我正在阅读EqualityComparer< T> .Default的源代码,发现它并不那么聪明.这是一个例子：
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)？
