int a=;

long b=a;        // 从int到long的隐式转换

int c=(int) b;   // 从long到int是显式转换

---------------------------------------------------------

class Base{}

class Derived:Base{}

class Program

{

     static void Main(string[] args)

     {

          Base myBaseType;

          // 派生类向基类的隐式强制类型转换

          myBaseType = new Derived ();

          // 在派生类型中存储基类引用必须显式强制类型转换

          Derived myDerivedType= (Derived)myBaseType ;

     }

}

public static explicit operator  xx类 (yy类 y)

{

      xx类 x= new xx类();

      y.porp =x.porp;

      return x;

}