使用条件运算符时的评估顺序[关闭](Order of evaluation when using conditional operator [closed])

使用条件运算符时的评估顺序[关闭](Order of evaluation when using conditional operator [closed])

是否有以下行为？

public class TestClass { public int IntValue { get; set; } } TestClass instance = null; // this line throws exception: Object reference not set to an instance of an object string test1 = "abc" + instance != null ? instance.IntValue.ToString() : "0"; // this works string test2 = instance != null ? instance.IntValue.ToString() : "0"; // this works string test3 = "abc" + (instance != null ? instance.IntValue.ToString() : "0");

更新 ：

为什么这不会抛出异常？

TestClass instance = null; string test4 = "abc" + instance; string test5 = "abc" + true;

Is the following behavior expected?

public class TestClass { public int IntValue { get; set; } } TestClass instance = null; // this line throws exception: Object reference not set to an instance of an object string test1 = "abc" + instance != null ? instance.IntValue.ToString() : "0"; // this works string test2 = instance != null ? instance.IntValue.ToString() : "0"; // this works string test3 = "abc" + (instance != null ? instance.IntValue.ToString() : "0");

UPDATE:

How come this doesn't throw an exception?

TestClass instance = null; string test4 = "abc" + instance; string test5 = "abc" + true;

最满意答案

您的问题是运营商优先级。 +排名高于!= 和 ?: 。 你的行无效也可以这样写：

string test1 = ("abc" + instance) != null ? instance.IntValue.ToString() : "0";

表达式"abc" + instance 永远不会为 null ，即使instance为null 。 这意味着您的表达式始终计算为true ，这使您在instance.IntValue.ToString()遇到NullReferenceException 。案例instance的NullReferenceException instance.IntValue.ToString()表达式为null 。

您可以使用parenthisation覆盖运算符优先级。 这意味着你必须确保instance != null ? instance.IntValue.ToString() instance != null ? instance.IntValue.ToString()首先通过括在括号中来计算：

string test1 = "abc" + (instance != null ? instance.IntValue.ToString() : "0");

更新：

就更新中的问题而言：这些表达式不会引发错误，因为关于字符串上的+运算符实现有两个细节：

只要在另一端有一个有效的字符串，它就可以容忍null值。 由于ToString方法在System.Object实现，因此任何值都可以转换为带ToString的字符串。 因此，可以将非字符串值添加到字符串（仅在右侧 ）。

Your problem is operator precedence. + ranks higher than != and ?:. Your line that is not working could be also written like this:

string test1 = ("abc" + instance) != null ? instance.IntValue.ToString() : "0";

The expression "abc" + instance is never null, even if instance is null. That means your expression always evaluates to true, which makes you run into a NullReferenceException in the instance.IntValue.ToString() expression in the case instance is null.

You can override operator precedence with parenthisation. That means you would have to make sure instance != null ? instance.IntValue.ToString() is evaluated first, by enclosing it in parentheses:

string test1 = "abc" + (instance != null ? instance.IntValue.ToString() : "0");

UPDATE:

As far as your questions in the update is concerned: These expressions don't throw an error, because there are two specifics about the + operator implementation on a string:

It's tolerant to null values as long as you have a valid string on the other side. Since the ToString method is implemented in System.Object, any value can be turned into a string with ToString. Therefore it is possible to add non-string values to a string (only on the right side).