C#

Constructor in C#

Constructor is a class method which is called when an object of class is created.Constructor differs from other methods:

• Name of constructor is same as class name
• Constructor method could not return any value

This is the simplest constructor we can declare:

public class ElectronicDevice
{
public ElectronicDevice ()
{
Console.WriteLine ("Creating ElectronicDevice");
}
}

Now when we create an object of the class,the constructor is automatically called:

var obj=new ElectronicDevice();

If we execute that above code then “Creating ElectronicDevice” will be printed

If you want to pass different number and types of values then you can use:

• Constructor Overloading
• Assign Default value to constructor parameter

Constructor Overloading

Assign Default value to constructor parameter

Now if we modify our constructor as below,then we will have to pass an argument when creating an object of the class

public ElectronicDevice(int id)
{
Console.WriteLine("Creating ElectronicDevice");
}

now if we try to create an object of the class without passing an argument then we will get an error.This will throw error:
var obj = new ElectronicDevice();
To prevent this error from occuring we can assign default value to constructor parameter as:

public ElectronicDevice(int id=1)
{
Console.WriteLine("Creating ElectronicDevice");
}

Constructor of Base class

Below we have declared a base class.It defines one parameterless constructor:

public class ElectronicDevice
{
public ElectronicDevice ()
{
Console.WriteLine ("Creating ElectronicDevice");
}
}

This is the derived class.It also defines one parameterless constructor

public class Laptop : ElectronicDevice
{

public Laptop()
{
Console.WriteLine ("Creating Laptop");
}

}

Now when we create an object of derived class,lets see which constructor is called.

using System;

public class Program
{
public static void Main()
{
var obj=new Laptop();
Console.WriteLine("Hello World");
}

}

The constructors are called in the following sequence if you execute the above code.

• ElectronicDevice
• Laptop

Though we have not explicitly called the constructor but it is automatically called when we create an object of the derived class.

If you have defined any parameterized constructors in the base class you need to call them from the derived class using the base keyword

public class Laptop : base(name)
{
}

 

 

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Looping means to iterate block of statements over and over based on some condition.C# has several loops based on the requirement.The two main types of loops are the while loop and the for loop.You use the while loop if you want to iterate block of statements based on a boolean condition.On the other hand,you use for loop if you want to loop over statement block based on a condition variable.

Loop variable This is the variable which controls the loop.If the value of this variable is true in the condition block then loop is executed.
If this value is false then the loop exits.We can have multiple loop variables.

There are three important constructs:

1. Initialization block this initializes the loop variable and is executed once at the begining.
2. Condition or Verification block this should return bool value
3. Update block updates the loop variable.this executes after the loop body executes

continue

This statement jumps the execution to next iteration

break

This statement is used to exit from the for loop

Following is the syntax of the for loop:

for(intiailize control variable;boolean condition;update control variable)
{
//statement block
}

The condition is reevaluated after every code block execution and if the condition is false the loop exits.

For example you can use the for loop to iterate from 1 to 15 as:

for(int counter=0;counter<15;counter++)
{
Console.WriteLine("Hello");
}

All of these blocks(1,2 and 3) are optional.So this is a valid for loop.Though this is not useful as it is an infinite loop.

for ( ; ; )
{
//body
}

 

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Both are used for implementing polymorphism in c#.One is compile time polymorphism-Method overloading.Other is runtime polymorphism-method overriding.

Method Overloading

Here you define two methods in the same class with the same name but different signatures. In the following example Common class defines two method PrintName which are used for printing name of an employee.
First overload takes the name to print while the other overload takes the Employee object as parameter.

class Common
{
public void PrintName(string FullName)
{

Console.WriteLine(FullName);
}

public void Print(Employee emp)
{
Console.WriteLine(emp.FirstName+emp.LastName);
}

Now when you call the PrintName() method then the method to call is determined on the bases of the arguments you pass while calling the method.For example if you use the following method call:

PrintName("Steve")

then first version of the method will be called.

Method Overriding

You define two methods with the same signature,one in derived class and other in base class.In the following example we have declared two classes Manager and Employee.The Employee class defines a Details() method which does not have any parameters.This method just prints the name of the Employee.In the derived Manager class we have overridden this method to print the names of all the employees which a Manager manages.

class Manager: Employee
{
public List<Employee> Employees{get;set;}
public virtual void Details()
  {
  foreach(var emp in Employees)
        {
      Console.WriteLine(emp.Name);
    }
  }
}

class Manager: Employee
{
public string Name{get;set;}
public virtual void Details()
{
Console.WriteLine(Name);
}
}

In method overloading the method to call is determined at runtime,based on the reference used to call the method.If you call the following method then base class method will be called

Employee emp=new Employee();
emp.Details()

If you use the below method call then the derived class method will be called.

Employee mgr=new Manager();
mgr.Details()

 

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Three types in System namespace are used for working with date and time values.People wrongly believe that DateTime is a class,all of these are structs.

Date time structs for working with date and time values:

• DateTime   Stores value as ticks and DateTimeKind enum for identifying date and time.
• DateTimeOffset Stores value as timestamp (called offset) from UTC.
• TimeSpan  Represents interval of time

DateTime struct

DateTime is the most commonly used type for working with date time values.It defines the following properties.All of these are get only,you can not modify after you create a DateTime value:

• Date
• Day
• Hour
• Minute
• Month
• Year
• Second

Creating DateTime object

Using constructors

You pass the values for day,month,year,hour,minute and second.Some of the constructors it provides are:

DateTime(int year, int month, int day);

DateTime(int year, int month, int day, int hour, int minute, int second);

public DateTime(int year, int month, int day, int hour, int minute, int second, int millisecond);

public DateTime(int year, int month, int day, int hour, int minute, int second, int millisecond, DateTimeKind kind);

public DateTime(int year, int month, int day, int hour, int minute, int second, int millisecond, Calendar calendar);

For example to create a date object for the year 2019,day 1 and month 9:

var dateObj = new DateTime(2019, 9, 1);

After you have created a datetime variable you can access its various properties,to get the appropriate value.

Adding DateTime values

To add datetime values you use the AddXXX methods.

At a minimum you need to specify at least day,month and year or tick.You specify day,month and year as int

Some of the common methods are:

AddDays(double value);
AddHours(double value);
AddMinutes(double value);
AddMilliseconds(double value);
AddMonths(int months);
AddYears(int value);

Examples

Adding days

You can add one day to a DateTime object as:

var dateObj = new DateTime(2019, 9, 1);
var newdateObj = dateObj.AddDays(1);

Substracting Days

Outputting DateTime

You can use ToString() method on any date time object to print its value.By default it prints the short datetime value.

var newdateObj = new DateTime(2019, 9, 2);

Console.WriteLine(newdateObj.ToString());

Output:

9/2/2019 12:00:00 AM

TimeSpan

You use TimeSpan to find the difference between two date time values.For example you can find the difference between the following beginning and stopDate as:

DateTime beginingDateTime = DateTime.Now;
DateTime stopDateTime = DateTime.Now.AddDays(1);
TimeSpan difference = stopDateTime - beginingDateTime;

 

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If you have a given string and you want to count the number of characters which are repeated in the string then you can use the following program.

public static void Main(string[] args)
{
int numberOfRepeatedChars=0;
char[] chars=new char[25];
string str="AAABCDEE";
char tocheck=(char)'a';
foreach(var i in str)
{
int charNo=0;
var asc=(char)i;
charNo++;
if(!chars.Contains((asc)))
{

chars[charNo]=asc;

}
else
{

numberOfRepeatedChars++;
}

}

Console.WriteLine(numberOfRepeatedChars);
}

the above program will print 2 as there are two characters which are repeated in the string which are A and E.

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