Check your nulls

I call it my billion-dollar mistake. It was the invention of the null
reference in 1965. At that time, I was designing the first
comprehensive type system for references in an object oriented language
(ALGOL W). My goal was to ensure that all use of references should be
absolutely safe, with checking performed automatically by the compiler.
But I couldn't resist the temptation to put in a null reference, simply
because it was so easy to implement. This has led to innumerable
errors, vulnerabilities, and system crashes, which have probably caused
a billion dollars of pain and damage in the last forty years. In recent
years, a number of program analysers like PREfix and PREfast in
Microsoft have been used to check references, and give warnings if
there is a risk they may be non-null. More recent programming languages
like Spec# have introduced declarations for non-null references. This
is the solution, which I rejected in 1965.