C # 条件 AND (& &) OR (| |)优先级

在我的工作中，我们总是陷入不必要的编码争论。今天我问条件 AND (& &)或 OR (| |)是否具有更高的优先级。我的一个同事坚持他们有同样的优先权，我有疑问，所以我查了一下。

根据 MSDN，AND (& &)的优先级比 OR (| |)高。但是，你能向持怀疑态度的同事证明吗？

Http://msdn.microsoft.com/en-us/library/aa691323(vs.71).aspx

bool result = false || true && false; // --> false
// is the same result as
bool result = (false || true) && false; // --> false
// even though I know that the first statement is evaluated as
bool result = false || (true && false); // --> false

所以我的问题是如何用代码证明 AND (& &)比 OR (| |)具有更高的优先级？如果你的答案是它不重要，那么为什么它在语言中是这样构建的？

61681 次浏览
小开

Wouldn't this get you what you're after? Or maybe I'm missing something...

bool result = true || false && false;
小开
最佳答案

Change the first false by true. I know it seems stupid to have (true || true) but it proves your point.

bool result = true || true && false;   // --> true
result = (true || true) && false; // --> false
result = true || (true && false); // --> true
小开

false || true && true

Yields: true

false && true || true

Yields: true

小开

You don't prove it with code but with logic. AND is boolean multiplication whereas OR is boolean addition. Now which one has higher precedence?

小开

If you really want to freak him out try:

bool result = True() | False() && False();


Console.WriteLine("-----");
Console.WriteLine(result);


static bool True()
{
Console.WriteLine(true);
return true;
}


static bool False()
{
Console.WriteLine(false);
return false;
}

This will print:

True
False
False
-----
False

Edit:

In response to the comment:

In C#, | is a logical operator that performs the same boolean logic as ||, but does not short-circuit. Also in C#, the | operator has a higher precedence than both || and &&.

By printing out the values, you can see that if I used the typical || operator, only the first True would be printed - followed by the result of the expression which would have been True also.

But because of the higher precedence of |, the true | false is evaluated first (resulting in true) and then that result is &&ed with false to yield false.

I wasn't trying to show the order of evaluation, just the fact that the right half of the | was evaluated period when it normally wouldn't be :)

小开

You cannot just show the end result when your boolean expressions are being short-circuited. Here's a snippet that settles your case.

It relies on implementing & and | operators used by && and ||, as stated in MSDN 7.11 Conditional logical operators

public static void Test()
{
B t = new B(true);
B f = new B(false);


B result = f || t && f;


Console.WriteLine("-----");
Console.WriteLine(result);
}


public class B {
bool val;
public B(bool val) { this.val = val; }
public static bool operator true(B b) { return b.val; }
public static bool operator false(B b) { return !b.val; }
public static B operator &(B lhs, B rhs) {
Console.WriteLine(lhs.ToString() + " & " + rhs.ToString());
return new B(lhs.val & rhs.val);
}
public static B operator |(B lhs, B rhs) {
Console.WriteLine(lhs.ToString() + " | " + rhs.ToString());
return new B(lhs.val | rhs.val);
}
public override string ToString() {
return val.ToString();
}
}

The output should show that && is evaluated first before ||.

True & False
False | False
-----
False

For extra fun, try it with result = t || t && f and see what happens with short-circuiting.


提交答案