6

假设我有一个父类和子类，如下所示

家长班：

class Parent
{
    public string _First;
    public string _Last;

    public override bool Equals(object obj)
    {
        if (ReferenceEquals(obj, null))
            return false;
        else if (ReferenceEquals(obj, this))
            return true;
        else if (obj is Parent == false)
            return false;
        else
            return this.Equals(obj as Parent) & base.Equals(obj);

    }

    public override int GetHashCode()
    {
        unchecked
        {
            return this._First.GetHashCode() ^ this._Last.GetHashCode() ^ base.GetHashCode();
        }
    }

    public bool Equals(Parent that)
    {
        if (ReferenceEquals(that, null))
            return false;
        else if (ReferenceEquals(that, this))
            return true;
        else
            return this._First.Equals(that._First) & this._Last.Equals(that._Last);
    }

    public static  bool operator ==(Parent p1, Parent p2)
    {
        return p1.Equals(p2);
    }

    public static  bool operator !=(Parent p1, Parent p2)
    {
        return !p1.Equals(p2);
    }


}

儿童班：

  class Child : Parent
{
    public string Address;

    public override bool Equals(object obj)
    {
        if (ReferenceEquals(obj, null))
            return false;
        else if (ReferenceEquals(obj, this))
            return true;
        else if (obj is Parent == false)
            return false;
        else
            return this.Equals(obj as Child);

    }

    public override int GetHashCode()
    {
        unchecked
        {
            return this._First.GetHashCode() ^ this._Last.GetHashCode() ^ base.GetHashCode();
        }
    }

    public bool Equals(Child that)
    {
        if (ReferenceEquals(that, null))
            return false;
        else if (ReferenceEquals(that, this))
            return true;
        else
            return this.Address.Equals(that.Address) & base.Equals(that);
    }

    public static  bool operator ==(Child p1,Child p2)
    {
        return p1.Equals(p2);
    }

    public static bool operator !=(Child p1, Child p2)
    {
        return !p1.Equals(p2);
    }

}

这是比较两个 child 实例的代码。

 Parent p = new Child() { _First = "Mitul", _Last = "Patel", Address="abc1"};
        Parent p1 = new Child() { _First = "Mitul", _Last = "Patel", Address = "abc" };

        Child c = new Child() { _First = "Mitul", _Last = "Patel", Address = "abc1" };
        Child c1 = new Child() { _First = "Mitul", _Last = "Patel", Address = "abc" };

        Console.WriteLine(p.Equals(p1));
        Console.WriteLine(p == p1);

        Console.WriteLine(c.Equals(c1));
        Console.WriteLine(c == c1);

        Console.Read();

输出

真 真 假 假

我知道为什么它在第一次比较时给出了真实的和真实的。因为它调用了父类的重载 ==() 运算符。我的问题是我想使用子类的 == 运算符，因为该对象是 Child 类型的，那怎么可能呢？对于静态方法，不能使用 virtual 关键字。

谢谢，

4

2 回答 2

12

运算符的实现是在编译时选择的。运算符不是虚拟方法——子类的==运算符不会覆盖父类的==运算符。

因此，让编译器选择子==运算符的唯一方法是让变量本身具有类型Child，例如

 Child p = new Child() { _First = "Mitul", _Last = "Patel", Address="abc1"};
 Child p1 = new Child() { _First = "Mitul", _Last = "Patel", Address = "abc" };

或者让==操作员调用 Equals 方法，并使 Equals 的子实现覆盖父实现：

在 Parent.cs 中：

// No need for these to be public- they should only be called internally.
protected virtual bool Equals(Parent that)
{
    if (ReferenceEquals(that, null))
        return false;
    else if (ReferenceEquals(that, this))
        return true;
    else
        return this._First.Equals(that._First) & this._Last.Equals(that._Last);
}

在 Child.cs 中：

// Notice I changed your argument type here...
protected override bool Equals(Parent that)
{
    // If we are not dealing with a Child instance, delegate to the base class.
    if (!(that is typeof(Child)))
        return base.Equals(that);

    if (ReferenceEquals(that, null))
        return false;
    else if (ReferenceEquals(that, this))
        return true;
    else
        return this.Address.Equals(that.Address) & base.Equals(that);
}
于 2012-05-09T20:34:22.557 回答
1

static方法在编译时解析，而不是在运行时解析。
因为你的代码p和p1都是Parent对象，所以它会调用类的==操作符Parent。
如果要调用派生类的运算符，则必须将它们声明为派生类实例。

于 2012-05-09T20:34:31.083 回答