C# 比较可为空的 DateTime？
声明：本页面是StackOverFlow热门问题的中英对照翻译，遵循CC BY-SA 4.0协议，如果您需要使用它，必须同样遵循CC BY-SA许可，注明原文地址和作者信息，同时你必须将它归于原作者（不是我）：StackOverFlow
原文地址: http://stackoverflow.com/questions/13673117/
Warning: these are provided under cc-by-sa 4.0 license. You are free to use/share it, But you must attribute it to the original authors (not me):
StackOverFlow
Comparing nullable DateTime?
提问by genxgeek
Looking for a better way to compare a nullable date time than the following:
寻找比以下更好的方法来比较可为空的日期时间：
Any suggestions?
有什么建议？
// myobject.ExpireDatetime is of DateTime?
//
if (!myobject.ExpireDateTime.IsNull() && DateTime.Compare((DateTime)myobject.ExpireDateTime, DateTime.Now.ToUniversalTime()) < 0)
{ //error! }
Edited: Sorry for confusion...myobject.ExpireDatetime is of type DateTime.
编辑：抱歉混淆...myobject.ExpireDatetime 是 DateTime 类型。
采纳答案by Jeppe Stig Nielsen
Your question is not quite clear to me, but if we have
你的问题对我来说不是很清楚，但如果我们有
DateTime? ExpireDateTime; // could be a variable or a property
it's OK to say just
可以说只是
if (ExpireDateTime < DateTime.UtcNow)
{
...
}
This will be OK if ExpireDateTimeis null(HasValueis false). Some inexperienced developers will struggle to understand lifted operators, so to make it more clear, you could write
如果ExpireDateTime是null(HasValue是假的) 就可以了。一些没有经验的开发人员会很难理解提升操作符，所以为了更清楚，你可以写
if (ExpireDateTime < (DateTime?)DateTime.UtcNow)
{
...
}
It's the same, but easier to read and understand.
它是一样的，但更容易阅读和理解。
Neverwrite .Valueif the nullable might be null, of course. You will get an InvalidOperationException"Nullable object must have a value"if you do so.
.Value当然，如果可空对象可能为空，则永远不要写。如果你这样做，你会得到一个InvalidOperationException“可空对象必须有一个值”。
回答by Oded
回答by Tim Schmelter
If ExpireDateTimeis a Nullable<DateTime>i would do following instead:
如果 ExpireDateTime是一个 Nullable<DateTime>我会做以下：
if (ExpireDateTime.HasValue && ExpireDateTime < DateTime.Now.ToUniversalTime())
{
}
回答by labroo
I would recommend you to use the following:
我建议您使用以下方法：
int y = Nullable.Compare<DateTime>(DateTime.UtcNow, x);
// x is the nullable date time object.
// y will be '-1' if x is greater than DateTime.UtcNow
回答by Patrick
The compiler liftsvariables and generates code to check for nulls.
编译器提升变量并生成代码以检查空值。
> new DateTime?()
null
> DateTime.Now > null
false
> DateTime.Now < null
false
> new int?()
null
> 10 >= null
false
> 10 =< null
false
> 10 == null
false
> 10 != null
true
Knowing this you can write simple code.
知道了这一点，您就可以编写简单的代码。
// d is a DateTime?
DateTime now = DateTime.Now;
bool after = d > now;
bool before = d < now;
bool today = d?.Date == now.Date;
If d is null everything will be false, else it will work like normal DateTime comparison.
如果 d 为空，一切都将是假的，否则它将像正常的 DateTime 比较一样工作。
