Throwing ArgumentNullException


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!");


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?

5/20/2013 9:09:11 PM

Accepted Answer

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 to use the containing object without reference to 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.

2/18/2016 4:31:04 PM

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.

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.

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