基本引用类型与派生引用类型之间有什么区别 [英] What is the difference between base reference type vs derived reference type
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问题描述
class Dog
{
}
class BullDog : Dog
{
}
class Program
{
static void Main()
{
Dog dog2 = new BullDog();
BullDog dog3 = new BullDog();
}
}
-
什么Dog作为参考与BullDog作为参考有什么区别?
What is the difference between using Dog as a reference vs BullDog as a reference?
我习惯于使用 var dog3 = new BullDog ();
,类似于 BullDog dog2 = new BullDog();
。什么时候需要使用 Dog dog2 = new BullDog();
?
I have an habit of using var dog3 = new BullDog();
, which is similar to BullDog dog2 = new BullDog();
. When do we need to use Dog dog2 = new BullDog();
?
推荐答案
编辑:要从评论中解决其他问题:
To address the additional question from the comments:
static void TakesDog(Dog theDog) { ... }
static void TakesBulldog(Bulldog theBulldog) { ... }
static void TakesObject(object theObject) { ... }
static void Main()
{
//Given these declarations...
object dog = new BullDog();
Dog dog2 = new BullDog();
BullDog dog3 = new BullDog();
//These calls will work because a BullDog is a Dog:
TakesDog(dog2);
TakesDog(dog3);
//this call will work because a Bulldog is a Bulldog:
TakesBulldog(dog3);
//and these calls will ALL work because all Dogs are Objects:
TakesObject(dog);
TakesObject(dog2);
TakesObject(dog3);
//However, these calls will fail because an Object or Dog is not
//necessarily a Bulldog,
//EVEN THOUGH our current dog and dog2 are indeed references to Bulldogs:
TakesBulldog(dog);
TakesBulldog(dog2);
//An explicit conversion is necessary to make the above calls work:
TakesBulldog(dog2 as Bulldog); //works given the current reference
TakesBulldog((Bulldog)dog2); //works given the current reference
TakesBulldog(dog as Bulldog); //ditto
TakesBulldog((Bulldog)dog); //ditto
//but if we change the value of dog2 to some other dog:
dog2 = new Labrador();
//the above calls now fail:
TakesBulldog(dog2 as Bulldog); //passes null into the method
TakesBulldog((Bulldog)dog2); //throws InvalidCastException
//you can avoid problems by checking the true type:
if(dog2 is Bulldog) //interrogates the type of the referenced object
TakesBulldog((Bulldog)dog2); //works
else
TakesDog(dog2); //general fallback case
//Object is similar but even more basic:
dog = "I'm a dog"; //now dog is a System.String
//this call still works:
TakesObject(dog);
//but these will fail:
TakesDog(dog);
TakesBulldog(dog);
}
需要了解的最后一件事:
One last thing to understand:
//given these declarations:
object dog = new BullDog();
BullDog dog2 = new BullDog();
//even though dog is a BullDog, attempting to call BullDog-specific
//members (methods, properties, fields) will fail:
dog.Drool();
//you may only call members as specific as the object type of the
//variable holding the reference:
dog.ToString(); //defined by Object. If you've overridden it in Dog or BullDog,
//you'll get that implementation
dog2.Drool(); //works because we know from the variable that dog2 is a BullDog.
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