为什么此转换无效？ [英] Why does this conversion doesn't work?

问题描述

以下代码行为异常（至少对我来说）：

following code behaves strange (at least for me):

int testValue = 1234;

this.ConversionTest( testValue );

private void ConversionTest( object value )
{
    long val_1 = (long) (int) value; // works
    long val_2 = (long) value;       // InvalidCastException
}

我不明白为什么直接长不工作。
有人可以解释这个行为吗？

I don't understand why the direct (explicit) cast to long doesn't work. Can someone explain this behaviour?

感谢

推荐答案

ConversionTest 方法的值参数类型为 object ;这意味着传递给方法的任何值类型 - 例如 int - 将被加框。

The value parameter of your ConversionTest method is typed as object; this means that any value types -- for example, int -- passed to the method will be boxed.

装箱值只能取消装箱到完全相同的类型：

Boxed values can only be unboxed to exactly the same type:

• 当您执行（long） 您首先将值解除到 int （其原始类型） 到长。


• 当<$ c $ int 到长 >，这是非法的。

• When you do (long)(int)value you're first unboxing value to an int (its original type) and then converting that int to a long.
• When you do (long)value you're attempting to unbox the boxed int to a long, which is illegal.

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