I would like to prevent further processing on an object if it is null.
In the following code I check if the object is null by either:
if (!data.Equals(null))
and
if (data != null)
However, I receive a NullReferenceException at dataList.Add(data). If the object was null, it should never have even entered the if-statement!
Thus, I'm asking if this is proper way of checking if an object is null:
public List<Object> dataList;
public bool AddData(ref Object data)
bool success = false;
try
{
// I've also used "if (data != null)" which hasn't worked either
if (!data.Equals(null))
{
//NullReferenceException occurs here ...
dataList.Add(data);
success = doOtherStuff(data);
}
}
catch (Exception e)
{
throw new Exception(e.ToString());
}
return success;
}
If this is the proper way of checking if the object is null, what am I doing wrong (how can I prevent further processing on the object to avoid the NullReferenceException)?
It's not data that is null, but dataList.
You need to create one with
public List<Object> dataList = new List<Object>();
Even better: since it's a field, make it private. And if there's nothing preventing you, make it also readonly. Just good practice.
Aside
The correct way to check for nullity is if(data != null). This kind of check is ubiquitous for reference types; even Nullable<T> overrides the equality operator to be a more convenient way of expressing nullable.HasValue when checking for nullity.
If you do if(!data.Equals(null)) then you will get a NullReferenceException if data == null. Which is kind of comical since avoiding this exception was the goal in the first place.
You are also doing this:
catch (Exception e)
{
throw new Exception(e.ToString());
}
This is definitely not good. I can imagine that you put it there just so you can break into the debugger while still inside the method, in which case ignore this paragraph. Otherwise, don't catch exceptions for nothing. And if you do, rethrow them using just throw;.
C# 6 has monadic null checking :)
before:
if (points != null) {
var next = points.FirstOrDefault();
if (next != null && next.X != null) return next.X;
}
return -1;
after:
var bestValue = points?.FirstOrDefault()?.X ?? -1;
in C# 7 the best is
if (obj is null) ...
This will ignore any == or != defined by the object (unless of course you want to use them ...)
For not equal you can if (!(obj is null)) (ugly)
Your dataList is null as it has not been instantiated, judging by the code you have posted.
Try:
public List<Object> dataList = new List<Object>();
public bool AddData(ref Object data)
bool success = false;
try
{
if (!data.Equals(null)) // I've also used if(data != null) which hasn't worked either
{
dataList.Add(data); //NullReferenceException occurs here
success = doOtherStuff(data);
}
}
catch (Exception e)
{
throw new Exception(e.ToString());
}
return success;
}
[Edited to reflect hint by @kelton52]
Simplest way is to do object.ReferenceEquals(null, data)
Since (null==data) is NOT guaranteed to work:
class Nully
{
public static bool operator ==(Nully n, object o)
{
Console.WriteLine("Comparing '" + n + "' with '" + o + "'");
return true;
}
public static bool operator !=(Nully n, object o) { return !(n==o); }
}
void Main()
{
var data = new Nully();
Console.WriteLine(null == data);
Console.WriteLine(object.ReferenceEquals(null, data));
}
Produces:
Comparing '' with 'Nully'
True
False
No, you should be using !=. If data is actually null then your program will just crash with a NullReferenceException as a result of attempting to call the Equals method on null. Also realize that, if you specifically want to check for reference equality, you should use the Object.ReferenceEquals method as you never know how Equals has been implemented.
Your program is crashing because dataList is null as you never initialize it.
The problem in this case is not that data is null. It is that dataList itself is null.
In the place where you declare dataList you should create a new List object and assign it to the variable.
List<object> dataList = new List<object>();
In addition to @Jose Ortega answer, its better for use extension method
public static bool IsNull(this object T)
{
return T == null;
}
And use IsNull method for all of object like:
object foo = new object(); //or any object from any class
if (foo.IsNull())
{
// blah blah //
}
Jeffrey L Whitledge is right. Your `dataList´-Object itself is null.
There is also another problem with your code: You are using the ref-keyword, which means the argument data cannot be null! The MSDN says:
An argument passed to a ref parameter must first be initialized. This differs from out, whose arguments do not have to be explicitly initialized before they are passed
It's also not a good idea to use generics with the type `Object´. Generics should avoid boxing/unboxing and also ensure type safety. If you want a common type make your method generic. Finally your code should look like this:
public class Foo<T> where T : MyTypeOrInterface {
public List<T> dataList = new List<T>();
public bool AddData(ref T data) {
bool success = false;
try {
dataList.Add(data);
success = doOtherStuff(data);
} catch (Exception e) {
throw new Exception(e.ToString());
}
return success;
}
private bool doOtherStuff(T data) {
//...
}
}
As others have already pointed out, it's not data but rather likely dataList that is null. In addition to that...
catch-throw is an antipattern that almost always makes me want to throw up every time that I see it. Imagine that something goes wrong deep in something that doOtherStuff() calls. All you get back is an Exception object, thrown at the throw in AddData(). No stack trace, no call information, no state, nothing at all to indicate the real source of the problem, unless you go in and switch your debugger to break on exception thrown rather than exception unhandled. If you are catching an exception and just re-throwing it in any way, particularly if the code in the try block is in any way nontrivial, do yourself (and your colleagues, present and future) a favor and throw out the entire try-catch block. Granted, throw; is better than the alternatives, but you are still giving yourself (or whoever else is trying to fix a bug in the code) completely unnecessary headaches. This is not to say that try-catch-throw is necessarily evil per se, as long as you do something relevant with the exception object that was thrown inside the catch block.
Then there's the potential problems of catching Exception in the first place, but that's another matter, particularly since in this particular case you throw an exception.
Another thing that strikes me as more than a little dangerous is that data could potentially change value during the execution of the function, since you are passing by reference. So the null check might pass but before the code gets to doing anything with the value, it's changed - perhaps to null. I'm not positive if this is a concern or not (it might not be), but it seems worth watching out for.
public static bool isnull(object T)
{
return T == null ? true : false;
}
use:
isnull(object.check.it)
Conditional use:
isnull(object.check.it) ? DoWhenItsTrue : DoWhenItsFalse;
Update (another way) updated 08/31/2017. Thanks for the comment.
public static bool isnull(object T)
{
return T ? true : false;
}
Whenever you are creating objects of class you have to check the whether the object is null or not using the below code.
Example: object1 is object of class
void myFunction(object1)
{
if(object1!=null)
{
object1.value1 //If we miss the null check then here we get the Null Reference exception
}
}
来源：https://stackoverflow.com/questions/6417902/checking-if-an-object-is-null-in-c-sharp