Moby Dick of exceptions

Question

There are multiple questions (1,2,3,4 etc. etc.) called "Why isn't this exception caught". Sadly, none of these solutions work for me... So I am stuck with a truly uncatchable exception.

I have a piece of code (.NET 4.0) that checks a large textfile for digits and numbers. Whilst testing I got a runtime exception:

What you see here is a try-catch pattern with a catchblock for an ArgumentOutOfRangeException. But during runtime, the try block throws an ArgumentOutOfRangeException that is not being caught.

I read the C# language specification section about the try-catch structure, and it says:

A catch block of a try statement is reachable if the try statement is reachable.

So in theory the above code should catch the exception.

Then I thought it might had something to do with the fact that this code is running in a task (during the processing of the textfile I also want to update the UI so I do it asynchronous). I searched around and then I found this answer by Jon Skeet. Basically suggesting I use Task.Wait in a try-catch block to catch any exceptions.

The problem I am facing now is that I can't really call Task.Wait because that would block the calling thread which is my UI thread! Then I figured that I could create an extra tasklayer to wait for that task:

//Code called from the UI
System.Threading.Tasks.Task.Factory.StartNew(()=>
{
    //Create a new task and use this task to catch any exceptions
    System.Threading.Tasks.Task task = System.Threading.Tasks.Task.Factory.StartNew(MethodWithException);
    try
    {
        task.Wait();
    }
    catch(Exception)
    {
        MessageBox.Show("Caught it!");
    }
});

But this still gives the same result... Then I thought that it could be because of the fact I am not specific enough with my Exceptiontype. But the C# Language Specification states:

Some programming languages may support exceptions that are not representable as an object derived from System.Exception, although such exceptions could never be generated by C# code.

So unless you use some sketchy third party API you're always good when you use Exception. So I found myself with an suggested answer by Jon Skeet that didn't quite work for me. That's when I knew I should just stop trying...

So does anyone know what is going on? And how can I fix this? I know I could just check if i is equal or bigger than text.Length but understanding what's happening is more important than working code.

Answer 1

This is simply an artifact of the debugger.

In the Debug menu, there's an option called Exceptions... Click it, and make sure to uncheck the "Thrown" checkbox here:

Many times, you'll want to see the error in context, even if it's inside a try/catch, which is what this setting is for. In this case, that is exactly what you ought to be doing, so that you can see compare i to the length of text and see where your problem is.

If you ran the code without the debugger (such as by double-clicking the executable or using the "Start without Debugging" option), you would "correctly" throw away the error without any alerts.

Answer 2

I just wrote the following test:

    [TestMethod]
    public void ArgumentOutOfRangeExceptionTest()
    {
        string test = "abc";
        int i = 0;
        try
        {
            while (true)
            {
                test.ElementAt(i);
                i++;
            }
        }
        catch (ArgumentOutOfRangeException)
        { }
    }

It is working fine. I believe that you had another exception, which is not called in your calling code.

There is only one exception I know which can't be caught. It's the StackOverflowException. See this question about it.