Question

我开始学习可空类型并遇到以下行为。

在尝试nullable int时，我看到比较运算符给了我意想不到的结果。例如，在我的下面的代码中，我得到的输出是＆＃34;两者和1相等＆＃34; 。注意，它不会打印＆＃34; null＆＃34;同样。

int? a = null;
int? b = 1;

if (a < b)
    Console.WriteLine("{0} is bigger than {1}", b, a);
else if (a > b)
    Console.WriteLine("{0} is bigger than {1}", a, b);
else
    Console.WriteLine("both {0} and {1} are equal", a, b);

我希望任何非负整数都大于null，我在这里错过了什么？

Answer 1

根据MSDN - 它位于“操作员”部分的页面下方：

当您对可空类型进行比较时，如果其中一个可空类型的值为null而另一个不可归类型，则除了false之外，所有比较都会计算为!=

因此，a > b和a < b评估为false，因为a为空...

Answer 2

正如MSDN所说

当您与可空类型进行比较时，如果值为1 可以为空的类型是null，而另一个不是，所有的比较评估为假，除了！=（不等于）。重要的是不要假设因为特定比较返回false，所以相反的情况返回true。在以下示例中，10不是大于，小于，等于null。只有num1！= num2 评估为真。

int? num1 = 10;
int? num2 = null;
if (num1 >= num2)
{
    Console.WriteLine("num1 is greater than or equal to num2");
}
else
{
    // This clause is selected, but num1 is not less than num2.
    Console.WriteLine("num1 >= num2 returned false (but num1 < num2 also is false)");
}

if (num1 < num2)
{
    Console.WriteLine("num1 is less than num2");
}
else
{
    // The else clause is selected again, but num1 is not greater than 
    // or equal to num2.
    Console.WriteLine("num1 < num2 returned false (but num1 >= num2 also is false)");
}

if (num1 != num2)
{
    // This comparison is true, num1 and num2 are not equal.
    Console.WriteLine("Finally, num1 != num2 returns true!");
}

// Change the value of num1, so that both num1 and num2 are null.
num1 = null;
if (num1 == num2)
{
    // The equality comparison returns true when both operands are null.
    Console.WriteLine("num1 == num2 returns true when the value of each is null");
}

/* Output:
 * num1 >= num2 returned false (but num1 < num2 also is false)
 * num1 < num2 returned false (but num1 >= num2 also is false)
 * Finally, num1 != num2 returns true!
 * num1 == num2 returns true when the value of each is null
 */

Answer 3

总结：即使两个操作数均为空，任何与null（salt-cp 'test1' /tmp/test* salt:// -l debug [DEBUG ] Reading configuration from /etc/salt/master [DEBUG ] Configuration file path: /etc/salt/master [WARNING ] Insecure logging configuration detected! Sensitive data may be logged. [DEBUG ] Reading configuration from /etc/salt/master [DEBUG ] Missing configuration file: /root/.saltrc [DEBUG ] MasterEvent PUB socket URI: /var/run/salt/master/master_event_pub.ipc [DEBUG ] MasterEvent PULL socket URI: /var/run/salt/master/master_event_pull.ipc [DEBUG ] Initializing new AsyncZeroMQReqChannel for ('/etc/salt/pki/master', 'XXX_master', 'tcp://127.0.0.1:44506', 'clear') [DEBUG ] Initializing new IPCClient for path: /var/run/salt/master/master_event_pub.ipc [DEBUG ] SaltReqTimeoutError, retrying. (1/3) [DEBUG ] SaltReqTimeoutError, retrying. (2/3) [DEBUG ] SaltReqTimeoutError, retrying. (3/3) [ERROR ] An un-handled exception was caught by salt's global exception handler: SaltClientError: Salt request timed out. The master is not responding. If this error persists after verifying the master is up, worker_threads may need to be increased. Traceback (most recent call last): File "/usr/bin/salt-cp", line 10, in <module> salt_cp() File "/usr/lib/python2.7/dist-packages/salt/scripts.py", line 359, in salt_cp client.run() File "/usr/lib/python2.7/dist-packages/salt/cli/cp.py", line 38, in run cp_.run() File "/usr/lib/python2.7/dist-packages/salt/cli/cp.py", line 105, in run ret = local.cmd(*args) File "/usr/lib/python2.7/dist-packages/salt/client/__init__.py", line 568, in cmd **kwargs) File "/usr/lib/python2.7/dist-packages/salt/client/__init__.py", line 317, in run_job raise SaltClientError(general_exception) SaltClientError: Salt request timed out. The master is not responding. If this error persists after verifying the master is up, worker_threads may need to be increased. salt.exceptions.SaltClientError: Salt request timed out. The master is not responding. If this error persists after verifying the master is up, worker_threads may need to be increased.，>=，<，<=）的不等式比较都会返回>。即。

false

与null（null > anyValue //false null <= null //false，==）的任何相等或不相等比较都“按预期”工作。即。

!=

Answer 4

将C＃与SQL进行比较

C＃：a = null和b = null => a == b => true

SQL：a = null和b = null => a == b =>假

比较运算符如何与null int一起使用？

4 个答案: