Difference between these two casts

Question

I'm really not fully familiar with casting. So feel free to edit or comment changes to my question.

Let's say I have a class that implements an interface:

public class Class1: Interface1
{
}

Whats the difference between these two?:

Interface1 myObject = new Class1();

and

Class1 myClassObject = new Class1();
Interface1 myObject = (Interface1) myClassObject;

Is the first one also a form of casting?

Edit:

What does each one do?

See this question: http://stackoverflow.com/questions/1085144/what-is-the-difference-between-boxing-unboxing-and-type-casting — rkrahl, Mar 08 '13 at 08:46
@rkrahl: boxing/unboxing is when you make a value type a reference type(f.e. `int` -> `object`) and vice-versa. — Tim Schmelter, Mar 08 '13 at 08:49

score 5 · Accepted Answer · answered Mar 08 '13 at 08:53

The first one is indeed an implicit cast. To cite Microsoft:

For reference types, an implicit conversion always exists from a class to any one of its direct or indirect base classes or interfaces. No special syntax is necessary because a derived class always contains all the members of a base class.

The second cast is an explicit conversion, and as already mentioned is not necessary for the reason stated above. Explicit conversions are necessary when some of the information could/would get lost during the casting and tells the compiler how to handle that.

Microsoft has a nice article about casting: http://msdn.microsoft.com/en-us/library/ms173105.aspx

Freeman · Answer 2 · 2013-03-08T08:52:22.387

2

The first is a polymorphism feature (though still implicit casting), because Class1 implements Interface1 . The second is explicit casting, and is not required as, MyClass1 is already an Interface1 (a matter of saying it) .

edited Mar 08 '13 at 08:52

answered Mar 08 '13 at 08:44

Freeman

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you've mispelled implicit to imlicit – Freddie Fabregas Mar 08 '13 at 08:45
Perhaps you should elaborate the difference and the uses? – JustAnotherUserYouMayKnow Mar 08 '13 at 08:46
the difference in this case is just in the syntax, one is implicit, the other is explicit. I believe they end up compiled in the same thing. – Freeman Mar 08 '13 at 08:53

VladL · Answer 3 · 2013-03-08T08:55:25.630

The first case the concept is called polymorphism

In computer science, polymorphism is a programming language feature that allows values of different data types to be handled using a uniform interface. The concept of parametric polymorphism applies to both data types and functions. A function that can evaluate to or be applied to values of different types is known as a polymorphic function. A data type that can appear to be of a generalized type (e.g., a list with elements of arbitrary type) is designated polymorphic data type like the generalized type from which such specializations are made.

It still is an implicit cast. – JustAnotherUserYouMayKnow Mar 08 '13 at 08:45 — JustAnotherUserYouMayKnow, Mar 08 '13 at 08:45

Difference between these two casts

3 Answers3