C# Problem Overview
Please help us settle the controversy of "Nearly" everything is an object (an answer to Stack Overflow question As a novice, is there anything I should beware of before learning C#?). I thought that was the case as everything in Visual Studio at least appears as a struct. Please post a reference, so that it doesn't become "modern jackass" (This American Life).
Note that this question refers to C#, not necessarily .NET, and how it handles the data under the hood (obviously it's all 1's and 0's).
Here are the comments to "everything is an object":
- Eh, no, it's not. – Binary Worrier
- I'd like an example... – scotty2012
- isn't everything derived from the base type Object? – rizzle
- Most things are objects... – Omar Kooheji
- Value types, ints, doubles, object references (not the objects them selves) etc aren't objects. They can be "boxed" to look like objects (e.g. i.ToString()) but really they're primitive types. Change the entry to "NEARLY everthing is an object" and I'll remove the downvote – Binary Worrier
- I appreciate the clarification. I think the lowest level that you can interact with, say an int, in C# is as a struct, which isn't an object? - http://msdn.microsoft.com/en-us/library/ms173109.aspx – rizzle
- Doesn't Int32 inherit from ValueType which inherits from Object? If so, despite the behavior, an int is an object. – Chris Farmer
- No, the boxed type for int inherits from ValueType, which inherits from Object. They're not objects in the traditional sense because a) an int isn't a reference to an int, IT IS the int. b) ints aren't garbage collected. If you declare an Int32, then that int is 4 bytes on the stack, end of story – Binary Worrier
Definition of object: "Object" as a inheritor of class System.Object vs. "object" as an instance of a type vs. "object" as a reference type."
Solution 1 - C#
The problem here is that this is really two questions - one question is about inheritance, in which case the answer is "nearly everything", and the other is about reference type vs value type/memory/boxing, which case the answer is "no".
In C#, the following is true:
- All value types, including enums and nullable types, are derived from
- All class, array, and delegate types are derived from
- Interface types are not derived from
System.Object. They are all convertible to
System.Object, but interfaces only derive from other interface types, and
System.Objectis not an interface type.
- No pointer types derive from
System.Object, nor are any of them directly convertible to
- "Open" type parameter types are also not derived from
System.Object. Type parameter types are not derived from anything; type arguments are constrained to be derived from the effective base class, but they themselves are not "derived" from anything.
> Supports all classes in the .NET > Framework class hierarchy and provides > low-level services to derived classes. > This is the ultimate base class of all > classes in the .NET Framework; it is > the root of the type hierarchy. > > Languages typically do not require a > class to declare inheritance from > Object because the inheritance is > implicit. > > Because all classes in the .NET > Framework are derived from Object, > every method defined in the Object > class is available in all objects in > the system. Derived classes can and do > override some of these methods.
So not every type in C# is derived from
System.Object. And even for those types that are, you still need to note the difference between reference types and value types, as they are treated very differently.
While value types do inherit from
System.Object, they are treated differently in memory from reference types, and the semantics of how they are passed through methods in your code are different as well. Indeed, a value type is not treated as an Object (a reference type), until you explicitly instruct your application to do so by boxing it as a reference type. See more information about boxing in C# here.
Solution 2 - C#
A little late to the party, but I came across this in a search result on SO and figured the link below would help future generations:
Eric Lippert discusses this very thoroughly, with a much better (qualified) statement:
> The way to correct this myth is to simply replace "derives from" with "is convertible to", and to ignore pointer types: every non-pointer type in C# is convertible to object.
The gist of it, if you hate reading well-illustrated explanations from people that write programming languages, is that (pointers aside), things like Interface, or generic parameter type declarations ("T") are not objects, but are guaranteed to be treatable as objects at runtime, because they have a definite instance, that will be an Object. Other types (Type, Enum, Delegate, classes, etc.) are all Objects. Including value types, which can be boxed to object as other answers have discussed.
Solution 3 - C#
Some people here have a strange notion of what an “object” in object-oriented programming is. In order for something to be an object it does not have to be a reference type or, more generally, follow any formal implementation.
All that means is that you can operate on it as a first-class citizen in an object-oriented world. Since you can do this on values in C# (thanks to autoboxing), everything is indeed an object. To some extend, this is even true for functions (but arguably not for classes).
Whether this is relevant in practice is another question but this is a general problem with OOP that I notice once again. Nobody is clear on the definition of OOP (yes, most people agree that it has something to do with polymorphism, inheritance and encapsulation, some throw in “abstraction” for good measure).
From a usage point of view, every value in C# handles like an object. That said, I like the currently accepted answer. It offers both technically important aspects.
Notice that in other contexts, e.g. C++, other aspects are stressed since C++ isn't necessarily object-oriented and furthermore is much more focused on low-level aspects. Therefore, the distinction between objects, POD and builtin primitives makes sometimes sense (then again, sometimes not).
Solution 4 - C#
You're confusing an object with a value or reference. Basically, everything is an object. An Int is an object, but it is also a value type. A class instance is an object, but it is also a reference type.
Methods aren't objects, nor are properties. The just operate on objects. And yes, pretty much everything inherits from the object class.
Solution 5 - C#
In C# (and in OOP in general) we have types (class - reference, struct - value, etc.). These are the definitions. And the "object" is the concrete instance of a given type.
So, if we read the question literally, yes, everything is an object when instantiated.
The confusion most probably begins with a bad choosing of the name of the very base class for everything. In .NET this is the Object class.
Solution 6 - C#
They are all treated as objects, but they are not all objects. The confusion comes in with Autoboxing.
See this for more information: http://en.wikipedia.org/wiki/Object_type
The abstraction confuses people apparently.
Solution 7 - C#
I thought that value types are NOT objects. They're stored differently in memory by the CLR - value types are stored on the stack, and objects are stored on the heap. You can cast value types to a reference type to make them act like an object, but the CLR takes the value off of the stack, wraps it in an object, and stores it on the heap. That is what happens when you "box" a variable.
Solution 8 - C#
From: Value Types (C# Reference) - MSDN 3.5 > All value types are derived implicitly > from the System.ValueType.
From: Value Type Class - MSDN 3.5 > ValueType overrides the virtual > methods from Object with more > appropriate implementations for value > types.
From: Enum Class - MSDN 3.5
> This class inherits from ValueType
The Inheritance Hierarchy is as follows:
Conclusion: Everything is an object
Solution 9 - C#
Based on all books that I read, everything in C# is an object.
Some are reference other are Value type. Value type object inherit from the class ValueType. They have different behavior but inherently ... objects.
This is the reason why you can store an Int32 in an object variable as well as everything that you can ever create in .NET.
For more detail... look at the following: http://msdn.microsoft.com/en-us/library/s1ax56ch(VS.71).aspx
> All value types are derived implicitly > from the Object class.
Solution 10 - C#
While everyone seems to be focusing on the value types vs. reference types debate, we are forgetting one type in C# that is neither reference nor value, it doesn't derive from object, and it can't be cast to object: pointers.
Unlike values and reference types, pointers cannot be cast to object.
According to the MSDN documentation on C# pointer types,
> Pointer types do not inherit from > object and no conversions exist > between pointer types and object. > Also, boxing and unboxing do not > support pointers. However, you can > convert between different pointer > types and between pointer types and > integral types.
Solution 11 - C#
Short answer: No.
The answer hinges on the definition of "object". Different languages have different definitions of what "object"-means, but the authoritative definition for C# is the official C# Language Specification:
> The types of the C# language are divided into two main categories: reference types and value types. (...) Value types differ from reference types in that variables > of the value types directly contain their data, whereas variables of > the reference types store references to their data, the latter being > known as objects. >
So according to the C#, an object is an instance of a reference type. Value type values are therefore not objects. So it is not true that everything is an object in C#.
> C#’s type > system is unified such that a value of any type can be treated as an > object. (...) Values of value types are treated as objects by > performing boxing and unboxing operations (§9.3.12).
So a value type can be treated as an object by boxing it (effectively turning into a reference type). But an unboxed value type is not in itself an object.
The CLR Specification [PDF] uses a definition very similar to C#:
> object: An instance of a reference type. An object has more to it than > a value. An object is self-typing; its type is explicitly stored in > its representation. It has an identity that distinguishes it from all > other objects, and it has slots that store other entities (which can > be either objects or values). While the contents of its slots can be > changed, the identity of an object never changes.
So in the CLR terminology, a value type value is not an object either.
Solution 12 - C#
Addressing the semantics, Why overload the word "object" so that it means "reference type" when we already have a perfectly good, unambiguous term for that -> "Reference Type", and the when, by overloading the word "Object" in this way we create the confusion this thread demonstrates... i.e., the mismatch between the fact that all Types, (including value types), inherit the implementation defined in the Type "System.Object". Clearly, this is at best unnecessary, and at worst extremely confusing.. Even the fact that the MS documentation is at times confusing on this issue is no excuse to propagate the confusion.
Much easier, and clearer, is to just define and use the term "object" to mean an instance of ANY type, value or reference, and the phrase "Reference Type" to describe the Types that use pointer variables and have their state stored on the Heap ...
Solution 13 - C#
The number 2 is not an object.
Solution 14 - C#
This a discussion of two worlds: language and memory.
To me language is like a layer of abstraction and the term object belongs to this level of abstraction. I don't see a point in talking about objects in terms of memeory organisation and if you do use the "object" term when talking about memory you actually are borrowing this term from a different layer of abstraction. Thus you shouldn't forget where it came from.
If we're talking about C# I don't undestand why someone would use memory organisation as an argument. Of course if I would answer this question to someone I would say "Yes, in C# everything is an object, but you also should know that under the hood it may work differently depending on...."
This may start an interesting argument but may also speak to some: in a similar discussion one could say that actually there is no object oriented programming, there's only procedural programming. Does you CPU understand objects? Even better, actually there is no software, there's only differnt hardware states :)
My point is that some terms don't translate to other layers of abstraction and you should stick the discussion to where it belongs (which in this case is a language, not memory).
Even the author of this question stated: "Note that this question refferes to C# not necessarily .NET and how it handles the data under the hood (obviously it's all 1's and 0's.)"
Solution 15 - C#
Value types are not objects, they obey different copying semantics, different passing semantics, and must be wrapped in a class (Object) in order to be treated as such.
Edit: I think the argument is somewhat vague, as you must qualify what you mean by 'object'. Is an object just something that inherits from Object, or is it something that obeys Object's use semantics? Or are we talking the most general definition of object, where it is anything that can contain data and operations on that data?
Solution 16 - C#
Considering that the question is referring to Object in a OOP sense, the answers is:
From a technical point of view the answer is: No
From a dogmatic point of view the answer is: Yes
Technically value types (primitives or structs) are not objects unless in "boxed" form, but because the .Net does seamless conversions of value types to their Object counterpart through the act of boxing/unboxing (creating a class instance that holds the value and is derived from Object) that means value types can be treated as both objects and simple values.
So value types are dual in nature, they behave as values and as objects. Values in .Net are Objects when they need to be, and they are not objects in the rest of cases.
The correct answer that takes the technical aspect into consideration is "Everything in .Net is as if it were an Object".
The dogmatic answer is "Everything is an Object".
Solution 17 - C#
One of the reasons why there are so many different answers is that the question is very imprecise. What does "everything" mean? Does it really mean every C# language element? Then the answer is clearly "no": Operators are not objects, the "using" keyword is not an object, comments are not objects, etc.
But if this was not meant, what was then meant? Maybe "everything apart from those things that are obviously not classes"? This is obviously not helpful as different people have different opinions on what is "obvious". Nevertheless, most answers seem to follow along this line of opinionated interpretation.
Another source of confusion is around the term "object". What is an object? There is no unique universal definition of this term and different people seem to use it in a different way. The only formal definition in the C# language is the definition of the System.Object type and which other types derive from it and which not. This documentation is readily available and more can't be said about it.