Constructors in C# - ablealias/asp.net GitHub Wiki

Constructors

When a class or struct is created, its constructor is called. Constructors have the same name as the class or struct, and they usually initialize the data members of the new object. In the following example, a class named Taxi is defined by using a simple constructor.

 public class Taxi
    {
        public bool isInitialized;
        public Taxi()
        {
            isInitialized = true;
        }
    }

    class TestTaxi
    {
        static void Main()
        {
            Taxi t = new Taxi();
            Console.WriteLine(t.isInitialized);
        }
    }

A constructor that takes no parameters is called a default constructor. Default constructors are invoked whenever an object is instantiated by using the new operator and no arguments are provided to new.

Instance Constructors

Instance constructors are used to create and initialize any instance member variables when you use the new expression to create an object of a class. The following example shows an instance constructor:

class CoOrds
        {
            public int x, y;

            // constructor
            public CoOrds()
            {
                x = 0;
                y = 0;
            }
        }

A constructor like this one, which takes no arguments, is called a default constructor. However, it is often useful to provide additional constructors. For example, we can add a constructor to the CoOrds class that allows us to specify the initial values for the data members:

 // A constructor with two arguments:
            public CoOrds(int x, int y)
            {
                this.x = x;
                this.y = y;
            }

Private Constructors

A private constructor is a special instance constructor. It is generally used in classes that contain static members only. If a class has one or more private constructors and no public constructors, other classes (except nested classes) cannot create instances of this class. For example:

 class NLog
    {
        // Private Constructor:
        private NLog() { }
        public static double e = Math.E;  //2.71828...
    }

The declaration of the empty constructor prevents the automatic generation of a default constructor. Note that if you do not use an access modifier with the constructor it will still be private by default. However, the private modifier is usually used explicitly to make it clear that the class cannot be instantiated.

Static Constructors

A static constructor is used to initialize any static data, or to perform a particular action that needs to be performed once only. It is called automatically before the first instance is created or any static members are referenced.

  class SimpleClass
    {
        // Static variable that must be initialized at run time.
        static readonly long baseline;

        // Static constructor is called at most one time, before any
        // instance constructor is invoked or member is accessed.
        static SimpleClass()
        {
            baseline = DateTime.Now.Ticks;
        }
    }

Static constructors have the following properties:

• A static constructor does not take access modifiers or have parameters.
• A static constructor is called automatically to initialize the class before the first instance is created or any static members are referenced.
• A static constructor cannot be called directly.
• The user has no control on when the static constructor is executed in the program.
• A typical use of static constructors is when the class is using a log file and the constructor is used to write entries to this file.
• Static constructors are also useful when creating wrapper classes for unmanaged code, when the constructor can call the LoadLibrary method.
• If a static constructor throws an exception, the runtime will not invoke it a second time, and the type will remain uninitialized for the lifetime of the application domain in which your program is running.