Beware: The null-coalescing (??) operator is low in the order of operator precedence.

Introduction

One of my favourite operators is the null-coalescing, or ?? operator.  It is used with a nullable type to evaluate to a non-null value when the nullable value is null.  For example: 

Background

int? foo = null; 
int bar = foo ?? 7;
// bar == 7

It is a shorthand for:

int? foo = null; 
int bar = foo == null ? 7 : foo;
// bar == 7

Order of Operator Precedence

It is important, however, to keep in mind the order of operator precedence:

int? top = 60;
int? bottom = 180;
int height = bottom ?? 0 - top ?? 0;

// So height == 120?

Nope. When you look at this code, it seems like it should be evaluated like:

int? top = 60;
int? bottom = 180;
int height = (bottom ?? 0) - (top ?? 0);

// height == 120

However, it is actually evaluated:

int? top = 60;
int? bottom = 180;
int height = bottom ?? (0 - top ?? 0);

// height == 180

This is caused me wasted debugging time on more than one occasion.