C# DateTime 为空字符串或 null ？如何检查？
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/8985854/
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
DateTime as empty String or null ?How to check?
提问by Anyname Donotcare
Q:
问：
I want to check the DateTime against null valueto empty the cell in my report if the datetime is null.but i don't know how to do this :it appears like this 1/1/0001if it was null.and i want it to be empty cell.
null value如果日期时间为空，我想检查日期时间以清空报告中的1/1/0001单元格。
This is the datatype in my dataset :
这是我的数据集中的数据类型：
and this is the expression value of my column :
这是我的列的表达式值：
=FormatDateTime(Fields!D_DateTime.Value,2)
采纳答案by Anyname Donotcare
=IIf(FormatDateTime(Fields!D_DateTime.Value,2)=CDate("1/1/0001"),"",FormatDateTime(Fields!D_DateTime.Value,2))
Thanks a lot ,i think this fixes my problem.
非常感谢，我认为这解决了我的问题。
回答by Marco
As I told you in my comment, you should check if your date is DateTime.MinValue(the minimum value a date can assume, which is exactly 01/01/0001).
正如我在评论中告诉你的，你应该检查你的日期是否是DateTime.MinValue（日期可以假设的最小值，也就是 01/01/0001）。
if (your_date_property == DateTime.MinValue)
{
// Do what you need
}
回答by Maheep
As datetime is a struct rather than class i.e. a value type rather than a reference type; it must be initialized with some value. It cannot have null values.
由于 datetime 是一个结构而不是类，即值类型而不是引用类型；它必须用某个值初始化。它不能有空值。
Hence to check the default value you should check the equality with DateTime.MinValue
因此要检查默认值，您应该检查与 DateTime.MinValue 的相等性
i.e.
IE
if(D_DateTime.Value == DateTime.MinValue)
{
//write code here for default value handling
}
回答by lc.
Change the type of the field in the dataset (rd:TypeName) to System.Nullable (Of System.DateTime). Then you can simply test =Fields!D_DateTime.Value Is Nothing.
将数据集中的字段类型 ( rd:TypeName) 更改为System.Nullable (Of System.DateTime)。然后你可以简单地测试=Fields!D_DateTime.Value Is Nothing.
回答by LCJ
Like @Marco suggested you can check for MinValue. And if you want to pass NULLto the nullable parameter, you can use the following code for reportviewerparameter.
就像@Marco 建议您可以检查MinValue. 而如果你想传递NULL给可为空的参数，你可以使用下面的reportviewer参数代码。
Vb.Net
网络
Dim rpFrom As New ReportParameter("FromDate", New String() {Nothing}, False)
C#
C＃
ReportParameter rpFrom = new ReportParameter("FromDate", new string[] { null }, false);
