英文:
True condition evaluates to false
问题
我正在编写一个基本的模型验证器，该验证器使用自定义的“Required”属性和反射。当一个模型传递给验证器方法时，会检查所有属性是否具有此必需属性。复杂类型也会进行递归验证。当发现带有此属性的属性时，如果它是可空类型，它将将值与null进行比较，否则将与默认值进行比较。
当将非可空类型与默认值进行比较时，我遇到了问题。以下条件对于int类型评估为false，但应该为true：
if (prop.GetValue(testObject) == default)
{
Console.WriteLine($"{VALIDATION_FAILED_MESSAGE}: {baseObjectName}{testObject.GetType().Name}.{prop.Name} is null.");
isNullorDefault = true;
}
我在调试器中看到，被比较的值毫无疑问是默认值 ->
default(int) == 0，但它评估为false。为什么？
英文:
I am writing a basic model validator that uses reflection with a custom "Required" attribute. When a model is passed into the validator method, all properties are checked for this required attribute. Complex types are recursively validated as well. When a property is found with this attribute, it will either compare the value to null if it is a nullable type, else it will compare to default.
I'm having issues when comparing non-nullable type to default. The following condition is evaluating to false for type int, but it should be true:
if (prop.GetValue(testObject) == default)
{
Console.WriteLine($"{VALIDATION_FAILED_MESSAGE}: {baseObjectName}{testObject.GetType().Name}.{prop.Name} is null.");
isNullorDefault = true;
}
I see in the debugger that the value being compared is without a doubt the default value ->
default(int) == 0, but it is evaluating to false. Why?
答案1
得分: 7
以下是翻译的内容：
好的，这里有一些事实：
GetValue返回一个object，它是一个引用类型。- 对象上的
==进行引用比较（所以如果两者是相同的对象则为真）。 - 值类型在转换为
object时总是装箱到一个新的对象中。 - 没有类型的
default根据上下文被简单地推断为object，所以是null。
因此，这种比较永远不会评估为真：装箱的值类型永远不会是 null，即使你执行 GetValue(x) == (object)0，它仍然为假，因为这个 0 会装箱到一个不同的对象中，因此不同的引用会不同地进行比较。
所以这就是为什么会发生这种情况的答案。至于你实际尝试做的事情的解决方案，请参考这个问题。
英文:
Well, here are a few facts:
GetValuereturns anobject, which is a reference type==on objects does a reference comparison (so true if both are the same object).- Value types are always boxed to a new object when cast to
object. defaultwithout a type is simply inferred based on context, and here it'sobject, sonull.
Therefore, this comparison can never evaluate to true: a boxed value type is never null, and even if you did GetValue(x) == (object)0, it would still be false because that 0 would get boxed to a different object, and as such, different references compare differently.
So that's your answer as for why this is happening. For a solution to the actual thing you're trying to do, see this question.
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