При приведении ссылочного типа к значимому будет анбоксинг с одновременным приведением типа и без конвертации, как уже писали выше. В IL-коде в этом месте будет инструкция unbox.any [mscorlib]System.Int64.
Jeffrey Richter, CLR via C#, Chapter 5Internally, here’s exactly what happens when a boxed value type instance is unboxed:
1. If the variable containing the reference to the boxed value type instance is null, a NullReferenceException is thrown.
2. If the reference doesn’t refer to an object that is a boxed instance of the desired value type, an InvalidCastException is thrown.
The second item above means that the following code will not work as you might expect:
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public static void Main() {
Int32 x = 5;
Object o = x; // Box x; o refers to the boxed object
Int16 y = (Int16) o; // Throws an InvalidCastException
}
Logically, it makes sense to take the boxed Int32 that o refers to and cast it to an Int16. However, when unboxing an object, the cast must be to the exact unboxed value type—Int32 in this case.
