C# Object Initialization

With our broader understanding of objects in memory, let’s re-examine something you’ve been working with already, how the values in that memory are initialized (set to their initial values). In C#, there are four primary ways a value is initialized:

1. By zeroing the memory
2. By setting a default value
3. By the constructor
4. With Initialization syntax

This also happens to be the order in which these operations occur - i.e. the default value can be overridden by code in the constructor. Only after all of these steps are completed is the initialized object returned from the constructor.

Zeroing the Memory

This step is actually done for you - it is a feature of the C# language. Remember, allocated memory is simply a series of bits. Those bits have likely been used previously to represent something else, so they will already be set to 0s or 1s. Once you treat it as a variable, it will have a specific meaning. Consider this statement:

int foo;

That statement allocates the space to hold the value of foo. But what is that value? In many older languages, it would be whatever is specified by how the bits were set previously - i.e. it could be any integer within the available range. And each time you ran the program, it would probably be a different value! This is why it is always a good idea to assign a value to a variable immediately after you declare it.

The creators of C# wanted to avoid this potential problem, so in C# any memory that is allocated by a variable declaration is also zeroed (all bits are set to 0). Exactly what this means depends on the variable’s type. Essentially, for numerics (integers, floating points, etc) the value would be 0, for booleans it would be false. And for reference types, the value would be null.

Default Values

A second way a field’s value can be set is by assigning a default value after it is declared. Thus, if we have a private backing _count in our CardDeck class, we could set it to have a default value of 52:

public class CardDeck
{
  private int _count = 52;

  public int Count 
  {
    get 
    {
      return _count;
    }
    set 
    {
      _count = value;
    }
  }
}

This ensures that _count starts with a value of 52, instead of 0.

We can also set a default value when using auto-property syntax:

public class CardDeck
{
  public int Count {get; set;} = 52;
}

public class CardDeck
{
  public int Count {get; set;} = 52;
  public int PricePerCard {get;} = 5m / Count; 
}

Constructors

This brings us to the constructor, the standard way for an object-oriented program to initialize the state of the object as it is created. In C#, the constructor always has the same name as the class it constructs and has no return type. For example, if we defined a class Square, we might type:

public class Square {
    public float length;

    public Square(float length) {
        this.length = length;
    }

    public float Area() {
        return length * length;
    }
}

Note that unlike the regular method, Area(), our constructor Square() does not have a return type. In the constructor, we set the length field of the newly constructed object to the value supplied as the parameter length. Note too that we use the this keyword to distinguish between the field length and the parameter length. Since both have the same name, the C# compiler assumes we mean the parameter, unless we use this.length to indicate the field that belongs to this - i.e. this object.

Parameterless Constructors

A parameterless constructor is one that does not have any parameters. For example:

public class Ball {
    private int x;
    private int y;

    public Ball() {
        x = 50;
        y = 10;
    }
}

Notice how no parameters are defined for Ball() - the parentheses are empty.

If we don’t provide a constructor for a class the C# compiler automatically creates a parameterless constructor for us, i.e. the class Bat:

public class Bat {
    private bool wood = true;
}

Can be created by invoking new Bat(), even though we did not define a constructor for the class. If we define any constructors, parameterless or otherwise, then the C# compiler will not create an automatic parameterless constructor.

Initializer Syntax

Finally, C# introduces some special syntax for setting initial values after the constructor code has run, but before initialization is completed - Object initializers. For example, if we have a class representing a rectangle:

public class Rectangle 
{
  public int Width {get; set;}
  public int Height {get; set;}
}

We could initialize it with:

Rectangle r = new Rectangle() {
  Width = 20,
  Height = 10
};

The resulting rectangle would have its width and height set to 20 and 10 respectively before it was assigned to the variable r.