What are datatypes in terms of OOP in c# ??? Are they classes?

Question

Every class that I make requires the NEW keyword to instantiate it. I wonder then is Oledbconnection a class (because it requires a new keyword) ??? If it is then are int, bool, char also classes ??? I just need to clear my concepts.

Answer 1

most types in .NET derive from object which is a class. So you could say that all those are classes but then some of those derive from ValueType which behave differently from reference types. And reference types are what most people refere to when talking about classes. In that respect Value type such as int, bool and any (other) struct are not classes.

an interfaces is neither a class nor a struct/ValueType it's an interface.

int x = 0; //the object represented by 0 is of type int which is a ValueType x.ToString(); //ToString is implemented by the base type Object which is a class (IComparable)x).CompareTo(1); //System.Int32 implements that interface

Boxing aside, the above illustrates that depending on how you see it an object might be classified as both ValueType, Class or interface

The real thing to worry about is not whether something is a class, an interface or something else. Because depending on how you see it most objects are in more than one set. A key thing to keep in mind is whether something has value semantics (structs/valuetypes) or reference semantics. When ever you assign something that has value semantics to something else a copy is made. If on the other hand you assign something that has reference semantics you are simply aliasing

public struct Foo{
   public int x;
   public Foo(int x){
       this.x = x;
   }
}

//value semantics
Foo x = new Foo(1)
Foo y = x;

//reference semantics
object xx = new object();
object yy = xx

Object.ReferenceEquals(x,y); // <-- returns false;
Object.ReferenceEquals(xx,yY); // <-- returns true;

If you are not careful then you can have some hard to find bugs when treating value types as if they had reference semantics or the other way around

 public class Baz {
     private Foo _myFoo = new Foo(1);
     public Foo MyFoo {
         get{
            return _myFoo;
         }
     }
 }

Foo bar = new Baz();
bar.MyFoo.x = 789; // <-- returning a copy of _myFoo not a reference to that object
if(barMyFoo.x == 1){
   Console.WriteLine("This will be written to console because bar.x is 1!");
}

Answer 2

In OOP terms, those are all "classes". Whether or not they require the new keyword, value type mechanics, etc. are all implementation details of the C# language, not OOP concepts. At a conceptual level, yes, they are all classes in the OO sense.

In C#, the OOP term "class" is devided up into two types, normal classes and structs. Normal classes are treated as reference types, while structs are treated as value types (see here for a discussion of the difference).

The primitive types you mention (int, bool, char) are aliases to structs. That basically means behind-the-scenes, the compiler creates a new instance of the struct type for you. For example, int is an alias to System.Int32, so you can use the two interchangeably. This again is an implementation detail - it doesn't mean they're not classes, it just means the compiler is trying to simplify things and make life easier for you.

Answer 3

In C# we have "Value types" ie. int, double, decimal, enums, structs and "Reference types" - those are classes.