为什么我不能从System.Object中提取一个ushort，然后将它作为一个uint？ （C＃） [英] Why can't I pull a ushort from a System.Object and then cast it as a uint? (C#)

问题描述

我正在操作 System.Management.ManagementObjectCollection 的列表中的项目。每个项目都是一个 System.Management.ManagementObject ，其中包含以字符串索引的属性。请参阅：

  foreach（searchObject中的ManagementObject queryObj）
 {
 string osversion = string）queryObj [Version]; 
 string os =（string）queryObj [Name]; 
 uint spmajor =（uint）queryObj [ServicePackMajorVersion]; 
 uint spminor =（uint）queryObj [ServicePackMinorVersion]; 
 ... 
 ... 
 ... 
} 
  

每个字典访问queryObj 返回一个C＃对象

问题是，我得到一个 InvalidCastException

code>在 uint 中强制转换。我必须使用真正的类型，这是 ushort 。从 ushort 到 uint 的演员不应该可以接受吗？

在这种情况下，我最终会将值转换为 string ，但如果我不得不将它们转换为 uint 或 int 或 long 变量？

解决方案

您正尝试取消包装，而且只能取消装箱到某个群组。

任何类型为T的盒装值只能被取消装箱为T（或Nullable） 。

Eric Lippert对这个确切的事情做了非常好的博客文章此处。

I'm manipulating the items in a list that's a System.Management.ManagementObjectCollection. Each of these items is a System.Management.ManagementObject which contains properties indexed by string. See:

foreach (ManagementObject queryObj in searcher.Get())
{
    string osversion = (string)queryObj["Version"];
    string os = (string)queryObj["Name"];
    uint spmajor = (uint)queryObj["ServicePackMajorVersion"];
    uint spminor = (uint)queryObj["ServicePackMinorVersion"];
    ...
    ...
    ...
}

Each "dictionary access" to queryObj returns a C# object which is in fact whatever the property is supposed to be -- I have to know their "real" type beforehand, and that's OK.

Problem is, I get a InvalidCastException in the uint casts. I have to use the real type, which is ushort. Shouldn't a cast from ushort to uint be acceptable and obvious?

In this case, I'll eventually convert the values to string, but what if I had to get them into uint or int or long variables?

解决方案

You're attempting to unbox a ushort, and it can only be unboxed to a ushort.

Once you've unboxed it you can then cast it as normal.

Any boxed value of type T can only be unboxed to a T (or a Nullable).

Eric Lippert did a very good blog post about this exact thing here.

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