为什么允许从超类到子类的隐式转换？

有人能告诉我为什么带有“//Compiles”的行可以编译，为什么带有“//Doesn't Compile”的行不能编译？

我不明白为什么 A 可以隐式转换为 B，而不是相反。

public class SomeClass {

 static public void Test() {
  AClass a = new AClass();
  BClass b = new BClass();

  a = b; // Compiles
  b = a; // Doesn't compile
 }
}

public class AClass {
 public void AMethod() { 
     Console.WriteLine("AMethod");
 }
}

public class BClass : AClass { 
 public void BMethod() {
  Console.WriteLine("BMethod");
 }
}

thanks!

因为 B 会做 A 所做的所有事情，但 A 不一定会做 B 所做的所有事情。可以这样想：

AClass --> Shape
BClass --> Circle

Shape a = new Shape();
Circle b = new Circle();

a = b; // works because a is of type "Shape" and a circle is a specific shape
b = a; // doesn't work because b is of type "Circle" and a could be a square.