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In C#, is it possible for an object to do suicide, that is set it self to null ?
this=null; didn't work....
The reason for wanting such ting is that if e.g. the object gets data in via constructor that is not good (for any reason), then it might be better off being null than rubbish.
A status flag is a workaround, but that is my second best alternative.
I suggest you throw an ArgumentException from the constructor in case you receive problematic data from the caller. Then catch that in the calling code.
No the object cannot set itself into null, it's the reference that can be set to null.
Just throw an ArgumentException insteed
If the object needs external data in order to complete its construction and gets something unusable (or no data at all) then what you need to do is throw an appropriate exception.
Changing the value of this is illegal, and even if it were not it would not offer any benefit: the caller still has to determine what happened during construction.
You should throw an exception in the constructor; this will prevent the object from being created all together:
class MyClass {
public MyClass(string value) {
if (value == null)
throw new ArgumentNullException("value");
...
}
}
In C++ you can play with this how you want.
In C# - only in restricted way. For example, you can assign struct's, but not the class' this to something
I'm really trying to figure out the best practices for reusable code that is easily debugged. I have ran into a common practice among developers that I don't quite understand yet.
public MyConstructor(Object myObject)
{
if (myObject == null)
throw new ArgumentNullException("myObject is null.");
_myObject = myObject;
}
It almost seems unnecessary to do this check. But I think it's because I don't completely understand what the benefits of doing this check are. It seems like a null reference exception would be thrown anyway? I am probably wrong, would really like to hear some thoughts on it.
Thank you.
To the compiler, null is a legitimate constructor argument.
Your class might be able to handle a null value for myObject. But if it can't - if your class will break when myObject is null - then checking in the constructor allows you to fail fast.
Passing a null object is perfectly legal in many cases - for this class the implementor wants to ensure that you cannot create an instance of the class w/o passing a valid Object instance though, so there have to be no checks later on - it's a good practice to ensure this as early as possible, which would be in the constructor.
if you under 4.0 you can do the following:
public ctor(IEnjection ninjaWeapon)
{
Contract.Requires<ArgumentNullException>(ninjaWeapon != null);
this.deadlyWeaponary.Add(ninjaWeapon);
}
if you under an older version, reference the Microsoft.Contract to do the same thing.
Others have already correctly stated that the passing of a null parameter may or may not be valid depending upon the functionality of the consuming code.
Where a null parameter is undesirable it is possible, from C# 7.0, to use throw expressions which allow us to rewrite null checking code much more succinctly as the following example shows:
public MyConstructor(Object myObject)
{
_myObject = myObject ?? throw new ArgumentNullException(nameof(myObject));
}
The above will set the value of _myObject to the parameter myObject unless that parameter is null, in which case an ArgumentNullException will be thrown.
The compilier has no idea about the value of an object, so you have to check this at runtime to ensure it doesn't get called with a null value.
It also depends on your particular solution. You don't need to throw the exception, I would only throw it if you can not have that value be null, and if it is null, that is an exceptional case.
I think that it is not possible to tell generally whether checking for null is necessary or not. It rather depends on whether you can live with null valued variables or not. Null is not per se a bad state. You might have situations where it is allowed for a variable to be null and other where it is not.
Ask yourself whether it makes sense to allow null values or not and design the constructor accordingly.
You could implement a simple ThrowIfNull extension method to reduce the code you write each time. Jon Skeet covered this in his blog and the referenced SO article here.
You need to explicitly check for null because the compiler doesn't know, but also because null can be a valid argument.
The benefit is that the exception will be thrown at the time of object construction, so you can easily trace which part of code is the culprit. If your code requires non-null myobject value and you don't validate it in the constructor, the NullReferenceException will be thrown when you use myObject_ and you will have to trace back manually to see who sent that null value in.
public static IFoo Bar<T>(this IFoo target, ...)
{
// I'm curious about how useful this check is.
if (target == null) throw new ArgumentNullException("target");
...
}
(1) The code above seems strange to me because I feel like in any case the calling code would be the one to check that. Is there some subtlety to extension methods in this area that applies?
(2) Is there a legitimate pattern that leverages the fact that the target can be null? I ask this as a consequence of wondering why calling an extension method on a null reference wouldn't generate the runtime exception the same way as if you called an instance method on a null reference.
Consider that null can be an argument to a method. Consider also that the extension method foo.Bar<int>(); is really just syntactic sugar for IFooExtensions.Bar<int>(foo); and you will see that, yes, the argument can indeed be null so if you're doing something with the argument, it may certainly be appropriate to test it for null (or simply let a NullReferenceException be thrown, take your pick).
Note: You would not get an exception merely by calling it with a null referenced object, because remember that the method does not actually belong to the object. You only get the exception if (a) you purposefully throw one yourself or (b) the method body actually causes it by trying to work with the instance that is null.
(2) Is there a legitimate pattern that
leverages the fact that the target can
be null? I ask this as a consequence
of wondering why calling an extension
method on a null reference wouldn't
generate the runtime exception the
same way as if you called an instance
method on a null reference.
Well, here's one use of it.
public static class UtilityExtensions
{
public static void ThrowIfNull( this object obj, string argName )
{
if ( obj == null )
{
throw new ArgumentNullException( argName );
}
}
}
Now you can write null checks easily and on one line (helpful if your coding convention forces you to use braces with all if statements).
public void Foo(string bar)
{
bar.ThrowIfNull("bar");
// ...
}
Whether or not you consider that a "legitimate" pattern is up to you, but I'd be very sad if the runtime was changed to make extension method invocations on null references throw exceptions.
Since extension methods really are only syntactic sugar for calling a static method with the object as the first parameter, I don't see why null for this parameter value shouldn't be allowed.
In my application I need to throw an exception if a property of a specific class is null or empty (in case it's a string). I'm not sure what is the best exception to use in this case. I would hate to create a new exception and I'm not sure if ArgumentNullException is appropriate in this case.
Should I create a new exception or there's an exception I can use?
I don't mind to throw an ApplicationException.
The MSDN guidelines for standard exceptions states:
Do use value for the name of the implicit value parameter of property
setters.
The following code example shows a
property that throws an exception if
the caller passes a null argument.
public IPAddress Address
{
get
{
return address;
}
set
{
if(value == null)
{
throw new ArgumentNullException("value");
}
address = value;
}
}
Additionally, the MSDN guidelines for property design say:
Avoid throwing exceptions from
property getters.
Property getters should be simple
operations without any preconditions.
If a getter might throw an exception,
consider redesigning the property to
be a method. This recommendation does
not apply to indexers. Indexers can
throw exceptions because of invalid
arguments.
It is valid and acceptable to throw
exceptions from a property setter.
So throw ArgumentNullException in the setter on null, and ArgumentException on the empty string, and do nothing in the getter. Since the setter throws and only you have access to the backing field, it's easy to make sure it won't contain an invalid value. Having the getter throw is then pointless. This might however be a good spot to use Debug.Assert.
If you really can't provide an appropriate default, then I suppose you have three options:
Just return whatever is in the property and document this behaviour as part of the usage contract. Let the caller deal with it. You might also demand a valid value in the constructor. This might be completely inappropriate for your application though.
Replace the property by methods: A setter method that throws when passed an invalid value, and a getter method that throws InvalidOperationException when the property was never assigned a valid value.
Throw InvalidOperationException from the getter, as you could consider 'property has never been assigned' an invalid state. While you shouldn't normally throw from getters, I suppose this might be a good reason to make an exception.
If you choose options 2 or 3, you should also include a TryGet- method that returns a bool which indicates if the property has been set to a valid value, and if so returns that value in an out parameter. Otherwise you force callers to be prepared to handle an InvalidOperationException, unless they have previously set the property themselves and thus know it won't throw. Compare int.Parse versus int.TryParse.
I'd suggest using option 2 with the TryGet method. It doesn't violate any guidelines and imposes minimal requirements on the calling code.
About the other suggestions
ApplicationException is way too general. ArgumentException is a bit too general for null, but fine otherwise. MSDN docs again:
Do throw the most specific (the most derived) exception that is
appropriate. For example, if a method
receives a null (Nothing in Visual
Basic) argument, it should throw
System.ArgumentNullException instead
of its base type
System.ArgumentException.
In fact you shouldn't use ApplicationException at all (docs):
Do derive custom exceptions from the T:System.Exception class rather than the T:System.ApplicationException class.
It was originally thought that custom exceptions should derive from the ApplicationException class; however, this has not been found to add significant value. For more information, see Best Practices for Handling Exceptions.
InvalidOperationException is intended not for when the arguments to a method or property are invalid, but for when the operation as a whole is invalid (docs). It should not be thrown from the setter:
Do throw a System.InvalidOperationException exception if in an inappropriate state. System.InvalidOperationException should be thrown if a property set or a method call is not appropriate given the object's current state. For example, writing to a System.IO.FileStream that has been opened for reading should throw a System.InvalidOperationException exception.
Incidentally, InvalidOperationException is for when the operation is invalid for the object's current state. If the operation is always invalid for the entire class, you should use NotSupportedException.
I would throw an InvalidOperationException. MSDN says it "is thrown when a method call is invalid for the object's current state."
If the problem is that a member of an argument, and not the argument itself, is null then I think the best choice is the more generic ArgumentException. ArgumentNullException does not work here because the argument is in fact not null. Instead you need the more generic "something is wrong with your argument" exception type.
A detailed message for the constructor would be very appropriate here
Well, it's not an argument, if you're referencing a property of a class. So, you shouldn't use ArgumentException or ArgumentNullException.
NullReferenceException would happen if you just leave things alone, so I assume that's not what you're looking for.
So, using ApplicationExeption or InvalidOperationException would probably be your best bet, making sure to give a meaningful string to describe the error.
It is quite appropriate to throw ArgumentNullException if anyone tries to assign null.
A property should never throw on a read operation.
If it can't be null or empty, have your setter not allow null or empty values, or throw an ArgumentException if that is the case.
Also, require that the property be set in the constructor.
This way you force a valid value, rather than coming back later and saying that that you can't determine account balance as the account isn't set.
But, I would agree with bduke's response.
Does the constructor set it to a non-null value? If so I would just throw ArgumentNullException from the setter.
There is a precedent for stretching the interpretation of ArgumentNullException to meaning "string argument is null or empty": System.Windows.Clipboard.SetText will throw an ArgumentNullException in this case.
So I wouldn't see anything wrong with using this rather than the more general ArgumentException in your property setter, provided you document it.
Just throw whatever as long as the error message is helpful to a developer. This class of exception should never happen outside of development, anyway.
Suppose I have a method that takes an object of some kind as an argument. Now say that if this method is passed a null argument, it's a fatal error and an exception should be thrown. Is it worth it for me to code something like this (keeping in mind this is a trivial example):
void someMethod(SomeClass x)
{
if (x == null){
throw new ArgumentNullException("someMethod received a null argument!");
}
x.doSomething();
}
Or is it safe for me to just rely on it throwing NullException when it calls x.doSomething()?
Secondly, suppose that someMethod is a constructor and x won't be used until another method is called. Should I throw the exception immediately or wait until x is needed and throw the exception then?
I prefer the ArgumentNullException over the NullReferenceException that not checking the argument would provide. In general, my preference is to always check for nullity before trying to invoke a method on a potentially null object.
If the method is a constructor, then it would depend on a couple of different factors: is there also a public setter for the property and how likely is it that the object will actually be used. If there is a public setter, then not providing a valid instance via the constructor would be reasonable and should not result in an exception.
If there is no public setter and it is possible to use the containing object without referencing the injected object, you may want to defer the checking/exception until its use is attempted. I would think that the general case, though, would be that injected object is essential to the functioning of the instance and thus an ArgumentNull exception is perfectly reasonable since the instance can't function without it.
I always follow the practice of fail fast. If your method is dependent on X and you understand X might be passed in null, null check it and raise the exception immediately instead of prolonging the point of failure.
2016 update:
Real world example. I strongly recommend the usage of JetBrains Annotations.
[Pure]
public static object Call([NotNull] Type declaringType,
[NotNull] string methodName,
[CanBeNull] object instance)
{
if (declaringType == null) throw new ArgumentNullException(nameof(declaringType));
if (methodName == null) throw new ArgumentNullException(nameof(methodName));
Guard statements have been vastly improved with C# 6 providing the nameof operator.
I prefer the explicit exception, for these reasons:
If the method has more than one SomeClass argument it gives you the opportunity to say which one it is (everything else is available in the call stack).
What if you do something that may have a side effect before referencing x?
No excuse not to make the check these days. C# has moved on and you can do this very neatly using a discard and a null coalescing operator:
_ = declaringType ?? throw new ArgumentNullException(nameof(declaringType));
_ = methodname ?? throw new ArgumentNullException(nameof(methodName));
I agree with the idea of failing fast - however it is wise to know why failing fast is practical. Consider this example:
void someMethod(SomeClass x)
{
x.Property.doSomething();
}
If you rely on the NullReferenceException to tell you that something was wrong, how will you know what was null? The stack trace will only give you a line number, not which reference was null. In this example x or x.Property could both have been null and without failing fast with aggressive checking beforehand, you will not know which it is.
I'd prefer the parameter check with the explicit ArgumentNullException, too.
Looking at the metadata:
//
// Summary:
// Initializes a new instance of the System.ArgumentNullException class with
// the name of the parameter that causes this exception.
//
// Parameters:
// paramName:
// The name of the parameter that caused the exception.
public ArgumentNullException(string paramName);
You can see, that the string should be the name of the parameter, that is null, and so give the developer a hint on what is going wrong.
I strongly agree with #tvanfosson. Adding to his answer, with .net 6 it’s very easy to throw the ArgumentNullException.
ArgumentNullException.ThrowIfNull(object);
Here’s the official documentation.
You should explicitly throw an ArgumentNullException if you are expecting the input to not be null. You might want to write a class called Guard that provides helper methods for this. So your code will be:
void someMethod(SomeClass x, SomeClass y)
{
Guard.NotNull(x,"x","someMethod received a null x argument!");
Guard.NotNull(y,"y","someMethod received a null y argument!");
x.doSomething();
y.doSomething();
}
The NonNull method would do the nullity check and throw a NullArgumentException with the error message specified in the call.
It is better to throw the ArgumentNullException sooner rather than later. If you throw it, you can provide more helpful information on the problem than a NullReferenceException.
Do it explicitly if you do not want a Null value. Otherwise, when someone else look at your code, they will think that passing a Null value is acceptable.
Do it as early as you can. This way, you do not propagate the "wrong" behavior of having a Null when it's not supposed to.
If you program defensively you should fail fast. So check your inputs and error out at the beginning of your code. You should be nice to your caller and give them the most descriptive error message you can.
I'll probably be downvoted for this, but I think completely different.
What about following a good practice called "never pass null" and remove the ugly exception checking?
If the parameter is an object, DO NOT PASS NULL. Also, DO NOT RETURN NULL. You can even use the Null object pattern to help with that.
If it's optional, use default values (if your language supports them) or create an overload.
Much cleaner than ugly exceptions.
Since .NET 6 you can throw the argument null exception in one line for instance:
int? foo = null;
ArgumentNullException.ThrowIfNull(foo);
This will check if foo is null and throw an error. You can pass a second parameter to set the parameter name, but this is not recommended.
https://learn.microsoft.com/en-us/dotnet/api/system.argumentnullexception.throwifnull?view=net-6.0
You can use syntax like the following to not just throw an ArgumentNullException but have that exception name the parameter as part of its error text as well. E.g.;
void SomeMethod(SomeObject someObject)
{
Throw.IfArgNull(() => someObject);
//... do more stuff
}
The class used to raise the exception is;
public static class Throw
{
public static void IfArgNull<T>(Expression<Func<T>> arg)
{
if (arg == null)
{
throw new ArgumentNullException(nameof(arg), "There is no expression with which to test the object's value.");
}
// get the variable name of the argument
MemberExpression metaData = arg.Body as MemberExpression;
if (metaData == null)
{
throw new ArgumentException("Unable to retrieve the name of the object being tested.", nameof(arg));
}
// can the data type be null at all
string argName = metaData.Member.Name;
Type type = typeof(T);
if (type.IsValueType && Nullable.GetUnderlyingType(type) == null)
{
throw new ArgumentException("The expression does not specify a nullible type.", argName);
}
// get the value and check for null
if (arg.Compile()() == null)
{
throw new ArgumentNullException(argName);
}
}
}
All code examples use error prone IF clause, where one use equal operator ==.
What happen, if type override == ?
On C# 7 and greater, use constant pattern matching.
example:
if (something is null)
{
throw new ArgumentNullException(nameof(something), "Can't be null.");
}