C# 如何使用三元运算符检查空值或日期时间值？
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How to check null or datetime value using ternary operator?
提问by Neo
I want to check passed value is null or datetime value using ternary operator in c#?
我想在 c# 中使用三元运算符检查传递的值是空值还是日期时间值？
I tried like this
我试过这样
fromDate == null ? null : Convert.ToDateTime(fromDate)
getting error:
得到错误：
type of conditional expression cannot be determined
I want to check whether variable fromDateis null or having date time value ?
我想检查变量是否fromDate为空或具有日期时间值？
variable fromDate is coming from Querystring and type of string.
变量 fromDate 来自 Querystring 和字符串类型。
采纳答案by Soner G?nül
From ?: Operator:
来自?: Operator：
Either the type of first_expression and second_expression must be the same, or an implicit conversion must exist from one type to the other.
first_expression 和 second_expression 的类型必须相同，或者必须存在从一种类型到另一种类型的隐式转换。
condition ? first_expression : second_expression;
Convert.ToDateTimereturns DateTimeand there is no implicit conversionbetween nulland DateTime. And the conditional operatoris an expression and that needs a return type.
Convert.ToDateTime返回DateTime并且在和之间没有隐式转换。而条件运算符是一个表达式，并且需要一个返回类型。nullDateTime
One option seems logical to me using DateTime.TryParse(which returns boolean) as a first expressionand use another boolean (trueor false) as a second expression. Damiths' answerseems logical.
一个选项对我来说似乎合乎逻辑，使用DateTime.TryParse（返回boolean）作为第一个表达式并使用另一个布尔值（true或false）作为第二个表达式。达米斯的回答似乎合乎逻辑。
Or instead you can use nullable DateTime like DateTime?
或者，您可以使用可为空的 DateTime 之类的 DateTime?
DateTime? foo;
if(foo.HasValue)
{
//Nullable DateTime has a value..
}
回答by Hamlet Hakobyan
romDate == null ? null : Convert.ToDateTime(fromDate)
nulland Convert.ToDateTime(fromDate)haven't common type. Compiler must be able to cast both expressions in canditional operator to same type.
null并且Convert.ToDateTime(fromDate)没有共同类型。编译器必须能够将条件运算符中的两个表达式转换为相同的类型。
回答by Ali Baghdadi
you must use String.IsNullOrEmpty to check if fromDate is null like this:
您必须使用 String.IsNullOrEmpty 来检查 fromDate 是否为空，如下所示：
DateTime? date = String.IsNullOrEmpty(fromDte) ? null : (DateTime?)Convert.ToDateTime(fromDate)
回答by Damith
if you have string value for fromDatedo as below
如果你有字符串值fromDate，如下做
DateTime dt;
bool isValidDate = String.IsNullOrEmpty(fromDate) ? false : DateTime.TryParse(fromDate, out dt);
if you know the datetime format/formats which your input having you better use DateTime.TryParseExactmethod
如果您知道输入的日期时间格式/格式让您更好地使用DateTime.TryParseExact方法
if fromDate is DateTime, then simple you can check as below
如果fromDate是DateTime，那么简单你可以检查如下
fromDate.HasValue
you don't need ?: Operator
你不需要 ?: Operator
回答by Sain Pradeep
I hope it will help you
我希望它会帮助你
string format = "ddd dd MMM h:mm tt yyyy";
DateTime dateTime;
fromDate=(DateTime.TryParseExact(fromDate, format, CultureInfo.InvariantCulture,
DateTimeStyles.None, out dateTime))?dateTime:null;
回答by Lummo
It looks like the main problem is that you are trying to assign nullto a DateTime
看起来主要问题是您试图分配null给DateTime
DateTime is a structure and not a reference type so this can't be done.
DateTime 是一个结构而不是一个引用类型，所以这不能完成。
Either use a nullable DateTime (DateTime?) or a specific value to indicate null, such as DateTime.MinValue
使用可为空的 DateTime ( DateTime?) 或特定值来指示空值，例如DateTime.MinValue
Have a look here for an example: http://www.dotnetperls.com/nullable-datetime
看看这里的一个例子：http: //www.dotnetperls.com/nullable-datetime
回答by TK-421
The problem is that Convert.ToDateTime(fromDate) is of type DateTime, which cannot accept a null value, that is why this code won't work in this form. You have two choices to make. Either change it to regular if:
问题是 Convert.ToDateTime(fromDate) 是 DateTime 类型，它不能接受空值，这就是为什么这段代码不能在这种形式下工作。你有两个选择。如果出现以下情况，请将其更改为常规：
if(fromDate != null)
{
Convert.ToDateTime(fromDate)
}
Or cast DateTime to nullable DateTime:
或将 DateTime 转换为可为空的 DateTime：
fromDate == null ? null : (DateTime?)Convert.ToDateTime(fromDate)
Ternary operator in C# needs both values to be of the same type and it is explained here.
C# 中的三元运算符需要两个值的类型相同，这里解释。
