- Constructor is a special method of a class which will invoke automatically whenever instance or object of class is created. Constructors are responsible for object initialization and memory allocation of its class. If we create any class without constructor, the compiler will automatically create one default constructor for that class. There is always at least one constructor in every class.
- Here you need to remember that a class can have any number of constructors and constructors don’t have any return type, not even void and within a class we can create only one static constructor.
- Generally constructor name should be same as class name. If we want to create constructor in a class we need to create a constructor method name same as class name check below sample method for constructor
class SampleA
{
public SampleA()
{
Console.WriteLine("Sample A Test Method");
}
}
Types of Constructors
Basically constructors are 5 types those are
1. Default Constructor
2. Parameterized Constructor
3. Copy Constructor
4. Static Constructor
5. Private Constructor
Default Constructor
A constructor without having any parameters called default constructor. In this constructor every instance of the class will be initialized without any parameter values like as shown below
using System;
namespace ConsoleApplication3
{
class Sample
{
public string param1, param2;
public Sample() // Default Constructor
{
param1 = "Welcome";
param2 = "Aspdotnet";
}
}
class Program
{
static void Main(string[] args)
{
Sample obj=new Sample(); // Once object of class created automatically constructor will be called
Console.WriteLine(obj.param1);
Console.WriteLine(obj.param2);
Console.ReadLine();
}
}
}
When we run above program it will show output like as shown below
Output
Welcome
Aspdotnet
Parameterized Constructors
A constructor with at least one parameter is called as parameterized constructor. In parameterized constructor we can initialize each instance of the class to different values like as shown below
using System;
namespace ConsoleApplication3
{
class Sample
{
public string param1, param2;
public Sample(string x, string y) // Declaring Parameterized constructor with Parameters
{
param1 = x;
param2 = y;
}
}
class Program
{
static void Main(string[] args)
{
Sample obj=new Sample("Welcome","Aspdotnet"); // Parameterized Constructor Called
Console.WriteLine(obj.param1 +" to "+ obj.param2);
Console.ReadLine();
}
}
}
When we run above program it will show output like as shown below
Output
Welcome to Aspdotnet
Constructor Overloading
In c# we can overload constructor by creating another constructor with same method name and different parameters like as shown below
using System;
namespace ConsoleApplication3
{
class Sample
{
public string param1, param2;
public Sample() // Default Constructor
{
param1 = "Hi";
param2 = "I am Default Constructor";
}
public Sample(string x, string y) // Declaring Parameterized constructor with Parameters
{
param1 = x;
param2 = y;
}
}
class Program
{
static void Main(string[] args)
{
Sample obj = new Sample(); // Default Constructor will Called
Sample obj1=new Sample("Welcome","Aspdotnet"); // Parameterized Constructor will Called
Console.WriteLine(obj.param1 + ", "+obj.param2);
Console.WriteLine(obj1.param1 +" to " + obj1.param2);
Console.ReadLine();
}
}
When we run above program it will show output like as shown below
Output
Hi, I am Default Constructor
Welcome to Aspdotnet
Copy Constructor
A parameterized constructor that contains a parameter of same class type is called as copy constructor. Main purpose of copy constructor is to initialize new instance to the values of an existing instance. Check below example for this
using System;
namespace ConsoleApplication3
{
class Sample
{
public string param1, param2;
public Sample(string x, string y)
{
param1 = x;
param2 = y;
}
public Sample(Sample obj) // Copy Constructor
{
param1 = obj.param1;
param2 = obj.param2;
}
}
class Program
{
static void Main(string[] args)
{
Sample obj = new Sample("Welcome", "Aspdotnet"); // Create instance to class Sample
Sample obj1=new Sample(obj); // Here obj details will copied to obj1
Console.WriteLine(obj1.param1 +" to " + obj1.param2);
Console.ReadLine();
}
}
}
When we run above program it will show output like as shown below
Output
Welcome to Aspdotnet
Static Constructor
When we declared constructor as static it will be invoked only once for any number of instances of the class and it’s during the creation of first instance of the class or the first reference to a static member in the class. Static constructor is used to initialize static fields of the class and to write the code that needs to be executed only once.
using System;
namespace ConsoleApplication3
{
class Sample
{
public string param1, param2;
static Sample()
{
Console.WriteLine("Static Constructor");
}
public Sample()
{
param1 = "Sample";
param2 = "Instance Constructor";
}
}
class Program
{
static void Main(string[] args)
{
// Here Both Static and instance constructors are invoked for first instance
Sample obj=new Sample();
Console.WriteLine(obj.param1 + " " + obj.param2);
// Here only instance constructor will be invoked
Sample obj1 = new Sample();
Console.WriteLine(obj1.param1 +" " + obj1.param2);
Console.ReadLine();
}
}
}
When we run above program we will get output like as shown below
Output
Static Constructor
Sample Instance Constructor
Sample Instance Constructor
Importance points of static constructor
- Static constructor will not accept any parameters because it is automatically called by CLR.
- Static constructor will not have any access modifiers.
- Static constructor will execute automatically whenever we create first instance of class
- Only one static constructor will allowed.
Private Constructor
Private constructor is a special instance constructor used in a class that contains static member only. If a class has one or more private constructor and no public constructor then other classes is not allowed to create instance of this class this mean we can neither create the object of the class nor it can be inherit by other class. The main purpose of creating private constructor is used to restrict the class from being instantiated when it contains every member as static.
using System;
namespace ConsoleApplication3
{
public class Sample
{
public string param1, param2;
public Sample(string a,string b)
{
param1 = a;
param2 = b;
}
private Sample() // Private Constructor Declaration
{
Console.WriteLine("Private Constructor with no prameters");
}
}
class Program
{
static void Main(string[] args)
{
// Here we don't have chance to create instace for private constructor
Sample obj = new Sample("Welcome","to Aspdotnet");
Console.WriteLine(obj.param1 +" " + obj.param2);
Console.ReadLine();
}
}
}
Output
Welcome to Aspdotnet
In above method we can create object of class with parameters will work fine. If create object of class without parameters it will not allow us create.
// it will works fine
Sample obj = new Sample("Welcome","to Aspdotnet");
// it will not work because of inaccessability
Sample obj=new Sample();
Important points of private constructor
- One use of private construct is when we have only static member.
- Once we provide a constructor that is either private or public or any, the compiler will not allow us to add public constructor without parameters to the class.
- If we want to create object of class even if we have private constructors then we need to have public constructor along with private constructor
