![](/img/trans.png)
[英]C# Threading: What's the difference between Block and WaitSleepJoin?
[英]What is the difference between using ?? and an if not null block in C#?
使用??
有區別??
以及在C#中if (foo == null) {...} else {...}
?
??
運算符只會評估您的值一次,您的版本可能會評估多次。
所以
var baz = foo ?? bar;
應該被評估為
var tmp = foo;
if(tmp == null)
{
tmp = bar;
}
var baz = tmp;
當foo
是在吸氣劑中具有副作用的函數或屬性時,這點很重要。
private int _counter1 = 0;
private int _counter2 = 0;
private string Example1()
{
_counter1++;
if(_counter1 % 2 == 0)
return _counter1.ToString();
else
return null;
}
private string Example2()
{
_counter2++;
return _counter2.ToString();
}
每次您執行var result = Example1() ?? Example2()
var result = Example1() ?? Example2()
_ _counter1
的值_counter1
應增加一個,而_counter2
的值_counter2
應在其他所有調用中增加。
//Equivalent if statement
var tmp = Example1();
if(tmp == null)
{
tmp = Example2();
}
var result = tmp;
聲明:本站的技術帖子網頁,遵循CC BY-SA 4.0協議,如果您需要轉載,請注明本站網址或者原文地址。任何問題請咨詢:yoyou2525@163.com.