Getting Error - "Object reference not set to an instance of an object".

Question

Hi In my C# application i am getting an error - Object reference not set to an instance of an object.. This application is running round the clock(stock appln) and i have just enabled additional logging. This error is happening for a particular sequence of events. I suspect at the end of the day when i colse and open objects/file streams/writers i encounter this. Any idea on the curcumstances under which this normally hapens? I believe you guys might have come across this. I will get back to you with more info after checking the log( need to wait one day for that error). Thanks in advance for the help. Regards Peri

periakaruppan_sg · Answer

Hi CodeBender and Ytram Thanks for the help. Managed to resolve it thru the debugger. As we do not catch all the exceptions i set a breakpoint under the 'debug' menu. There is an option to specify breakpoint for any thrown exception. Using that i managed to pinpoint the line causing the problem. The problem was occurring when i tried to access the hashtable variable for a key which was not there. Regards Peri

ytram5 · Answer

The easiest ways I've found to fix this problem is to run in debug mode and then find the exact line where the NullReference is happening. Put a break point at this line and then run again in debug mode. Check every object in that line to see if it is null using the command window. The most common instances of this happening to me is if an object is not instantiated in a constructor or an incorrect instantiation of a global variable. For example: MyObject mo; public Constructor() { MyObject mo = new MyObject(); //this should just be mo = new MyObject(); } public void DoSomething() { mo.DoSomething(); //This causes a NullReference since the mo object is declared again inside the constructor instead of just being instantiated. } So basically just make sure that all global variables are being instantiated and instantiated correctly.

crausch · Answer

The error is fairly self explainatory your code is referencing something that has not been instantiated yet. This usually ends up being an object whos value is undefined or null at the point of the error. A debugger will pick this up fairly quickly.