Updated July 7, 2023
Introduction to Constructor in C#
Constructor plays a very important role in Object-Oriented Programming. Let us understand the role of constructor in C# with the help of the following points:
- A constructor is a special method present inside a class responsible for initializing the class variables.
- Its name is the same as the class name.
- It automatically gets executed when we create an instance of the class.
- A constructor does not return any value.
- If we do not define a constructor, an implicit constructor is always provided by the class, which is called the default constructor.
Syntax:
public class Student()
{
//constructor
public Student()
{
//code
}
}Here, public Student() is a method that does not have any return type, not even void, and its name is the same as the class name, i.e. ‘Student’. Thus, this method is the constructor of this class.
When we create an object of this class using:
Student obj = new Student();Then the code inside the constructor will be executed.
Working of Constructor in C#
1. The constructor initializes data members for the new object. It is invoked by the ‘new’ operator immediately after memory gets allocated to the new object.
2. Explicit constructors (constructors defined by the user) can be parameterless or parameterized. If it is parameterized, then the values passed to the constructor can be assigned to the class’s data members.
3. The implicit constructor initializes variables of the class with the same value even if we create multiple instances of that class.
Example:
Code:
using System;
public class ConstructorDemo
{
public int num = 10;
public static void Main()
{
ConstructorDemo obj1 = new ConstructorDemo();
ConstructorDemo obj2 = new ConstructorDemo();
ConstructorDemo obj3 = new ConstructorDemo();
Console.WriteLine("obj1.num = "+obj1.num+"\nobj2.num = "+obj2.num
+"\nobj3.num = "+obj3.num);
}
}Output:
Pictorial Representation of Above Program:
4. Explicit constructor with parameters allows us to initialize variables of the class with a different value each time we create an instance of that class.
Example:
Code:
using System;
public class ConstructorDemo
{
public int num;
//explicit constructor
public ConstructorDemo(int num)
{
this.num = num;
}
public static void Main()
{
ConstructorDemo obj1 = new ConstructorDemo(10);
ConstructorDemo obj2 = new ConstructorDemo(20);
ConstructorDemo obj3 = new ConstructorDemo(30);
Console.WriteLine("obj1.num = "+obj1.num+"\nobj2.num = "+obj2.num
+"\nobj3.num = "+obj3.num);
}
}Output:
Pictorial Representation of Above Program:
Top 5 Types of Constructor in C#
C# provides five types of constructors. They are as follows:
1. Default Constructor
- A constructor without any parameter is called Default Constructor. If we do not define it explicitly, then it will be implicitly provided by the compiler.
- In such a case, we can call it an implicit constructor. Default or implicit constructor initializes all data members of the class with their default values, such as all numeric fields to zero and all string and object fields to null.
Example:
Code:
using System;
public class DefaultConstructor
{
public int num;
public string str;
}
public class Demo
{
public static void Main()
{
DefaultConstructor obj = new DefaultConstructor();
Console.WriteLine("obj.num = "+obj.num+"\nobj.str = "+obj.str);
}
}Output:
2. Parameterized Constructor
A constructor with at least one parameter is called Parameterized Constructor. Parameters to the constructor can be passed while creating the instance of the class. It allows us to initialize each instance of a class with different values.
Example:
Code:
using System;
public class ParameterizedConstructor
{
public int num;
public string str;
//parameterized constructor
public ParameterizedConstructor(int num, string str)
{
this.num = num;
this.str = str;
}
}
public class Demo
{
public static void Main()
{
//passing values to constructor while creating instance
ParameterizedConstructor obj = new ParameterizedConstructor(50, "constructor");
Console.WriteLine("obj.num = "+obj.num+"\nobj.str = "+obj.str);
}
}Output:
3. Copy Constructor
It is a parameterized constructor that takes the same class’s object as a parameter. It copies the existing object’s value (which is passed as a parameter) to the newly created object instantiated by the constructor. We can say that it copies data of one object to another object.
Example:
Code:
using System;
public class CopyConstructor
{
public int num;
public CopyConstructor(int num)
{
this.num = num;
}
//copy constructor
public CopyConstructor(CopyConstructor obj)
{
num = obj.num;
}
}
public class Demo
{
public static void Main()
{
CopyConstructor obj1 = new CopyConstructor(50);
//passing same class's object as parameter
CopyConstructor obj2 = new CopyConstructor(obj1);
Console.WriteLine("Original object:");
Console.WriteLine("obj1.num = "+obj1.num);
Console.WriteLine("\nCopied object:");
Console.WriteLine("obj2.num = "+obj2.num);
}
}Output:
4. Static Constructor
- It can be defined by prefixing the constructor name with a keyword. It is implicitly defined by the compiler (if not defined explicitly) if the class contains any static variable.
- It is the first block to be executed in the class and will be called automatically. It will be executed only once, irrespective of the number of class instances. It is parameterless and doesn’t accept any access modifier.
Example:
Code:
using System;
public class StaticConstructor
{
//static constructor
static StaticConstructor()
{
Console.WriteLine("Static constructor executed");
}
public static void Display()
{
Console.WriteLine("\nDisplay method executed");
}
}
public class Demo
{
public static void Main()
{
StaticConstructor.Display();
}
}Output:
5. Private Constructor
A constructor created with a private specifier is called a private constructor. We cannot create an instance of the class if it contains only a private constructor, and it does not allow other classes to derive from this class. Used in class that contains only static members.
Example:
Code:
using System;
public class PrivateConstructor
{
public static int num = 100;
//private constructor
private PrivateConstructor()
{
}
}
public class Demo
{
public static void Main()
{
//PrivateConstructor obj = new PrivateConstructor(); //Error
Console.WriteLine("num = "+PrivateConstructor.num);
}
}Output:
Conclusion
If we define any type of constructor in a class, then there will not be any implicit constructor in the class provided by the compiler. Like methods, parameterized constructors can also be overloaded with different numbers of parameters. Constructors defined implicitly by the compiler are always public.
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