answer to this question.

thanks

Re: how read file from Open with in C#

Dave Kreskowiak30-Sep-21 8:08

Dave Kreskowiak

30-Sep-21 8:08

You're going to have to be more specific. "Right-click" in what?

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Dave Kreskowiak

Re: how read file from Open with in C#

Luc Pattyn30-Sep-21 8:30

Luc Pattyn

30-Sep-21 8:30

windows add my program to "open with" in context menu - Google Search[^]

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Question About Async

Kevin Marois29-Sep-21 10:21

Kevin Marois

29-Sep-21 10:21

I see a lot of tutorials and examples that define controller methods like this:

[HttpGet]
public async Task<IEnumerable<Book>> GetBooks()
{
    return await _bookRepository.Get();
}

[HttpGet("{id}")]
public async Task<ActionResult<Book>> GetBooks(int id)
{
    return await _bookRepository.Get(id);
}

In these examples, the method signature returns a Task so that the method is async.

But can I just do this in my front end?

private async void LoadBooks()
{
    List books = null;

    await Task.Run(() =>
    {
        books = _proxy.GetBooks()

    }).ContinueWith(x => 
    {
        Books = new List(books);
    });
}

Why make the controller methods return task and use await? Doesn't my front end code acheive that?

If it's not broken, fix it until it is.
Everything makes sense in someone's mind.
Ya can't fix stupid.

modified 29-Sep-21 17:47pm.

Re: Question About Async

Pete O'Hanlon29-Sep-21 20:22

29-Sep-21 20:22

The reason that you return Task from an async method rather than void relates to the fact that returning void consumes exceptions that are being thrown. If you return Task, you have the ability to handle the exception gracefully. As for your code sample, you don't need to add an await on a Task.Run to get the ContinueWith to happen. In fact, you should remove that await altogether - if you're manually running up a Task here, you don't need async/await constructs.