添加约束为空的枚举 [英] Adding Constraints for Nullable Enum

问题描述

我写了一些枚举功能，并有以下几点：

I'm writing some Enum functionality, and have the following:

public static T ConvertStringToEnumValue<T>(string valueToConvert, bool isCaseSensitive)
{
    if (typeof(T).BaseType.FullName != "System.Enum" && typeof(T).BaseType.FullName != "System.ValueType")
    {
       throw new ArgumentException("Type must be of Enum and not " + typeof (T).BaseType.FullName);
    }

    if (String.IsNullOrWhiteSpace(valueToConvert))
      return (T)typeof(T).TypeInitializer.Invoke(null);

    valueToConvert = valueToConvert.Replace(" ", "");              

    if (typeof(T).BaseType.FullName == "System.ValueType")
    {
        return (T)Enum.Parse(Nullable.GetUnderlyingType(typeof(T)), valueToConvert, !isCaseSensitive);
    }

    return (T)Enum.Parse(typeof(T), valueToConvert, !isCaseSensitive);
}

我把它称为是这样的：

I call it like this:

EnumHelper.ConvertStringToEnumValue<Enums.Animals?>("Cat");

我现在想添加约束到T到一个枚举，如（这是我从<一个得到HREF =http://stackoverflow.com/questions/79126/create-generic-method-constraining-t-to-an-enum>文章＃1 ）：其中T：结构，IConvertible ，但为T需要能够把可空枚举我有问题。错误消息说：

I now want to add constraints to T to an Enum, such as (which I got from Stackoverflow article): where T : struct, IConvertible but I am having problems as T needs to be able to take nullable enums. Error message says:

该型'Enums.Animals？必须是一个非空的值类型为使用它作为参数 T'中的泛型类型或方法

The type 'Enums.Animals?' must be a non-nullable value type in order to use it as parameter 'T' in the generic type or method

有没有办法做到这一点，或者我需要仅仅依靠运行时检查我有方法内？

Is there a way to do this, or do I need to just rely on the runtime checking which I have inside the method?

谢谢大家！

推荐答案

T必须是值类型，包括空值类型没有，没有约束它说

No, there's no constraint which says "T must be a value type, including nullable value types."

一个选项，但是，这是该方法分为到：

One option, however, would be to split the method into to:

public static T ConvertStringToEnumValue<T>(...) where T : struct
public static T? ConvertStringToNullableEnumValue<T>(...) where T : struct

除了别的，每个方法的实现然后会更简单，太

Aside from anything else, each method's implementation would then be simpler, too.

当然，我们不知道你将如何使用此代码 - 但如果你要叫它直接从非泛型方法的，这将是我建议的方法。

Of course, we don't know how you're going to use this code - but if you're going to call it directly from non-generic methods, this would be my suggested approach.

当然，这仍然不会从<调用它阻止别人code> T = INT 或类似的东西...你可能想要看的无约束旋律了解刚性约束。

Of course, that's still not going to stop someone from calling it with T=int or something like that... you might want to look at Unconstrained Melody for more rigid constraints.

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