使用用户定义的转换将字符串转换为类型安全枚举 [英] Casting string to type-safe-enum using user-defined conversion

问题描述

为了使用枚举与字符串相结合，我实现了一个基于 http://stackoverflow.com/a/的StringEnum类424414/1293385 。

In order to use Enum's in combination with strings, I implemented a StringEnum class based on http://stackoverflow.com/a/424414/1293385.

但是，当我尝试实现建议的用户定义的转换操作时遇到问题。

However I run into problems when I try to implement the suggested user-defined conversion operations.

StringEnum类定义如下：

The StringEnum class is defined as follows:

public abstract class StringEnum
{
    private readonly String name;
    private readonly int value;

    protected static Dictionary<string, StringEnum> instances
        = new Dictionary<string, StringEnum>();

    protected StringEnum(int value, string name)
    {
        this.value = value;
        this.name = name;
        instances.Add(name.ToLower(), this);
    }

    public static explicit operator StringEnum(string name)
    {
        StringEnum se;
        if (instances.TryGetValue(name.ToLower(), out se))
        {
            return se;
        }
        throw new InvalidCastException();
    }

    public override string ToString()
    {
        return name;
    }
}

我使用这个类作为这样的基础： / p>

I use this class as a base like this:

public class DerivedStringEnum : StringEnum
{
    public static readonly DerivedStringEnum EnumValue1
        = new DerivedStringEnum (0, "EnumValue1");
    public static readonly DerivedStringEnum EnumValue2
        = new DerivedStringEnum (1, "EnumValue2");

    private DerivedStringEnum (int value, string name) : base(value, name) { }
}

但是，当我尝试使用

string s = "EnumValue1"
DerivedStringEnum e = (DerivedStringEnum) s;

返回一个InvalidCastException。检查代码显示StringEnum类的instances属性从未被填充。

An InvalidCastException is returned. Inspection of the code shows that the instances attribute of the StringEnum class is never filled.

有没有办法解决这个问题？

Is there an easy way to fix this?

我不喜欢使用[StringValue（EnumValue1）]的C＃属性magic。

I prefer not to use C# attribute "magic" such as [StringValue("EnumValue1")].

谢谢！

推荐答案

您还必须在派生类上定义一个显式转换运算符。基类不会知道如何转换为派生类。

You have to define an explicit cast operator on the derived class as well. The base class is not expected to know how to cast to a derived class.

由于运算符是静态的，它们不被继承 - 显式转换运算符仅在 string 和 StringEnum 。你可以这样做比较丑陋的双重自己：

Since operators are static they are not inherited - the explicit cast operator is only defined between string and StringEnum. You can do this rather ugly double-cast yourself:

DerivedStringEnum e = (DerivedStringEnum)(StringEnum)s

或者在您的派生类中，您可以：（@ @指出我自己的监督之后编辑）

Or in your derived class you can put: (edited after @ili pointed out my own oversight)

public static explicit operator DerivedStringEnum(string name) 
{ 
  return (DerivedStringEnum)(StringEnum)name; 
} 

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