abstract Console override record struct virtual

`Class`

C# is an object-oriented language, and it uses classes extensively. The class keyword specifies a type that can be instantiated, and the term "object" refers to class instances.

To continue, a key benefit of classes is that a class hides its information from other parts of a program. Unlike a struct, a class is always located on the heap region of memory.

`Class` example

This program defines a class named Box, with one public method Open(). This class is a type, a template, that does nothing.

Start The program begins in Main. Here the Box is created with "new" and Open() is called, printing a message with Console.WriteLine.

class Box
{
    public void Open()
    {
        System.Console.WriteLine("Box opened");
    }
}

class Program
{
    static void Main()
    {
        // The program begins here.
        // ... Create a new Box and call Open on it.
        Box box = new Box();
        box.Open();
    }
}
Box opened

Nested `class`

Here we see a nested class instance. Nested classes refer to class declarations that occur in other class declarations.

Info Class B is enclosed inside the declaration of class A. It is thus a nested class.

And Because it has a public accessibility modifier, class B can be accessed in places other than class A's scope.

Here We create an instance of A and an instance of A.B. The instance of A does not contain an instance of B.

class A
{
    public int _v1;

    public class B
    {
        public int _v2;
    }
}

class Program
{
    static void Main()
    {
        A a = new A();
        a._v1++;

        A.B ab = new A.B();
        ab._v2++;
    }
}

Inheritance

The C# language supports inheritance—we can inherit from regular classes, and from abstract classes. Here we have a Vehicle base class, and Car derives from Vehicle.

Then When we create an instance of Car, we call Test, and the override method on the Car class is called.

using System;

class Vehicle
{
    public virtual void Test()
    {
        Console.WriteLine("Vehicle Test");
    }
}

class Car : Vehicle
{
    public override void Test()
    {
        Console.WriteLine("Car Test");
    }
}

class Program
{
    static void Main()
    {
        // Use the derived class and call its override Test method.
        Vehicle c = new Car();
        c.Test();
    }
}
Car Test

Field ordering

In a class, does the ordering of fields matter? In older languages, sometimes it does—bytes must be next to each other to fit into 4-byte words.

Here I test how effectively the C# compiler "packs" fields. I measure memory usage of classes. I break down all the allocations.

Info In class "A," the byte and int fields are not grouped by their type. But in class "B," the bytes and ints are grouped by type.

Detail The Main method allocates 100,000 instances of both classes. The array memory usage is measured.

using System;

class A
{
    byte b1;
    int i1;
    byte b2;
    int i2;
    byte b3;
    int i3;
    byte b4;
    int i4;
}

class B
{
    byte b1;
    byte b2;
    byte b3;
    byte b4;
    int i1;
    int i2;
    int i3;
    int i4;
}

class Program
{
    static void Main()
    {
        // ... Measure class "A".
        long m1 = GC.GetTotalMemory(true);
        A[] ar1 = new A[100000];
        for (int i = 0; i < ar1.Length; i++)
        {
            ar1[i] = new A();
        }
        long m2 = GC.GetTotalMemory(true);
        Console.WriteLine(m2 - m1);
        Console.WriteLine((m2 - m1) / 100000);
        ar1[0] = null;
        ar1 = null;

        // ... Measure class "B".
        long m3 = GC.GetTotalMemory(true);
        B[] ar2 = new B[100000];
        for (int i = 0; i < ar2.Length; i++)
        {
            ar2[i] = new B();
        }
        long m4 = GC.GetTotalMemory(true);
        Console.WriteLine(m4 - m3);
        Console.WriteLine((m4 - m3) / 100000);
        ar2[0] = null;
        ar2 = null;
    }
}3200016  [bytes total, A]
32       [bytes per element, A]
3200016  [bytes total, B]
32       [bytes per element, B]

Field, results

Ordering did not matter for memory usage. So you can place your fields in any order you like in the source code file—it makes no difference.

Detail The class reference (within the arrays) accounted for 4 bytes per object. An empty class (with no fields) required 12 bytes.

However A class with just one int field also required 12 bytes. This is the smallest class size possible.

Info I included the 4 bytes from the first int into the minimum class size. Then I added all the remaining fields.

And The four byte fields occupied only four bytes. They were effectively packed together in memory.

Class reference:  4 bytes
Class, empty:     12 bytes
Class with 1 int: 12 bytes

Int: 4 bytes
     4 bytes
     4 bytes
     4 bytes

Byte: 1 byte
      1 byte
      1 byte
      1 byte

Total: 4 bytes (reference)
       12 bytes (class with 1 int)
       12 bytes (3 remaining ints)
       4 bytes (fields)
       = 32 bytes

With classes, we create instances of custom types. This gives a great power to model data. C# is a language based around classes, and they help keep it usable.

Class