问题描述

考虑以下代码:

void Handler(object o, EventArgs e)
{
   // I swear o is a string
   string s = (string)o; // 1
   //-OR-
   string s = o as string; // 2
   // -OR-
   string s = o.ToString(); // 3
}

这三种类型的施法有什么区别(好吧，第三种不是施法，但你明白了).应该首选哪一个?

What is the difference between the three types of casting (okay, the 3rd one is not a casting, but you get the intent). Which one should be preferred?

推荐答案

string s = (string)o; // 1

如果 o 是，则抛出 InvalidCastException不是字符串.否则，将 o 分配给 s，即使 o 为 null.

Throws InvalidCastException if o is not a string. Otherwise, assigns o to s, even if o is null.

string s = o as string; // 2

如果 o 不是 string 或者如果 o 则将 null 分配给 s> 是 null.因此，您不能将它与值类型一起使用(在这种情况下，运算符永远不会返回 null).否则，将 o 分配给 s.

Assigns null to s if o is not a string or if o is null. For this reason, you cannot use it with value types (the operator could never return null in that case). Otherwise, assigns o to s.

string s = o.ToString(); // 3

如果 o 会导致 NullReferenceException是 null.将任何 o.ToString() 返回的内容分配给 s，无论 o 是什么类型.

Causes a NullReferenceException if o is null. Assigns whatever o.ToString() returns to s, no matter what type o is.

对大多数转换使用 1 - 它简单明了.我几乎从不使用 2 ，因为如果某些类型不正确，我通常希望会发生异常.我只看到使用错误代码(例如返回 null = 错误，而不是使用异常)的糟糕设计的库需要这种返回 null 类型的功能.

Use 1 for most conversions - it's simple and straightforward. I tend to almost never use 2 since if something is not the right type, I usually expect an exception to occur. I have only seen a need for this return-null type of functionality with badly designed libraries which use error codes (e.g. return null = error, instead of using exceptions).

3 不是强制转换，只是一个方法调用.当您需要非字符串对象的字符串表示时使用它.

3 is not a cast and is just a method invocation. Use it for when you need the string representation of a non-string object.

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