为什么Convert.ToString（NULL）返回，如果你施放空不同的值？ [英] Why does Convert.ToString(null) return a different value if you cast null?

问题描述

Convert.ToString(null)

返回

null

正如我所料。

As I expected.

但

Convert.ToString(null as object)

返回

""

为什么这些不同？

Why are these different?

推荐答案

有2重载的ToString 的这里都有了用武之地

There are 2 overloads of ToString that come into play here

Convert.ToString(object o);
Convert.ToString(string s);

C＃编译器实质上是尝试挑选最具体的超负荷工作，并会与输入。 A 空值转换为任何引用类型。在这种情况下字符串比对象，因此它将被选为优胜者更具体。

The C# compiler essentially tries to pick the most specific overload which will work with the input. A null value is convertible to any reference type. In this case string is more specific than object and hence it will be picked as the winner.

在 null作为对象您已经凝固的EX pression类型为对象。这意味着它不再与字符串兼容过载和编译器采对象超载，因为它是唯一兼容的一个剩余。

In the null as object you've solidified the type of the expression as object. This means it's no longer compatible with the string overload and the compiler picks the object overload as it's the only compatible one remaining.

这是如何打破平局的作品涵盖了C＃语言规范第7.4.3真的毛茸茸的细节。

The really hairy details of how this tie breaking works is covered in section 7.4.3 of the C# language spec.

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