C# int- or object-to-double 转换错误解释

Question

以下代码在最后一次赋值时失败:

static void Main(string[] args)
{
    int a = 5;
    object b = 5;

    System.Diagnostics.Debug.Assert( a is int && b is int );

    double x = (double)a;
    double y = (double)b;
}

如果a和b都是int，这个错误的原因是什么？

Answer 1

这是一个非常常见的问题。参见 https://ericlippert.com/2009/03/03/representation-and-identity/寻求解释。

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片段:

I get a fair number of questions about the C# cast operator. The most frequent question I get is:

short sss = 123;
object ooo = sss;            // Box the short.
int iii = (int) sss;         // Perfectly legal.
int jjj = (int) (short) ooo; // Perfectly legal
int kkk = (int) ooo;         // Invalid cast exception?! Why?

Why? Because a boxed T can only be unboxed to T. (*) Once it is unboxed, it’s just a value that can be cast as usual, so the double cast works just fine.

(*) Or Nullable<T>.