avoiding null reference exceptions

Question

Apparently the vast majority of errors in code are null reference exceptions. Are there any general techniques to avoid encountering null reference errors?

Unless I

Answer 1

You don't.

Or rather, there's nothing special to do to try to 'prevent' NREs in C#. For the most part an NRE is just some type of logic error. You can firewall these off at interface boundaries by checking parameters and having lots of code like

void Foo(Something x) {
    if (x==null)
        throw new ArgumentNullException("x");
    ...
}

all over the place (much of the .Net Framework does this), so that when you do screw up, you get a slightly more informative diagnostic (the stack trace is even more valuable, though, and an NRE provides that too). But you still just end up with an exception.

(Aside: Exceptions like these - NullReferenceException, ArgumentNullException, ArgumentException, ... - typically should not be caught by the program, but rather just means "developer of this code, there is a bug, please fix it". I refer to these as a 'design time' exceptions; contrast these with true 'run time' exceptions that happen as a result of the run time environment (e.g. FileNotFound) and are intended to potentially be caught and handled by the program.)

But at the end of the day, you just have to code it right.

Ideally the majority of NREs would never happen because 'null' is a nonsensical value for many types/variables, and ideally the static type system would disallow 'null' as a value for those particular types/variables. Then the compiler would prevent you from introducing this type of accidental error (ruling out certain classes of errors are what compilers and type systems are best at). This is where certain languages and type systems excel.

But without those features, you just test your code to ensure you don't have code paths with this type of error (or possibly use some outside tools that can do extra analysis for you).