Object, Difference between a struct and a class

David Smith

14y1.8kVisual Basic .NET

1.What is "Object"

2.(C# only) What is the difference between a class and a struct?

6 Replies

Know the answer? Post it — somebody with the same question will find it here.

Vulpes14y ago
A good example of a struct is System.Int32 for which the C# keyword 'int' is an alias.

This requires 4 bytes of memory to store all possible integer values between -2,147,483,648 and 2,147,483,647.

Consider now this line of code:

int i = 3;

The memory slot for the variable 'i' actually contains the 4 bytes which represent the number 3 which is why structs are called value types. It doesn't contain a reference to where the number 3 is stored on the heap which would be the case if 'int' were a reference type.

However, you can convert value types to reference types using a process known as 'boxing'. In this line of code:

object o = 3;

because object (an alias for System.object) is a reference type and is the ultimate base type of all other types in .NET, then a variable of that type needs to hold a reference to an object not a value.

To achieve this, an object is created on the heap with a single field of type int. The value 3 is then copied into this field. The variable 'o' is then assigned a reference to this 'boxed' object. All this is, of course, done automatically behind the scenes.

To unbox the value you need to use a cast:

int j = (int)o;

The value type variable j now hold the value 3.

Structs are typically small types (ideally no more than 16 bytes) which are created frequently and are relatively short lived. As value types require no overhead (reference types require 8 bytes of overhead on a 32 bit system) and are not subject to garbage collection, this reduces an application's memory footprint and eases pressure on the garbage collector.

However, structs need to be small to avoid the performance cost of copying large blocks of memory - you only need to copy the reference (4 bytes on a 32 bit system) when copying a reference type, not the object itself.

All the other basic types in C# : byte, small, long, bool, char, float, double, decimal etc. are aliases for structs in the System namespace of the .NET Framework as are some other simple types such as DateTime, TimeSpan and Guid.
Jignesh Trivedi14y ago
hi,

Please Refer..

http://msdn.microsoft.com/en-us/library/ah19swz4.aspx
http://www.codeproject.com/Articles/8612/Structs-in-C

hope this help.
David Smith14y ago
What do you guys mean you say structures are value types, can you explain in detail. Also can you give me an example of an value type?
Senthilkumar14y ago
The class is object type, stored in the heap. Garbage collector runs to do clean up process
The stack is value type, stored in the stack memory.

Class can base class to others and can inherit. The structure can inherit the interface, but can't be base to any type.

Class can have constructor and destructor, but the struct can have only constructor.

Without new, the struct can create the instance of the class.
Jignesh Trivedi14y ago
hi,

An object is basically a block of memory that has been allocated and configured according to the blueprint. A program may create many objects of the same class. Objects are also called instances, and they can be stored in either a named variable or in an array or collection.
- Classes are Reference types and Structures are Values types.
- Classes will support an Inheritance whereas Structures won't.
- Classes can have explicitly parameterless constructors whereas structures haven't.
- Member variable initialization is possible in class whereas in Structures, it is not.
- It is not possible to declare destructor in structure but in class it is possible.
- Process of converting structure type into object type is called boxing and process of converting object type into structure type is called unboxing.
hope this help.
Vulpes14y ago
In the usual sense of the word an object is simply an instance of a class. Hence the term 'object-oriented' programming.

In .NET, the 'Object' class (with a capital 'O') in the System namespace is also the ultimate parent of all other .NET types. The C# keyword 'object' (with a lower case 'o') is simply an alias for this type.

In C# (or .NET generally) classes define reference types and structs define value types.

What this means is that a variable of a class type always contains a reference (or a pointer) to where an object of that class is stored on the managed heap. If such a variable doesn't currently refer to an object, then it is given the special value 'null' instead.

In contrast, a variable of a struct type always contains the actual value of the struct instance itself. Broadly, local variables and 'by value' parameters of struct types are stored on the stack and all other instances of structs are stored on the heap.