Which Null Exception to throw?

Question

Consider the following class and method:

public class MyDto
{
    public MyDtoChild Child {get; set;}
}

and

public void ProcessDto(MyDto myDto)
{
    if(myDto == null) throw new ArgumentNullException("myDto");
    this.CheckSomething(myDto.Child.ChildProperty);
}

If called with a MyDto with a null Child left to it's own devices this will throw a NullReferenceException which can be extremely difficult to diagnose in more complex methods.

Typically I throw an ArgumentNullException at the start of the method if myDto is null but what is the appropriate exception to throw if myDto.Children is null? An ArgumentNullException? A NullReferenceException? A custom exception?

Answer 1

As mentioned by the previous answers, it should not be an ArgumentNullException since the argument myDTO is not NULL. To me it makes more sense to throw an ArgumentException since the argument passed to the method did not meet the requirements (you expect Children to not be null). Moreover, your situation fits the ArgumentException's description:

ArgumentException is thrown when a method is invoked and at least one of the passed arguments does not meet the parameter specification of the called method.