Why is "null" present in C# and Java?

Question

We noticed that lots of bugs in our software developed in C# (or Java) cause a NullReferenceException.

Is there a reason why "null" has even been included in the language?

After all, if there were no "null", I would have no bug, right?

In other words, what feature in the language couldn't work without null?

Answer 1

Anders Hejlsberg, "C# father", just spoke about that point in his Computerworld interview:

For example, in the type system we do not have separation between value and reference types and nullability of types. This may sound a little wonky or a little technical, but in C# reference types can be null, such as strings, but value types cannot be null. It sure would be nice to have had non-nullable reference types, so you could declare that ‘this string can never be null, and I want you compiler to check that I can never hit a null pointer here’.

50% of the bugs that people run into today, coding with C# in our platform, and the same is true of Java for that matter, are probably null reference exceptions. If we had had a stronger type system that would allow you to say that ‘this parameter may never be null, and you compiler please check that at every call, by doing static analysis of the code’. Then we could have stamped out classes of bugs.

Cyrus Najmabadi, a former software design engineer on the C# team (now working at Google) discuss on that subject on his blog: (1st, 2nd, 3rd, 4th). It seems that the biggest hindrance to the adoption of non-nullable types is that notation would disturb programmers’ habits and code base. Something like 70% of references of C# programs are likely to end-up as non-nullable ones.

If you really want to have non-nullable reference type in C# you should try to use Spec# which is a C# extension that allow the use of "!" as a non-nullable sign.

static string AcceptNotNullObject(object! s)
{
    return s.ToString();
}