blocks|key|342833|text|因为C#中的%2B操作符在内部转换为String.Concat，这是一个静态方法。此方法恰好将null视为空字符串。如果你在Reflector中查看String.Concat的源代码，你会看到它：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|342834|//+while+looping+through+the+parameters
strArray[i]+=+(str+==+null)+?+Empty+:+str;
//+then+concatenate+that+string+array|code-block|syntax|javascript|342835|(MSDN也提到了：http://msdn.microsoft.com/en-us/library/k9c94ey1.aspx)|342836|另一方面，ToString()是一个实例方法，您不能在null上调用它(null应该使用什么类型？)。|342837|entityMap|0|LINK|mutability|MUTABLE|url|http://msdn.microsoft.com/en-us/library/k9c94ey1.aspx^0|6|1|G|D|19|4|20|D|0|0|A|1H|0|0|5|A|R|4|10|4|0^^$0|@$1|2|3|4|5|6|7|W|8|@$9|X|A|Y|B|C]|$9|Z|A|10|B|C]|$9|11|A|12|B|C]|$9|13|A|14|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|15|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|16|8|@]|D|@$9|17|A|18|1|19]]|E|$]]|$1|M|3|N|5|6|7|1A|8|@$9|1B|A|1C|B|C]|$9|1D|A|1E|B|C]|$9|1F|A|1G|B|C]]|D|@]|E|$]]|$1|O|3|-4|5|6|7|1H|8|@]|D|@]|E|$]]]|P|$Q|$5|R|S|T|E|$U|V]]]]

Because the <code>+</code> operator in C# internally translates to <code>String.Concat</code>, which is a static method. And this method happens to treat <code>null</code> like an empty string. If you look at the source of <code>String.Concat</code> in Reflector, you'll see it: 

<pre><code>// while looping through the parameters
strArray[i] = (str == null) ? Empty : str;
// then concatenate that string array
</code></pre>

(MSDN mentions it, too: <a href="http://msdn.microsoft.com/en-us/library/k9c94ey1.aspx">http://msdn.microsoft.com/en-us/library/k9c94ey1.aspx</a>) 

On the other hand, <code>ToString()</code> is an instance method, which you cannot call on <code>null</code> (what type should be used for <code>null</code>?).

blocks|key|342879|text|将null添加到字符串将被忽略。null+(在您的第二个示例中)不是任何对象的实例，因此它甚至没有ToString()方法。这只是个字面意思。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|342880|entityMap^0|1|4|G|4|1D|A|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]|$9|K|A|L|B|C]|$9|M|A|N|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|O|8|@]|D|@]|E|$]]]|G|$]]

Adding <code>null</code> to a string is simply ignored. <code>null</code> (in your second example) isn't an instance of any object, so it doesn't even have a <code>ToString()</code> method. It's just a literal.

blocks|key|559000|text|因为在连接字符串时，string.Empty和null之间没有区别。您也可以将null传递给string.Format。但是您正在尝试调用null上的方法，这将始终导致NullReferenceException，因此会生成编译器错误。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|559001|如果出于某种原因您确实想这样做，您可以编写一个扩展方法，该方法检查null，然后返回string.Empty。但像这样的扩展应该只在绝对必要的时候使用(在我看来)。|559002|entityMap^0|A|C|N|4|1A|D|1X|4|2C|M|0|X|4|16|C|0^^$0|@$1|2|3|4|5|6|7|J|8|@$9|K|A|L|B|C]|$9|M|A|N|B|C]|$9|O|A|P|B|C]|$9|Q|A|R|B|C]|$9|S|A|T|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|U|8|@$9|V|A|W|B|C]|$9|X|A|Y|B|C]]|D|@]|E|$]]|$1|H|3|-4|5|6|7|Z|8|@]|D|@]|E|$]]]|I|$]]

Because there is no difference between <code>string.Empty</code> and <code>null</code> when you concat strings.
You can pass null into <code>string.Format</code> as well. But you are trying to call a method on <code>null</code>, which would always result in a <code>NullReferenceException</code> and therefore generates a compiler error. 
If for some reason you really want to do it, you could write an extension method, that checks for <code>null</code> and then returns <code>string.Empty</code>. But an extension like that should only be used when absolutly necessary (in my opinion).

blocks|key|343040|text|在.net之前的COM框架中，任何接收到字符串的例程都必须在处理完字符串后将其释放。由于在例程中传入和传出空字符串非常常见，而且尝试“释放”空指针被定义为合法的无操作操作，因此Microsoft决定使用空字符串指针来表示空字符串。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|343041|为了实现与COM的一些兼容性，.net中的许多例程将空对象解释为空字符串的合法表示形式。通过对.net及其语言(最显著的是允许实例成员指示“不作为虚拟调用”)进行一些细微的更改，微软可以使声明类型字符串的null对象行为更像空字符串。如果微软这样做了，它还必须使Nullable<T>的工作方式稍有不同(以便允许Nullable<String>--这是他们无论如何都应该做的事情)和/或定义一个NullableString类型，该类型基本上可以与String互换，但不会将null视为有效的空字符串。|offset|length|style|CODE|343042|实际上，在某些情况下，null会被视为合法的空字符串，而在另一些情况下，则不会被视为合法的空字符串。通常，如果stringValue为null，则stringValue.someMember形式的表达式将失败，但是大多数接受字符串作为参数的框架方法和操作符都会将null视为空字符串。|343043|entityMap^0|0|2U|4|3N|B|4C|G|5H|E|67|6|6K|4|0|B|4|1J|B|1V|4|21|M|3N|4|0^^$0|@$1|2|3|4|5|6|7|L|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|M|8|@$D|N|E|O|F|G]|$D|P|E|Q|F|G]|$D|R|E|S|F|G]|$D|T|E|U|F|G]|$D|V|E|W|F|G]|$D|X|E|Y|F|G]]|9|@]|A|$]]|$1|H|3|I|5|6|7|Z|8|@$D|10|E|11|F|G]|$D|12|E|13|F|G]|$D|14|E|15|F|G]|$D|16|E|17|F|G]|$D|18|E|19|F|G]]|9|@]|A|$]]|$1|J|3|-4|5|6|7|1A|8|@]|9|@]|A|$]]]|K|$]]

In the COM framework which preceded .net, it was necessary for any routine which received a string to free it when it was done with it. Because it was very common for empty strings to be passed into and out of routines, and because attempting to "free" a null pointer was defined as a legitimate do-nothing operation, Microsoft decided to have a null string pointer represent an empty string.

To allow for some compatibility with COM, many routines in .net will interpret a null object as a legal representation as an empty string. With a couple of slight changes .net and its languages (most notably allowing instance members to indicate "do not invoke as virtual"), Microsoft could have made <code>null</code> objects of declared type String behave even more like empty strings. If Microsoft had done that, it would have also had to make <code>Nullable&lt;T&gt;</code> work somewhat differently (so as to allow <code>Nullable&lt;String&gt;</code>--something they should IMHO have done anyway) and/or define a <code>NullableString</code> type which would be mostly interchangeable with <code>String</code>, but which would not regard a <code>null</code> as a valid empty string.

As it is, there are some contexts in which a <code>null</code> will be regarded as a legitimate empty string and others in which it won't. Not a terribly helpful situation, but one which programmers should be aware of. In general, expressions of the form <code>stringValue.someMember</code> will fail if <code>stringValue</code> is <code>null</code>, but most framework methods and operators which accept strings as parameters will regard <code>null</code> as an empty string.

blocks|key|343078|text|'%2B'是一个中缀运算符。就像任何操作符一样，它实际上是在调用一个方法。您可以想象一个非中缀版本的"wow".Plus(null)+==+"wow"|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|343079|实现者已经决定了这样的事情...|343080|class+String
{
++...
++String+Plus(ending)
++{
+++++if(ending+==+null)+return+this;
+++++...
++}
}+|code-block|syntax|javascript|343081|所以..。你的例子变成了|343082|var+bob+=+"abc".Plus(null).Plus(null).Plus(null).Plus("123");++//+abc123|343083|，它与|343084|var+bob+=+"abc".Plus("123");++//+abc123|343085|null在任何时候都不会变成字符串。所以null.ToString()和null.VoteMyAnswer()没有什么不同。;)|343086|entityMap^0|0|3|1C|P|0|0|0|0|0|0|0|K|F|10|J|0^^$0|@$1|2|3|4|5|6|7|Y|8|@$9|Z|A|10|B|C]|$9|11|A|12|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|13|8|@]|D|@]|E|$]]|$1|H|3|I|5|J|7|14|8|@]|D|@]|E|$K|L]]|$1|M|3|N|5|6|7|15|8|@]|D|@]|E|$]]|$1|O|3|P|5|J|7|16|8|@]|D|@]|E|$K|L]]|$1|Q|3|R|5|6|7|17|8|@]|D|@]|E|$]]|$1|S|3|T|5|J|7|18|8|@]|D|@]|E|$K|L]]|$1|U|3|V|5|6|7|19|8|@$9|1A|A|1B|B|C]|$9|1C|A|1D|B|C]]|D|@]|E|$]]|$1|W|3|-4|5|6|7|1E|8|@]|D|@]|E|$]]]|X|$]]

<code>'+'</code> is an infix operator. Like any operator it is really calling a method. You could imagine a the non-infix version <code>"wow".Plus(null) == "wow"</code>

The implementer has decided on something like this...

<pre><code>class String
{
 ...
 String Plus(ending)
 {
 if(ending == null) return this;
 ...
 }
} 
</code></pre>

So.. your example becomes

<pre><code>var bob = "abc".Plus(null).Plus(null).Plus(null).Plus("123"); // abc123
</code></pre>

which is the same as 

<pre><code>var bob = "abc".Plus("123"); // abc123
</code></pre>

At no point does null become a string. So <code>null.ToString()</code> is no different that <code>null.VoteMyAnswer()</code>. ;)

This is valid C# code

<pre><code>var bob = "abc" + null + null + null + "123"; // abc123
</code></pre>

This is not valid C# code

<pre><code>var wtf = null.ToString(); // compiler error
</code></pre>

Why is the first statement valid?

Why is it Valid to Concatenate Null Strings but not to Call "null.ToString()"?

编译器

这是有效的C#代码var bob = "abc" + null + null + null + "123";  // abc123这不是有效的C#代码var wtf = null.ToString(); // compiler error为什么第一个语句是有效的？

问为什么连接空字符串而不调用"null.ToString()“是有效的？
EN

社区

活动

资源

关于

腾讯云开发者

热门产品

热门推荐

更多推荐

问为什么连接空字符串而不调用"null.ToString()“是有效的？EN

社区

活动

资源

关于

腾讯云开发者

热门产品

热门推荐

更多推荐

问为什么连接空字符串而不调用"null.ToString()“是有效的？
EN