Caught exception is null itself !

Question

I have an ASP.NET applications. Everything was fine, but recently I get exceptions that are null themselves:

try
{
    // do something
}
catch (Exception ex)
{
    Logger.Log("Error while tried to do something. Error: " + ex.Message);
}

Sometimes ex is null itself !

Any idea?

Solution 1

For anyone ending up here, I've found an instance where this is possible (If only detectable in the debugger). VS2013 Update 4.

Broken:

try
{
    // do something
}
catch (WebException ex) // <- both variables are named 'ex'
{
    Logger.Log("Error while tried to do something. Error: " + ex.Message);
}
catch (Exception ex) // <- this 'ex' is null
{
    Logger.Log("Error while tried to do something. Error: " + ex.Message);
}

The solution is to name your exception variables differently.

Fixed:

try
{
    // do something
}
catch (WebException webEx) // <- all good in the hood
{
    Logger.Log("Error while tried to do something. Error: " + webEx.Message); // <-
}
catch (Exception ex) // <- this 'ex' correctly contains the exception
{
    Logger.Log("Error while tried to do something. Error: " + ex.Message);
}

Solution 2

In my case, the cause was a StackOverflowException. Such exceptions normally don't reach the catch block at all, but this time, for some reason I don't understand, it did reach the catch block, but the exception was null.

Solution 3

I just ran into an issue where someone was passing ex.InnerException to a method, where ex was the root. Since the parameter was also called ex it led to some confusion in the debugger when I looked at the originally caught exception. This was likely the result of some careless refactoring.

e.g.:

public void MyMethod(string input)
{
    try {
        Process(input);
    } catch (Exception ex) { // <- (2) Attempting to view ex here would show null
        _logger.log(ex);
        LogInner(ex.InnerException);
    }
}

private void LogInner(Exception ex)
{
    _logger.log(ex); // <- (1) NullReferenceExeption thrown here
    if(ex.InnerException != null)
        LogInner(ex.InnerException);
}

This was refactored as such:

public void MyMethod(string input)
{
    try {
        Process(input);
    } catch (Exception ex) {
        LogExceptionTree(ex);
    }
}

private void LogExceptionTree(Exception exception)
{
    _logger.log(exception);
    if(exception.InnerException != null)
        LogExceptionTree(exception.InnerException);
}

Comments

[Andrew Barber]

+1 Stack Overflow questions make me test the darndest things!

[CodesInChaos]

I wouldn't be surprised if it's possible to throw null somehow. I think it's already possible to throw classes not derived from Exception. But I'd be surprised if catch(Exception ex) would catch such a null exception.

[Andrew Barber]

Nope. It's just like SLaks said: If you throw a null reference, you get a NullReferenceException. I just tried myself, too.

[Xaqron]

I don't throw the exception. Also I put a Break Point there and ex was null. Maybe it's not a CLR issue and it's the Visual Studio 2010.

[Xaqron]

Recently I used custom exceptions in web.config under httpErrors> and put a <clear/> as the first node and then <error statusCode="400" responseMode="Redirect" path="Error.aspx?Code=400"/> but I commented it after this problem.

[Andrew Barber]

@Xaqron - We are not saying you are throwing a null exception. We're saying it's not possible to do so. Also, that area of web.config has nothing at all to do with your problem, in all likelihood.

[SLaks]

You are wrong. A NullReferenceException will not have a null Message.

[Pavel Vyazankin]

It's about Java. But you cat found it interesting adarshr.com/papers/npe

[BatteryBackupUnit]

I experience the same issue with VS2013 Update 4. have you already reported this bug to microsoft? I've tried to, but it seems hard to create a repro. When i add a simple throw new InvalidOperationException() into the try clause the issue does not occur.

[BatteryBackupUnit]

I've reported the issue on MS Connect here. You're welcome to upvote it.

[Manfred]

Same problem and fix with Visual Studio 2013 Update 5.

[Wali]

This is the right answer... i have suffered from it. The issue still exists in VS2015.

[Kohanz]

This definitely CAN happen. Have experienced it myself.

[Amitava Karan]

for me, there is only one catch block. still, I got the issue. not sure what's wrong with the code?

[lucuma]

I ran into the same issue. The outer exception was named Exception ex and the inner was too and returning null.

[Mel]

We just saw this not ten minutes ago. Renaming the second catch variable made it go away. This is just... wow. How have we never run into this before. I name ALL my exception catch variables "ex", and I've never seen this before today, but we can absolutely 100% reproduce this.

[dbardakov]

The issue also currently could be observed in Jetbrainbs Rider IDE.

[BBlake]

I have just run into something similar in VS 2019. I have an ASP.NET web app where a controller action has two separate try/catch blocks. When the first block doesn't catch the error, but the second block does, and both define the catch as catch(Exception ex), then ex is always null in the second block. Renaming the second block's ex variable solved the problem.

[Bas]

This still happens in VS 2019 16.6.3. I have a WebSocketException catch block followed by a System.Exception catch block, and if they are both named "ex", the System.Exception catch will always be null.

[Wouter]

still in 16.9.2

[Marco Leite]

Running VS 2022 Version 17.0.1 and this is still an issue :facepalm: