C#枚举协方差不起作用 [英] c# enum covariance doesn't work

问题描述

我需要使用enum作为协变类型.假设我有以下代码:

I need to use enum as covariant type. Let's say i have this code:

public enum EnumColor
{
    Blue = 0,
    Red = 1,
}

public class Car : IColoredObject<EnumColor>
{
    private EnumColor m_Color;
    public EnumColor Color
    {
        get
        {
            return m_Color;
        }
        set
        {
            m_Color = value;
        }
    }

    public Car()
    {
    }
}

class Program
{
    static void Main()
    {
        Car car = new Car();
        IndependentClass.DoesItWork( car );
    }
}

和此代码:

public interface IColoredObject<out EnumColorType>
{
    EnumColorType Color
    {
        get;
    }
}

public static class IndependentClass
{
    public static void DoesItWork( object car )
    {
        IColoredObject<object> coloredObject = car as IColoredObject<object>;

        if( coloredObject == null )
            Console.WriteLine( "It doesn't work." );
        else
        {
            Console.WriteLine( "It works." );

            int colorNumber = (int)( coloredObject.Color );

            Console.WriteLine( "Car has got color number " + colorNumber + "." );
        }
    }
}

我正在尝试使用Enum.

I was trying to use Enum.

IColoredObject<Enum> coloredObject = car as IColoredObject<Enum>;

我试图使用IConvertible，它是Enum的接口.

I was trying to use IConvertible which is interface of Enum.

IColoredObject<IConvertible> coloredObject = car as IColoredObject<IConvertible>;

但是每次都行不通(它为空).

But everytime it doesn't work (it was null).

我应该使用什么?或者我该怎么办?

What should i use ? Or how can i do it ?

(我不想在代码的第二部分中使用EnumColor，因为我希望两个独立的代码仅通过接口连接.)

(I don't want to use EnumColor in second part of code, because i want two independent codes joined only with interface.)

推荐答案

值类型"不支持协方差.枚举属于此类别，因此不起作用.

Covariance is not supported with "Value types" Enum falls under this category and hence doesn't work.

仅引用类型支持通用接口中的差异.值类型不支持方差.例如， IEnumerable< int> (Visual Basic中的IEnumerable(In Integer))不能隐式转换为 IEnumerable< object> (Visual Studio中的IEnumerable(Of Object)基本)，因为整数由值类型表示.

Variance in generic interfaces is supported for reference types only. Value types do not support variance. For example, IEnumerable<int> (IEnumerable(Of Integer) in Visual Basic) cannot be implicitly converted to IEnumerable<object> (IEnumerable(Of Object) in Visual Basic), because integers are represented by a value type.

源MSDN

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