为什么是 Nullable<T>当 T 是时不是有效的自定义属性参数?

本文介绍了为什么是 Nullable<T>当 T 是时不是有效的自定义属性参数?的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！ 问题描述

If I have an enum like this

public enum Hungry { Somewhat, Very, CouldEatMySocks }

and a custom attribute like this

public class HungerAttribute : Attribute { public Hungry HungerLevel { get; set; } public Hungry? NullableHungerLevel { get; set; } }

I can do this

[Hunger(HungerLevel = Hungry.CouldEatMySocks)] public class Thing1

but I can't do this

[Hunger(NullableHungerLevel = Hungry.CouldEatMySocks)] public class Thing2

It generates an error that says "'NullableHungerLevel' is not a valid named attribute argument because it is not a valid attribute parameter type".

Why is that not allowed? I understand that fundamentally it just isn't on the list of accepted types. The valid types seem to be primitives, enums, string, type, and one dimensional arrays of the preceding types.

Is this just an old rule that did not get updated when Nullable came along?

解决方案

Hungry? is equal to Nullable<Hungry>, which in terms mean that

[Hunger(NullableHungerLevel = Hungry.CouldEatMySocks)]

is equal to

[Hunger(NullableHungerLevel = new Nullable<Hungry>(Hungry.CouldEatMySocks))]

Since you can only use constant values in named attribute arguments you will have to resort to Shimmy's solution.