When I get an Object Ref error, it can sometimes be a real pain to find out which variable is causing the error (when you can't debug). Is there a way for this error to throw the classname that isn't assigned?
So: I want the name of the type of the variable that was unexpectedly null.
Thanks in advance.
I dont think you can get the class name, the closes I get is to get the class and method name, then the stack trace:
try
{
}
catch ( Exception ex )
{
xxx.API.ErrorHandler.Handler.HandleError( ex, System.Reflection.MethodBase.GetCurrentMethod().Name, System.Reflection.MethodBase.GetCurrentMethod().DeclaringType.FullName );
}
Well, its only the information in the stack trace which would be the first step in finding out where the error has originated. Also you should make sure you check the complete stack trace (all inner exceptions too). This would give you the method name with complete namespace. So that should be fairly good step to see where the error is, unless the standard coding is really bad.
Related
I have some code and when I run it produces an error, saying:
NoSuchMethod: the method 'XYZ' was called on null
What does that mean and how do I fix it?
Why do I get this error?
Example
As a real world comparison, what just happened is this conversation:
Hey, how much gas is left in the tank of the car?
What are you talking about, we don't have a car.
That is exactly what is happening in your program. You wanted to call a function like _car.getGasLevel(); but there is no car, the variable _car is null.
Obviously, in your program it might not be a car. It could be a list or a string or anything else really.
Technical explanation
You are trying to use a variable that is null. Either you have explicitly set it to null, or you just never set it at all, the default value is null.
Like any variable, it can be passed into other functions. The place where you get the error might not be the source. You will have to follow the leads from the actual null value to where it originally came from, to find what the problem is and what the solution might be.
null can have different meanings: variables not set to another value will be null, but sometimes null values are used by programmers intentionally to signal that there is no value. Databases have nullable fields, JSON has missing values. Missing information may indeed be the information itself. The variable bool userWantsPizzaForDinner; for example might be used for true when the user said yes, false when the user declined and it might still be null when the user has not yet picked something. That's not a mistake, it's intentionally used and needs to be handled accordingly.
How do I fix it?
Find it
Use the stack trace that came with the error message to find out exactly which line the error was on. Then set a breakpoint on that line. When the program hits the breakpoint, inspect all the values of the variables. One of them is null, find out which one.
Fix it
Once you know which variable it is, find out how it ended up being null. Where did it come from? Was the value never set in the first place? Was the value another variable? How did that variable got it's value. It's like a line of breadcrumbs you can follow until you arrive at a point where you find that some variable was never set, or maybe you arrive at a point where you find that a variable was intentionally set to null. If it was unintentional, just fix it. Set it to the value you want it to have. If it was intentional, then you need to handle it further down in the program. Maybe you need another if to do something special for this case. If in doubt, you can ask the person that intentionally set it to null what they wanted to achieve.
simply the variable/function you are trying to access from the class does not exist
someClass.xyz();
above will give the error
NoSuchMethod: the method 'xyz' was called on null
because the class someClass does not exist
The following will work fine
// SomeClass created
// SomeClass has a function xyz
class SomeClass {
SomeClass();
void xyz() {
print('xyz');
}
}
void main() {
// create an instance of the class
final someClass = SomeClass();
// access the xyz function
someClass.xyz();
}
I have some code and when I run it produces an error, saying:
NoSuchMethod: the method 'XYZ' was called on null
What does that mean and how do I fix it?
Why do I get this error?
Example
As a real world comparison, what just happened is this conversation:
Hey, how much gas is left in the tank of the car?
What are you talking about, we don't have a car.
That is exactly what is happening in your program. You wanted to call a function like _car.getGasLevel(); but there is no car, the variable _car is null.
Obviously, in your program it might not be a car. It could be a list or a string or anything else really.
Technical explanation
You are trying to use a variable that is null. Either you have explicitly set it to null, or you just never set it at all, the default value is null.
Like any variable, it can be passed into other functions. The place where you get the error might not be the source. You will have to follow the leads from the actual null value to where it originally came from, to find what the problem is and what the solution might be.
null can have different meanings: variables not set to another value will be null, but sometimes null values are used by programmers intentionally to signal that there is no value. Databases have nullable fields, JSON has missing values. Missing information may indeed be the information itself. The variable bool userWantsPizzaForDinner; for example might be used for true when the user said yes, false when the user declined and it might still be null when the user has not yet picked something. That's not a mistake, it's intentionally used and needs to be handled accordingly.
How do I fix it?
Find it
Use the stack trace that came with the error message to find out exactly which line the error was on. Then set a breakpoint on that line. When the program hits the breakpoint, inspect all the values of the variables. One of them is null, find out which one.
Fix it
Once you know which variable it is, find out how it ended up being null. Where did it come from? Was the value never set in the first place? Was the value another variable? How did that variable got it's value. It's like a line of breadcrumbs you can follow until you arrive at a point where you find that some variable was never set, or maybe you arrive at a point where you find that a variable was intentionally set to null. If it was unintentional, just fix it. Set it to the value you want it to have. If it was intentional, then you need to handle it further down in the program. Maybe you need another if to do something special for this case. If in doubt, you can ask the person that intentionally set it to null what they wanted to achieve.
simply the variable/function you are trying to access from the class does not exist
someClass.xyz();
above will give the error
NoSuchMethod: the method 'xyz' was called on null
because the class someClass does not exist
The following will work fine
// SomeClass created
// SomeClass has a function xyz
class SomeClass {
SomeClass();
void xyz() {
print('xyz');
}
}
void main() {
// create an instance of the class
final someClass = SomeClass();
// access the xyz function
someClass.xyz();
}
I'm getting error while running a game I created with flex.
I know there has been some question about this, but my case is quite weird. I created a simple typing game that is running OK on my computer, but when I tried to deploy it online to facebook, I got those errors. I used code from the tutorial from adobe here http://www.adobe.com/devnet/facebook/articles/flex_fbgraph_pt4.html to deploy my flex game to facebook
This is the error message:
TypeError: Error #1009: Cannot access a property or method of a null object reference.
at FacebookUserStatusWeb/init()
at FacebookUserStatusWeb/___FacebookUserStatusWeb_Application1_creationComplete()
at flash.events::EventDispatcher/dispatchEventFunction()
at flash.events::EventDispatcher/dispatchEvent()
at mx.core::UIComponent/dispatchEvent()
at mx.core::UIComponent/set initialized()
at mx.managers::LayoutManager/doPhasedInstantiation()
at mx.managers::LayoutManager/doPhasedInstantiationCallback()
And here is snippet of the init() function:
protected var text1:String="Text to be typed"; //hard-coded temporarily
protected const TIMER_INTERVAL:int = 10;
protected var t:Timer = new Timer(TIMER_INTERVAL);
protected var topURL:String=ExternalInterface.call('top.location.toString');
protected function init():void
{
t.addEventListener(TimerEvent.TIMER, updateTimer);
ProblemText.text = new String(text1);
Facebook.init("<my app id>",loginHandler);
currentState = (topURL) ? "loggedout": "loggedoutonfacebook";
}
Some notes:
1.my app id is my facebook app id which I prefer not to show
2.ProblemText is a richtext which I placed the paragraph to be typed by the player.
3.I have deleted the method Application1_creationComplete() but it still appears at the error listing
And also I am curious about the errors other than the first two. What do they mean?
Ah, and if it is helpful, I can post some more of the code
First: You're only seeing one error. Everything you see below the #1009 error is your stack trace, not additional errors.
The stack trace basically tells you the series of things that happened prior to the error occurring, with the most recent at the top. This is useful because often things which happen prior to the actual error you see will contribute to said error.
Second: The null object reference is occurring because something in your init() function tried to access a property in an object that doesn't exist, or an object that doesn't exist. One (slightly messy but effective) way to debug this would be to drop some trace statements in the code to see how far it gets before barfing with the error -- the idea being to isolate the specific line that's causing the problem. Once you've done that, you need to work backwards to figure out why the object or property you're trying to use is null. It could be something simple, like a typo, or it could be more complex. You'll have to sleuth it out, one way or another =)
Good luck!
Finally got the bug. Just in case people have the same case with me, what exactly happened is at my computer I simulated the game with just one state, but when I'm deploying to facebook I have several states (loggedin,loggedout,etc). In the init() I tried to access ProblemText Label that is not present in the current state.
I have a global error trap in the Application_Error method of Global.asax that writes any unhandled exception to a log file.
Is there anyway of reporting the variable names and their values as part of the exception report?
Unless you do something really tricky with Aspect-Oriented Programming, you pretty much need to make sure that you manually introduce any relevant information into the stack trace when exceptions are thrown. For example:
public void DoSomething(int number, string name)
{
try
{
...
}
catch (Exception e)
{
throw new Exception("Error occurred while doing something: " +
new {number, name}, e);
}
}
This way, the number and name will be included in the stack trace when this exception trickles up to the top level.
Edit
After reading David Stratton's answer, I felt the need to expand on this a little. I get the sense that even some very experienced C# programmers haven't learned some of the tricks that I've learned.
First of all, I wanted to point out that the exception-handling system in .NET was designed with the idea of an InnerException specifically for this purpose (providing additional information at various points of the stack trace), and it is not at all hacky to do. However, you should definitely provide the exception as the innerException constructor parameter, rather than appending e.ToString() to the new exception's message.
Secondly, based on various comments and answers I've read on StackOverflow, as well as my own experience, it's best to:
Avoid catching an exception if there's nothing specific you plan to do with it.
When you catch an exception, rethrow it unless you know why the exception was thrown and you're in a scope where you know how to gracefully retreat from what you were trying to do. Simply pretending nothing went wrong is asking for more trouble down the road.
When re-throwing exceptions, either just throw; to preserve the original exception's stack trace, or include the original exception as a new exception's InnerException.
Consistently log exceptions that don't get re-thrown. This will generally only happen at the UI level, so you can tell the user something unexpected happened rather than allowing the program to crash.
Finally, I wanted to mention that the anonymous type declaration syntax is ideal for this kind of thing because it is very concise and it will automatically produce a string that uses the given variable names and values. For example, new {number, name}.ToString() might produce "{ number = 1, name = Test }".
We do this in some cases, but in a roundabout way. We use a try/catch at the granular level and if we want to pass the exception up to the global error handler, we build the error message. For example:
int someCounterValue = 0;
string someStringValue = "Some string we want to track to send to the global error handler."
private void SomeFunction()
{
try
{
someStringValue = "in the try block";
someCounterValue = 1.5 // should thrown an exception
}
catch(Exception ex)
{
throw new Exception("Error in SomeFunction. someStringValue = " + someStringValue + "; someCounterValue = " + someCounterValue.ToString() + "\r\nException details: " + ex.ToString());
}
}
It's a lot of work so we really don't do this very often. Usually our error handling is better handled at the local level but in those very rate cases where you want to pass it off to the global error handler, this is about the only way we've found, since the variables would be out of scope otherwise, and therefore, inaccessible.
An easier/less hack-like option would be to set up a static class in your web app called ErrorLogger or something similar, and just handle the exceptions better locally and pass them up to the global handler ONLY when you can't do it locally.
In my opinion (and in the way we do it here by policy) a global error handler should be used to catch exceptions that you forgot to handler better at a more granular level. It should NOT just be an easy way to be lazy about proper exception handling.
Also it feels like a hack to me, so if anyone has a better option I'd like to know it, too.
Object reference not set to an instance of an object
I am getting this error sometimes.
I have a page where I load questions and for some question system generate this error. Why do I get this error? Please help me.
It means that somewhere something dereferences a null reference. Crude example:
Foobar foo = null;
foo.DoSomething(); // this line will trigger a NullReferenceException
Here's what you can do:
compile in debug mode
run it until it crashes
examine the stack trace; it will tell you where the exception originates (unless you're doing stupid things like catching and rethrowing improperly). You may have to follow the chain of inner exceptions
if that doesn't provide sufficient information, step through your code with a debugger until right before the call that makes it crash. Examine any variables that might be null.
Once you have found the culprit, you need to find out why it is null and what to do about it. The solution can be one of:
fix incorrect program logic that causes something to be null that shouldn't be
check if something is null before dereferencing it
handle the exception at a point that makes sense, and translate it into something more meaningful (probably some sort of custom exception)
You got null reference and if it can't handle it will show that message.
It can caused by some case
The return value of a method is null
A local variable or member field is declared but not initialized
An object in a collection or array is null
An object is not created because of a condition
An object passed by reference to a method is set to null
I suggest you to track the null value using a debug mode
You get this error, whenever you are trying to make use of variable with Null value in it.