Question

I am porting a C++ utility to C#. When I run the following statement in C++, I get the correct operation. When I run the same statement in C#, however…

Does anyone know why ‘begin++’ is executed? The crazy thing is that if I run (i % 2) == 0 with i=0, the Immediate Window returns true.

Answer 1

Operator precedence is irrelevant in this question. It’s evaluation order that causes this behavior.

In C# i++ is evaluated before i % 2 since it’s on the left side. Thus i % 2 is false and the right side of the if gets evaluated.

First you use precedence to get the syntax tree:

=
  buffer[i++]
  if i % 2
    then temp[end--]
    else temp[begin++]

On each node you evaluate the children from left to right. This implies that i++ is evaluated before i % 2.

Eric Lippert has plenty of posts on this, both here on SO, and on his blog:

• Precedence vs Associativity vs Order
• Precedence vs order, redux

Personally I’d avoid such code. It’s much nicer to split it into multiple expressions, or even use a plain if statement instead of ? :

---

In C++ accessing a variable that was written to without a sequence point in between is undefined behavior. I think = is no sequence point, so I guess your expression is undefined in C++ and just happened to work.