C＃ - 与拳击/拆箱/整数类型转换的问题。我不明白 [英] C# - Issues with boxing / unboxing / typecasting ints. I don't understand

问题描述

我有一个很难理解这一点。请看下面的例子：

I'm having a hard time understanding this. Consider the following example:

protected void Page_Load(object sender, EventArgs e)
{
    // No surprise that this works
    Int16 firstTest = Convert.ToInt16(0);
    int firstTest2 = (int)firstTest;

    // This also works
    object secondTest = 0;
    int secondTest2 = (int)secondTest;

    // But this fails!
    object thirdTest = Convert.ToInt16(0);
    int thirdtest2 = (int)thirdTest;  // It blows up on this line.
}

这是我在运行时获取特定的错误是指定转换无效。如果我快速监视（INT）thirdTest 在Visual Studio中，我得到的值不能拆箱 thirdTest'作为'廉政'。

The specific error that I get at runtime is Specified cast is not valid. If I QuickWatch (int)thirdTest in Visual Studio, I get a value of Cannot unbox 'thirdTest' as a 'int'.

到底是什么怎么回事？

推荐答案

拆箱检查作为文档中解释的确切类型

拆箱是从object类型的值
型或从接口类型为显式转换实现
接口值类型。一个拆箱操作包括：

Unboxing is an explicit conversion from the type object to a value type or from an interface type to a value type that implements the interface. An unboxing operation consists of:

• 检查对象实例，以确保它是$ B $的装箱值b给定值的类型。

• Checking the object instance to make sure that it is a boxed value of the given value type.

复制从实例中的值转换成数值型变量。

Copying the value from the instance into the value-type variable.

正如你所看到的第一个步骤是检查对象实例的目标类型相匹配。

As you can see the first step is to check that the object instance matches the target type.

另外从文档报价：

有关值类型的拆箱在运行时取得成功，该项目是
拆箱必须是以前通过拳击该值类型的实例创建
对象的引用。试图拆箱空
会导致一个NullReferenceException。试图拆箱到
不兼容的值类型的引用导致一个InvalidCastException。

For the unboxing of value types to succeed at run time, the item being unboxed must be a reference to an object that was previously created by boxing an instance of that value type. Attempting to unbox null causes a NullReferenceException. Attempting to unbox a reference to an incompatible value type causes an InvalidCastException.

因此​​，要解决这个错误请确保类型尝试拆箱之前匹配：

So to fix this error make sure that the type matches before attempting to unbox:

object thirdTest = Convert.ToInt16(0);
short thirdtest2 = (short)thirdTest;  

这篇关于C＃ - 与拳击/拆箱/整数类型转换的问题。我不明白的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！