模式匹配中的is vs equals()vs==

class Shape {}
class Circle : Shape {}

var s = new Shape();
var c = new Circle();
Shape nullShape = null;

var isCaCircle = c is Circle; // -> true
var isSaCircle = s is Circle; // -> false
var isCaShape = c is Shape; // -> true
var isNullShapeNull = nullShape is null; // -> true;

if (c is Shape s) { 
     // use s of type Shape here 
}