Null conditional operator to "nullify" array element existence

Question

The new C# 6.0 null-conditional operator is a handy vehicle for writing more concise and less convoluted code. Assuming one has an array of customers, then you could get null instead of a length if customers is null using this (examples from MSDN):

int? length = customers?.Length;

Similarly you could get null instead of a customer with this:

Customer first = customers?[0];

And for a more elaborate expression, this yields null if customers is null, the first customer is null, or the first customer's Orders object is null:

int? count = customers?[0]?.Orders?.Count();

But then there is the interesting case of the non-existent customer that the null-conditional operator does not seem to address. We saw above that a null customer is covered, i.e. if an entry in the customers array is null. But that is quite distinct from a non-existent customer, e.g. looking for customer 5 in a 3-element array or customer n in a 0-element list. (Note that the same discussion applies to Dictionary lookup as well.)

It seems to me that the null-conditional operator is focused exclusively on negating the effects of a NullReferenceException; IndexOutOfRangeException or KeyNotFoundException are alone, exposed, cowering in the corner, and needing to fend for themselves! I submit, that in the spirit of the null-conditional operator, it should be able to handle those cases as well... which leads to my question.

Did I miss it? Does the null-conditional provide any elegant way to truly cover this expression...

customers?[0]?.Orders?.Count();

...when there is no zeroth element?

Answer 1

No, because it is a null-conditional operator, not an indexoutofrange-conditional operator and is merely syntactic sugar to something like the following:

int? count = customers?[0]?.Orders?.Count();

if (customers != null && customers[0] != null && customers[0].Orders != null)
{
    int count = customers[0].Orders.Count();
}

You can see that if there is no zeroth customer, you will get your regular IndexOutOfRangeException.

One way you could work around it is to have an extension method that checks for the index and returns null if it doesn't exist:

public static Customer? GetCustomer(this List<Customer> customers, int index)
{
    return customers.ElementAtOrDefault(index); // using System.Linq
}

Then your check could be:

int? count = customers?.GetCustomer(0)?.Orders?.Count();