C# 左移 255（作为一个字节）
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Left bit shifting 255 (as a byte)
提问by Chris S
Can anyone explain why the following doesn't compile?
谁能解释为什么以下不能编译？
byte b = 255 << 1
The error:
错误：
Constant value '510' cannot be converted to a 'byte'
常量值“510”无法转换为“字节”
I'm expecting the following in binary:
我期待以下二进制：
1111 1110
The type conversion has stumped me.
类型转换难倒了我。
采纳答案by Joey
Numeric literals in C# are int, not byte(and the bit shift will be evaluated by the compiler, hence only the 510 remains). You are therefore trying to assign a value to a bytewhich does not fit. You can mask with 255:
C# 中的数字文字是int，不是byte（并且位移位将由编译器计算，因此只剩下 510）。因此，您试图为byte不合适的a 分配一个值。您可以使用 255 进行屏蔽：
byte b = (255 << 1) & 0xFF
to reduce the result to 8 bits again. Unlike Java, C# does not allow overflows to go by undetected. Basically you'd have two sensible options when trying to assign 510 to a byte: Either clamp at the maximum value, then you'd get 255, or throw away the bits that do not fit, in which case you'd get 254.
再次将结果减少到 8 位。与 Java 不同，C# 不允许未检测到的溢出。基本上，在尝试将 510 分配给一个字节时，您有两个明智的选择：要么钳制最大值，然后得到 255，要么丢弃不适合的位，在这种情况下，您将得到 254。
You can also use unchecked, as lassevk mentioned:
你也可以使用unchecked，正如lassevk 提到的：
byte b = unchecked((byte)(255 << 1));
回答by Otávio Décio
255 << 1
will give you more than one byte.
会给你不止一个字节。
回答by Daniel A. White
have you tried casting it?
你试过铸造它吗？
byte b = (byte)(255 << 1)
This is an interesting approach - the above code will work if wrapped in a uncheckedblock like this:
这是一个有趣的方法 - 如果包裹在这样的unchecked块中，上面的代码将起作用：
unchecked
{
byte b = (byte)(255 << 1);
}
Since it is uncheckedthe value is truncated to the intended value of 254. So it is possible to do this with a cast!
由于它unchecked的值被截断为预期值 254。因此可以通过强制转换来做到这一点！
回答by Steve
You are shifting 255 by 1 bit, then trying to assign it to a byte.
255 << 1 is 510, and 510 won't fit in to a byte.
您将 255 移动 1 位，然后尝试将其分配给一个字节。
255 << 1 is 510, 并且 510 不适合一个字节。
回答by Michael Burr
byte b = 0xff & (255 << 1);
回答by Lasse V. Karlsen
The result of the <<operator is an Int32, not what you put into it.
<<运算符的结果是Int32，而不是您放入其中的内容。
You need to cast the result of the shift, not the input. Additionally, it will produce an overflow (it is larger than a byte afterall), so you need to specify that you need an unchecked cast.
您需要转换转换的结果，而不是输入。此外，它会产生溢出（毕竟它大于一个字节），因此您需要指定您需要未经检查的强制转换。
In other words, this will work:
换句话说，这将起作用：
Byte b = unchecked((Byte)(255 << 1));
回答by sipwiz
And since << has a higher precedence than & you can save the brackets:
由于 << 的优先级高于 & 您可以保存括号：
byte b = 255 << 1 & 0xff;
