C#：与 null 比较
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C#: Comparing with null
提问by
Are these equivalent:
这些是等价的：
if (null==myobject)
{
//do something
}
and
和
if (myobject==null)
{
//do something
}
or will they produce different code?
或者他们会产生不同的代码？
回答by JaredPar
In the 99% case this code is equivalent.
在 99% 的情况下，此代码是等效的。
The one exception is when the type of the object in question overrides the == operator. It's possible for a bug to be introduced in == which causes problems when one parameter is null.
一个例外是当所讨论的对象的类型覆盖 == 运算符时。== 中可能会引入一个错误，当一个参数为空时会导致问题。
A particular example I've seen before is the following
我以前见过的一个特定示例如下
public static bool operator==(Student left, Student right) {
return left.Equals(right);
}
This will fail when null is on the left, but likely not when null in on the right.
当 null 在左边时这将失败，但当 null 在右边时可能不会。
This is a pretty far out corner case though.
不过，这是一个相当遥远的角落案例。
回答by ReinstateMonica Larry Osterman
The form of "if" statement that puts the constant at the left is a holdover from C/C++ where you could have an arbitrary expression in an if statement.
if将常量放在左侧的 " " 语句的形式是 C/C++ 的保留，您可以在 if 语句中使用任意表达式。
C#'s syntax for if statements requires that the expression evaluate to a bool which means that if (foo = 0)won't compile.
C# 的 if 语句语法要求表达式计算为 bool，这意味着if (foo = 0)不会编译。
回答by Kenneth Cochran
As pointed out by others they are mostly equivalent.
正如其他人所指出的，它们大多是等效的。
You should also take a look at: http://en.wikipedia.org/wiki/Null_Object_pattern
您还应该看看：http: //en.wikipedia.org/wiki/Null_Object_pattern
It is a very useful alternative to simply checking for a null reference.
这是简单检查空引用的非常有用的替代方法。
回答by Kenneth Cochran
The
这
if (null==myobject) {
is a safe way of writing an if statement. It comes from C/C++ where the condition is an expression evaluated to an int. If the result is zero that means false, anything else is true. You could write something like
是编写 if 语句的安全方式。它来自 C/C++，其中条件是评估为int. 如果结果为零，那意味着false，其他任何事情都是true。你可以写类似的东西
if (variable == 1) {
but if you weren't careful you could also write
但如果你不小心你也可以写
if (variable = 1) {
in which case you have an assignment that always evaluates to 1 and thus is always true.
在这种情况下，您的赋值始终为 1，因此始终为真。
You could compile this and run it with no problems, but the result wouldn't be what you expected. So C/C++ programmers started to write things like
你可以编译它并毫无问题地运行它，但结果不会是你所期望的。所以 C/C++ 程序员开始写这样的东西
if (1 == variable) {
This won't compile if you misspell it, so you always had to write it as you meant to write it. This later becomes a (good) habit and you use it in all the languages you program with, C# for example.
如果您拼错了它，它将无法编译，因此您总是必须按照您的意图编写它。这后来成为一种（好）习惯，您可以在使用的所有语言（例如 C#）中使用它。
回答by Glenn Slayden
For those who miss it, of if you're looking to reduce clutter, it's also possible to enable C-language-style null-checking for your C#classes:
对于那些错过它的人，如果您想减少混乱，也可以C-language为您的C#类启用-style 空检查：
class MyObj
{
public void implicit_null_comparison_demo()
{
MyObj mo = null;
// ...
if (mo) // <-- like so (can also use !mo)
{
// it's not null...
}
else
{
// it's null...
}
}
public static implicit operator bool(MyObj mo)
{
return (Object)mo != null;
}
};
