The is operator only considers reference conversions,Boxing
conversions,and unBoxing conversions. Other conversions,such as
user-defined conversions,are not considered.

在实践中意味着什么？
用它来检查结构是否是某种类型是错误的吗？
例如,

public struct Point2D
{
    public int X;
    public int Y;

    ...

    public override bool Equals(Object value)
    {
        if (value != null && value is Point2D)   // or if (value != null && GetType() == value.GetType())
        {
            Point2D right = (Point2D)value;
            return (X == right.X && Y == right.Y);
        }
        else return false;
    }

    ...
}