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I have a strange error after catch the error "A generic error occurred in GDI+."

when i try to open an excel file trought the instruction :

Workbook workbook = new Workbook();

but few minute after catching this exception my application craches (NullReferenceException )without specifying the location of the error !!

knowing that it happens when i try to scan an excel file, but it work for others excel files !!!

Could someone help me in this topic ?
Posted 12-Jun-13 6:22am
Ron Beyer 12-Jun-13 12:59pm
Did you try looking at the InnerException property of the exceptions?
Sergey Alexandrovich Kryukov 12-Jun-13 17:04pm
In what line? Check under the debugger.

1 solution

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Solution 1

You did not show where the exception with the message "Object reference not set to an instance of an object" is thrown.

Not to worry. This is one of the very easiest cases to detect and fix. It simply means that some member/variable of some reference type is dereferenced by using and of its instance (non-static) members, which requires this member/variable to be non-null, but in fact it appears to be null. Simply execute it under debugger, it will stop the execution where the exception is thrown. Put a break point on that line, restart the application and come to this point again. Evaluate all references involved in next line and see which one is null while it needs to be not null. After you figure this out, fix the code: either make sure the member/variable is properly initialized to a non-null reference, or check it for null and, in case of null, do something else.

Please see also: want to display next record on button click. but got an error in if condition of next record function "object reference not set to an instance of an object"[^].

Sometimes, you cannot do it under debugger, by one or another reason. One really nasty case is when the problem is only manifested if software is built when debug information is not available. In this case, you have to use the harder way. First, you need to make sure that you never block propagation of exceptions by handling them silently (this is a crime of developers against themselves, yet very usual). The you need to catch absolutely all exceptions on the very top stack frame of each thread. You can do it if you handle the exceptions of the type System.Exception. In the handler, you need to log all the exception information, especially the System.Exception.StackTrace:[^],[^].

The stack trace is just a string showing the full path of exception propagation from the throw statement to the handler. By reading it, you can always find ends. For logging, it's the best (in most cases) to use the class System.Diagnostics.EventLog:[^].

Good luck,
waliddamri1 27-Sep-13 6:44am
Tnx for your answer Sergey.

I can't locate where the exception with the message "Object reference not set to an instance of an object" is thrown .

Visual Studio show that it can't locate it !!!
(see picture in this link[^] )

So even if i try to handle exceptions in my code i'll have nothing.

Do you have any idea about that
Does you debuggers stops on a break point put on the exception "catch"? On top of stack? Is exception handler called? You can check it up by logging?
waliddamri1 27-Sep-13 10:10am
there's no logging no point break of exception
but the weird thing is that it work for all excel files i have in my machine except one !

look at this :
Sergey Alexandrovich Kryukov 27-Sep-13 10:13am
Then add it all... at the very top-level stack frame... Get an exception stack trace...

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