Question

class Dog
{
}
class BullDog : Dog
{
}
class Program
{
    static void Main()
    {
        Dog dog2 = new BullDog();
        BullDog dog3 = new BullDog();
    }
}

1. What is the difference between using Dog as a reference vs BullDog as a reference?

2. 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();?

Answer 1

EDIT: 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);
}

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.