Tip3 区别对待强制转型as和is

Tip3 区别对待强制转型as和is

如果类型之间都上溯到了某个共同的基类，那么根据此基类进行的转型应该使用as。子类于子类之间的转型，则应该提供转型操作符，以便进行强制转型

class FirstType
{
public string Name { get; set; }
}

class SecondType
{
public string Name { get; set; }
public static explicit operator SecondType(FirstType firstType)
{
SecondType secondType = new SecondType() { Name = "转型自：" + firstType.Name };
return secondType;
}
}

FirstType firstType = new FirstType() { Name = "First Type" };
SecondType secondType = (SecondType)firstType;         //转型成功
//secondType = firstType as SecondType;     //编译期转型失败，编译不通过

static void DoWithSomeType(object obj)
{
//SecondType secondType = (SecondType)obj;//若传入参数为FirstType对象，会引发异常
//因为编译器运行时会检查obj是不是SecondType，绕过了转换操作符
SecondType secondType = obj as SecondType;
if (secondType != null)
{
// do someting
}
}

若FirstType是SecondType的基类，则as和强转都可以用

//is的用法
static void DoWithSomeType(object obj)
{
if (obj is SecondType)
{
SecondType secondType = obj as SecondType;
//do something
}
}