[英]== vs Equals in C#
What is the difference between the evaluation of == and Equals in C#? 在C#中评估==和Equals有什么区别?
For Ex, 对于Ex,
if(x==x++)//Always returns true
but 但
if(x.Equals(x++))//Always returns false
Edited: 编辑:
int x=0;
int y=0;
if(x.Equals(y++))// Returns True
According to the specification, this is expected behavior. 根据规范,这是预期的行为。
The behavior of the first is governed by section 7.3 of the spec: 第一部分的行为受规范第7.3节的约束:
Operands in an expression are evaluated from left to right. 表达式中的操作数从左到右进行计算。 For example, in
F(i) + G(i++) * H(i)
, method F is called using the old value of i, then method G is called with the old value of i, and, finally, method H is called with the new value of i. 例如,在F(i) + G(i++) * H(i)
,使用旧值i调用方法F,然后使用旧值i调用方法G,最后调用方法H与i的新价值。 This is separate from and unrelated to operator precedence. 这与运算符优先级分开并且与运算符优先级无关。
Thus in x==x++
, first the left operand is evaluated ( 0
), then the right-hand is evaluated ( 0
, x
becomes 1
), then the comparison is done: 0 == 0
is true. 因此,在x==x++
,首先评估左操作数( 0
),然后评估右手( 0
, x
变为1
),然后进行比较: 0 == 0
为真。
The behavior of the second is governed by section 7.5.5: 第二部分的行为受第7.5.5节的约束:
- If M is an instance function member declared in a value-type : 如果M是在value-type中声明的实例函数成员:
- E is evaluated. E被评估。 If this evaluation causes an exception, then no further steps are executed. 如果此评估导致异常,则不执行进一步的步骤。
- If E is not classified as a variable, then a temporary local variable of E's type is created and the value of E is assigned to that variable. 如果E未被归类为变量,则创建E类型的临时局部变量,并将E的值分配给该变量。 E is then reclassified as a reference to that temporary local variable. 然后将E重新分类为对该临时局部变量的引用。 The temporary variable is accessible as this within M, but not in any other way. 临时变量在M中可以访问,但不能以任何其他方式访问。 Thus, only when E is a true variable is it possible for the caller to observe the changes that M makes to this. 因此,只有当E是真变量时,呼叫者才有可能观察到M对此做出的变化。
- The argument list is evaluated as described in §7.5.1. 参数列表按照§7.5.1中的描述进行评估。
- M is invoked. M被调用。 The variable referenced by E becomes the variable referenced by this. E引用的变量成为由此引用的变量。
Note that value types are passed by reference to their own methods. 请注意, 值类型通过引用传递给它们自己的方法。
Thus in x.Equals(x++)
, first the target is evaluated (E is x
, a variable), then the arguments are evaluated ( 0
, x
becomes 1
), then the comparison is done: x.Equals(0)
is false. 因此,在x.Equals(x++)
,首先计算目标(E是x
,变量),然后计算参数( 0
, x
变为1
),然后进行比较: x.Equals(0)
为false 。
EDIT: I also wanted to give credit to dtb's now-retracted comment, posted while the question was closed. 编辑:我也想赞扬dtb现在收回的评论,在问题结束时发布。 I think he was saying the same thing, but with the length limitation on comments he wasn't able to express it fully. 我认为他说的是同样的事情,但由于评论的长度限制,他无法完全表达。
Order of evaluation. 评估顺序。 ++ evaluates first (second example). ++首先评估(第二个例子)。 But in the first example, == executes first. 但在第一个例子中,==首先执行。
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