How to know where the exception has occurred?

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VS2010

I get the following exception and don't know in what line it occurs so that i resolve it:
A first chance exception of type 'System.NullReferenceException' occurred in UserControls.dll

What I have tried:

I need to know what line in code causes the message to be shown in the Output window and I've no clue.

Posted 6-Oct-18 3:09am

ilostmyid2

Updated 6-Oct-18 5:59am

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Comments

F-ES Sitecore 6-Oct-18 9:14am

Add exception handling and logging to the methods and capture the stack trace. You get than code when you try

someObject.SomeProperty

where "someObject" is null.

ilostmyid2 6-Oct-18 9:27am

sure, but don't know where it occurs in a solution with lots of projects. so that i don't know where to put the try/catch block. this is the problem.

Eric Lynch 6-Oct-18 10:13am

Simply add a "last chance" try/catch block. Where exactly, depends on the type of application. For most types, you can place it around the logic in the Main method. For web apps, its a bit different, but same concept.

Or, you could simply run it in the debugger, which will normally break at the offending line, when an exception occurs.

ilostmyid2 7-Oct-18 4:04am

i think there's a problem. the exception is thrown somewhere in the code, but it's catched there, without being handled. so i can't find it. the exception is muted this way.

2 solutions

Solution 2

C#

try
{
   // code that may raise an exception
}
catch (NullReferenceException ex)
{
   msgbox($"{ex.Message} {ex.StackTrace}")
}

The ex.StackTrace will show where the error occurred (inc. line no)

Posted 6-Oct-18 5:59am

jsc42

Comments

ilostmyid2 7-Oct-18 4:07am

the problem is that i can't find the code block which raises the exception, because it's being muted.

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OriginalGriff · Accepted Answer · 2018-10-06T05:22:00

The simplest way is to just use the debugger - when it hits an unhandled exception, it will stop and show you exactly what it was executing - you can then use the stack trace to work back up to your code (if it occurs in the framework for whatever reason).

This is one of the most common problems we get asked, and it's also the one we are least equipped to answer, but you are most equipped to answer yourself.

Let me just explain what the error means: You have tried to use a variable, property, or a method return value but it contains null - which means that there is no instance of a class in the variable.
It's a bit like a pocket: you have a pocket in your shirt, which you use to hold a pen. If you reach into the pocket and find there isn't a pen there, you can't sign your name on a piece of paper - and you will get very funny looks if you try! The empty pocket is giving you a null value (no pen here!) so you can't do anything that you would normally do once you retrieved your pen. Why is it empty? That's the question - it may be that you forgot to pick up your pen when you left the house this morning, or possibly you left the pen in the pocket of yesterdays shirt when you took it off last night.

We can't tell, because we weren't there, and even more importantly, we can't even see your shirt, much less what is in the pocket!

Back to computers, and you have done the same thing, somehow - and we can't see your code, much less run it and find out what contains null when it shouldn't.
But you can - and Visual Studio will help you here. Run your program in the debugger and when it fails, VS 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, VS 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!