class TwoDPoint : System.Object
{
public readonly int x, y;

public TwoDPoint(int x, int y) //constructor
{
this.x = x;
this.y = y;
}

public override bool Equals(System.Object obj)
{
// If parameter is null return false.
if (obj == null)
{
return false;
}

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

// Return true if the fields match:
return (x == p.x) && (y == p.y);
}

public bool Equals(TwoDPoint p)
{
// If parameter is null return false:
if ((object)p == null)
{
return false;
}

// Return true if the fields match:
return (x == p.x) && (y == p.y);
}

public override int GetHashCode()
{
return x ^ y;
}
}

class ThreeDPoint : TwoDPoint
{
public readonly int z;

public ThreeDPoint(int x, int y, int z)
: base(x, y)
{
this.z = z;
}

public override bool Equals(System.Object obj)
{
// If parameter cannot be cast to ThreeDPoint return false:
ThreeDPoint p = obj as ThreeDPoint;
if ((object)p == null)
{
return false;
}

// Return true if the fields match:
return base.Equals(obj) && z == p.z;
}

public bool Equals(ThreeDPoint p)
{
// Return true if the fields match:
return base.Equals((TwoDPoint)p) && z == p.z;
}

public override int GetHashCode()
{
return base.GetHashCode() ^ z;
}
}

---

默认情况下，运算符 == 通过判断两个引用是否指示同一对象来测试引用是否相等，因此引用类型不需要实现运算符 == 就能获得此功能。当类型不可变时，意味着实例中包含的数据不可更改，此时通过重载运算符 == 来比较值是否相等而不是比较引用是否相等可能会很有用，因为作为不可变的对象，只要它们具有相同的值，就可以将它们看作是相同的。建议不要在非不可变类型中重写运算符 ==。

重载的运算符 == 实现不应引发异常。重载运算符 == 的任何类型还应重载运算符 !=。例如：

//add this code to class ThreeDPoint as defined previously
//
public static bool operator ==(ThreeDPoint a, ThreeDPoint b)
{
// If both are null, or both are same instance, return true.
if (System.Object.ReferenceEquals(a, b))
{
return true;
}

// If one is null, but not both, return false.
if (((object)a == null) || ((object)b == null))
{
return false;
}

// Return true if the fields match:
return a.x == b.x && a.y == b.y && a.z == b.z;
}

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