问题描述
我在 Sharepoint 中使用了一小段代码，如下所示:
I was working with a small piece of code in Sharepoint, that goes like this:
int? do_ = (int?)re["mi"]; // consider "re" a dictionary<string, object>;
等号右侧的表达式计算结果为 Double.为了确保double，我在观察窗口中double-检查了它(:D).是的，它是一个 System.Double，进进出出.
The expression to the right of the equal sign evaluates to a Double. To make double sure, I double-checked it (:D) in the watch window. And yeah, it's a System.Double, in and out.
但是，该行引发异常:
指定的演员表无效.
当我意识到是的，您可以将 double 转换为 int 时，我正要用我的额头在我的桌子上做个凹痕，但会丢失值的非整数部分(这实际上是我的目的).我在编程的第一天就学会了这一点，当时世界还没有发生变化.
I was about to make a dent on my desk with my forehead when it dawned on me that yes, you can cast double to int, though losing the non-integer part of the value (which is actually intended in my case). I learned this in my first days of programming, before the world moved on.
为了更加确定，我创建了另一个解决方案，一个简单的控制台应用程序.我把这段代码放进去:
So to be even more sure, I created another solution, a simple console app. I put this code into it:
double dragon = 2.0;
int? i = (int?)dragon;
它可以正常工作，正如预期的那样.
And it works, just as expected.
为什么它在一种设置中有效，而在另一种设置中失败?
Why would it work in one setting but fail in the other one?
推荐答案
这是一个非常常见的问题.除了 T 或 T? 之外，您不能将装箱的 T 拆箱到任何东西.
This is a very frequently asked question. You cannot unbox a boxed T to anything other than T or T?.
我在这里解释原因:
http://ericlippert.com/2009/03/03/representation-and-identity/
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