带枚举键类型的KeyedCollection不会按int索引

I have classes

我有课

public class FooKeyedCollection : KeyedCollection<FooType,Foo>
{
}

public enum FooType
{
   FooType1,
   FooType2,
   FooType3
}

public Foo
{
   public FooType Type { get; set; }
   public string Value { get; set; }
}
public class FooKeyedCol