C＃NullReference异常和ReSharper建议

This is what I have written:

这就是我写的:

if ((lstProperty[i].PropertyIdentifier as string).CompareTo("Name") == 0)

Resharper put me an error (I am new with ReSharper... I am trying it) and it suggests me :

Resharper给我一个错误(我是ReSharper的新手...我正在尝试它)它建议我:

  if (((string) lstProperty[i].PropertyIdentifier).CompareTo("Name") == 0)

Why is the second is NullException safe? For me both will crash if null value appear?

为什么第二个是NullException安全?对我来说,如果出现空值,两者都会崩溃?

2 个解决方案

#1

The 'as' operator will return null if the cast cannot be executed, while a C-style cast will throw an exception if it can't cast.

如果无法执行强制转换,则“as”运算符将返回null,而如果无法强制转换,则C样式强制转换将抛出异常。

I suggest breaking this out into multiple statements:

我建议将其分解为多个语句:

string propertyIdentifier = lstProperty[u].PropertyIdentifier as string;
if(propertyIdentifier != null && propertyIdentifier.CompareTo("Name") == 0)
{
    ... your if statement ...
}

Resharper shouldn't complain about this, and you also won't get a NullReferenceException if the PropertyIdentifier is null or not a string.

Resharper不应该抱怨这一点,如果PropertyIdentifier为null或者不是字符串,你也不会得到NullReferenceException。

#2

Both examples will succeed or fail in the same circumstances, and when they succeed, the behavior will be identical.

这两个示例在相同的情况下都会成功或失败,并且当它们成功时,行为将是相同的。

When they fail, the result will be slightly different: the second example fails slightly earlier (at the cast), and with a more specific exception (InvalidCastException vs. NullReferenceException).

当它们失败时,结果会略有不同:第二个示例稍早(在转换时)失败,并且具有更具体的异常(InvalidCastException与NullReferenceException)。

The main benefit is for debugging: when they fail, you have more information about why it failed in the second example than in the first. Specifically, if the PropertyIdentifier is null vs. non-string, you can tell in the second case, but not in the first case.

调试的主要好处是:当它们失败时,您可以获得有关第二个示例中失败原因的更多信息,而不是第一个示例。具体来说,如果PropertyIdentifier为null而非string,则可以在第二种情况下告知,但不能在第一种情况下告诉。

Also, if you are in a try/catch, you can handle the non-string case in a separate code path than the null case. However, you probably shouldn't be coding this way: if you are, you're doing something else wrong.

此外,如果您在try / catch中,则可以在单独的代码路径中处理非字符串大小写,而不是null大小写。但是,您可能不应该以这种方式编码:如果您是,那么您正在做其他错误的事情。

It might help illuminate the situation if you step through the following code in the various cases:

如果您在各种情况下单步执行以下代码,它可能有助于说明情况:

var propertyI = lstProperty[i];
var propertyIdentifier = propertyI.PropertyIdentifier;

// pick one of these:
var propertyIdentifierAsString = propertyIdentifier as string;
var propertyIdentifierAsString = (string)propertyIdentifier;

if (propertyIdentifierAsString.CompareTo("Name") == 0)

#1