c# – EqualityComparer.Default不够聪明

enum MyEnum : int { A,B }
EqualityComparer<MyEnum>.Default.Equals(MyEnum.A,MyEnum.B)
//is as fast as 
EqualityComparer<int>.Default.Equals(0,1)

enum AnotherEnum : long { A = 1L,B = 2L }
//is 8x slower than
EqualityComparer<long>.Default.Equals(1L,2L)

从EqualityComparer中私有方法的源代码可以看出原因.

private static EqualityComparer<T> CreateComparer()
{
    //non-important codes are ignored
    if (c.IsEnum && (Enum.GetUnderlyingType(c) == typeof(int)))
    {
        return (EqualityComparer<T>) RuntimeTypeHandle.CreateInstanceForAnotherGenericParameter((RuntimeType) typeof(EnumEqualityComparer<int>),c);
    }
    return new ObjectEqualityComparer<T>();
}

我们可以看到EqualityComparer< int> .Default,EqualityComparer< MyEnum> .Default和EqualityComparer< long> .Default得到一个明智的比较器,其Equals方法如下所示：

public static bool Equals(int x,int y)
{
    return x == y;  //or return x.Equals(y); here 
                    //I'm not sure,but neither causes boxing
}

public static bool Equals(MyEnum x,MyEnum y)
{
    return x == y;  //it's impossible to use x.Equals(y) here 
                    //because that causes boxing
}

上面两个是聪明的,但EqualityComparer< AnotherEnum> .Default是不吉利的,从我们最终可以看到的方法得到一个ObjectEqualityComparer< T>(),它的Equals方法可能看起来像：

public static bool Equals(AnotherEnum x,AnotherEnum y)
{
    return x.Equals(y);   //too bad,the Equals method is from System.Object
                       //and it's not override,boxing here!
                       //that's why it's so slow
}

我认为这个条件Enum.GetUnderlyingType(c)== typeof(int)是没有意义的,如果枚举的基础类型是int类型,该方法可以将int的默认比较器转换为此枚举.但为什么不能基于长期的枚举？我觉得这不难吗？任何特殊原因？构造像x == y这样的比较器对枚举来说并不那么难,对吧？为什么最后它给出了一个慢的ObjectEqualityComparer< T>对于枚举(即使它正常工作)？