c# - ((System.Object)p == null)

Question

为什么这样做:

    // If parameter cannot be cast to Point return false.
    TwoDPoint p = obj as TwoDPoint;
    if ((System.Object)p == null)
    {
        return false;
    }

取而代之的是:

    // If parameter cannot be cast to Point return false.
    TwoDPoint p = obj as TwoDPoint;
    if (p == null)
    {
        return false;
    }

我不明白你为什么要写 ((System.Object)p)？

问候，

丹

Answer 1

当你不知道或不能确定原始类是否覆盖了operator ==时，你转换为object:

using System;
class AlwaysEqual
{
    public static bool operator ==(AlwaysEqual a, AlwaysEqual b)
    {
        return true;
    }

    public static bool operator !=(AlwaysEqual a, AlwaysEqual b)
    {
        return true;
    }
}


class Program
{
    static void Main()
    {
        object o = new AlwaysEqual();
        AlwaysEqual ae = o as AlwaysEqual;

        if (ae == null)
        {
            Console.WriteLine("ae is null");
        }

        if ((object)ae == null)
        {
            Console.WriteLine("(object)ae is null");
        }
    }
}

这段代码只输出 “ae is null”，显然不是这样的。转换为 object 避免了 AlwaysEqual 类的 operator ==，因此是针对 null 的真正引用检查。