平等运营商 - 总是左联合的，是否重要？

Question

In c# Language Specification , its to be found "Except for the assignment operators, all binary operators are left-associative"

But will it make at difference for Equality operators? == and != ?

Say: bool X,Y,Z ...

return x==Y==z ... and so on?

Wont the result always be the same even if the order of x,y,z ... differs in the return statement, always return the same value? (if the init values of x,y,z is the same obviously)

(x==y)==z Equals to x==(Y==z) and for (x!=y)==z Equals to x!=(Y==z)

Right?

Answer 1

In the C# 4.0 Language Specification, its to be found "Except for the assignment operators, all binary operators are left-associative"

FYI that line is an error. The null coalescing operator is also right-associative -- not that it matters really. See https://stackoverflow.com/a/6269773/88656 for Jon's great analysis of this issue.

Will it make at difference for equality operators == and != operating only on bools?

No. As you correctly note, operator associativity is irrelevant when dealing only with equality operators and only on bools. Though that is an interesting bit of trivia, I'm not sure why it is particularly relevant.

A number of people have answered that it makes a difference because there could be side effects in the operands. These people are wrong; this is an extremely common error. The order in which operands are evaluated has nothing whatsoever to do with the precedence or associativity of the operators. In C, for example, a conforming implementation is allowed to compute side effects in any order. If you say:

x = (A() != B()) == C();
y = D() != (E() == F());

In C then you are guaranteed that D(), E() and F() do not run before A(), B() and C(). But you are not guaranteed that side effects of B() are computed before C(). A(), B() and C() can be executed in any order in C, legally. Operator associativity, whether 'natural' or imposed by parentheses, does not impose any restriction on the order in which side effects may be computed.

The situation is somewhat improved in C#. In C# you are guaranteed that side effects of operands are observed to happen left to right regardless of the precedence or associativity of the operators . That is, if you have A() + B() * C() then you are guaranteed that A() happens before B() and that B() happens before C(), regardless of the fact that A() is involved in an addition which is lower precedence than a multiplication.

Operator precedence and associativity controls the order in which the operators run, not the order in which the operands are computed.

Answer 2

bool x = false;
int y = 0;
int z = 1;

Console.WriteLine(x == y == z);//won't compile
Console.WriteLine(y == z == x);//will compile

All down to the associativity of == .

Answer 3

In your example, it would matter. x==y==z would mean "Z is a bool, and the line will return true if 'x==y && z==true' or if 'x!=y && z==false'".

So yes, it matters.

Answer 4

把它想象为（x ^ y ^ z），你就会有答案。

Answer 5

The result will be the same. The associativity of == and != does not matter, but an associativity must be chosen.

Answer 6

It could potentially make a difference if you provided an overload for the equality operator that had side effects. A terrible and pointless idea, obviously, but possible.