可空对象必须有一个价值 [英] nullable object must have a value

问题描述

有矛盾是在异常的描述：
可空对象必须有一个值（？！）

There is paradox in the exception description: Nullable object must have a value (?!)

这就是问题所在：

我有一个 DateTimeExtended 类，
有

I have a DateTimeExtended class, that has

{
  DateTime? MyDataTime;
  int? otherdata;

}

和构造

DateTimeExtended(DateTimeExtended myNewDT)
{
   this.MyDateTime = myNewDT.MyDateTime.Value;
   this.otherdata = myNewDT.otherdata;
}

运行此code

running this code

DateTimeExtended res = new DateTimeExtended(oldDTE);

引发出现InvalidOperationException 与消息：

可空对象必须有一个值。

Nullable object must have a value.

myNewDT.MyDateTime.Value - 是有效的，并包含正规的DateTime 对象

myNewDT.MyDateTime.Value - is valid and contain a regular DateTime object.

这是什么消息的含义，什么我做错了？

What is the meaning of this message and what am I doing wrong?

注意 oldDTE 不是空。我已删除了值从 myNewDT.MyDateTime 但同样的异常抛出因于所产生的制定者。

Note that oldDTE is not null. I've removed the Value from myNewDT.MyDateTime but the same exception is thrown due to a generated setter.

推荐答案

您应该更改行 this.MyDateTime = myNewDT.MyDateTime.Value; 只是 this.MyDateTime = myNewDT.MyDateTime;

您正在接受可空的DateTime的.Value属性被抛出，因为它是需要返回一个DateTime（因为那是在合同.value的状态），但它不能这样做，因为没有例外日期时间返回，所以它抛出一个异常。

The exception you were receiving was thrown in the .Value property of the Nullable DateTime, as it is required to return a DateTime (since that's what the contract for .Value states), but it can't do so because there's no DateTime to return, so it throws an exception.

在一般情况下，这是一个坏主意，一味呼吁。价值上可空类型，除非你有一些事先知道该变量必须包含一个值（即通过的HasValue检查）。

In general, it is a bad idea to blindly call .Value on a nullable type, unless you have some prior knowledge that that variable MUST contain a value (i.e. through a HasValue check).

修改

这里的code为DateTimeExtended不抛出异常：

Here's the code for DateTimeExtended that does not throw an exception:

class DateTimeExtended
{
    public DateTime? MyDateTime;
    public int? otherdata;

    public DateTimeExtended() { }

    public DateTimeExtended(DateTimeExtended other)
    {
        this.MyDateTime = other.MyDateTime;
        this.otherdata = other.otherdata;
    }
}

我测试了它是这样的：

I tested it like this:

DateTimeExtended dt1 = new DateTimeExtended();
DateTimeExtended dt2 = new DateTimeExtended(dt1);

在other.MyDateTime添加.value的导致异常。删除它摆脱了异常。我想你找错了地方。

Adding the .Value on other.MyDateTime causes an exception. Removing it gets rid of the exception. I think you're looking in the wrong place.

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