Basics
In this part of the C# tutorial, we will cover basic programming concepts
of the C# language. We introduce the very basic programs. We will work with variables,
constants and basic data types. We will read and write to the console; we will mention
variable interpolation.
We start with a very simple code example.
using System;
public class Simple
{
static void Main()
{
Console.WriteLine("This is C#");
}
}
This is our first C# program. It will print "This is C#" message to
the console. We will explain it line by line.
using System;
The using keyword imports a specific namespace to
our program. Namespaces are created to group and/or distinguish named entities
from other ones. This prevents name conflicts. This line is a C# statement.
Each statement is ended with a semicolon.
public class Simple
{
...
}
Each C# program is structured. It consists of classes and its members.
A class is a basic building block of a C# program. The
public keyword gives unrestricted access to this
class. The above code is a class definition. The definition has a body,
which starts with a left curly brace ({) and ends with a right curly brace (}).
static void Main()
{
...
}
The Main() is a method. A method is a piece of code
created to do a specific job. Instead of putting all code into one place,
we divide it into pieces, called methods. This brings modularity to our
application. Each method has a body, in which we place statements. The body
of a method is enclosed by curly brackets. The specific job for the Main()
method is to start the application.
It is the entry point to each console C# program. The method is declared to be
static. This static method can be called without the need
to create an instance of the CSharApp class. First we need start the application
and after that, we are able to create instances of classes.
Console.WriteLine("This is C#");
In this code line, we print the "This is C#" string
to the console. To print a message to the console, we use the
WriteLine() method of the
Console class.
The class represents the standard input, output, and error streams
for console applications. Note that Console class is part of the System namespace.
This line was the reason to import the namespace with the
using System;
statement. If we didn't use the statement, we would have to use the fully qualified
name of the WriteLine() method.
This would be
System.Console.WriteLine("This is C#");.
$ ./simple.exe
This is C#
Executing the program gives the above output.
Reading values
We can use the Console class to read values as well.
using System;
public class ReadLine
{
static void Main()
{
string name;
Console.Write("Enter your name: ");
name = Console.ReadLine();
Console.WriteLine("Hello {0}", name);
}
}
The second program will read a value from a console
and print it.
string name;
We declare a variable. The variable is called 'name'. Unlike constants,
which store only one value during the life of the program, variables may
store various different values of the same type. The
string
keyword defines the data type of the variable. Our variable will hold
string values
name = Console.ReadLine();
We read a line from the terminal. When we hit the Enter key, the
input is assigned to the name variable.
.
Console.WriteLine("Hello {0}", name);
In this code line, we perform variable interpolation.
Variable interpolation
is replacing variables with their values inside string literals.
Another names for variable interpolation are: variable substitution
and variable expansion.
$ ./readline.exe
Enter your name: Rama devi
Hello Rama devi
This is the output of the second program.
Command line arguments
C# programs can receive command line arguments. They
follow the name of the program, when we run it.
using System;
public class CSharpApp
{
static void Main(string[] args)
{
for (int i=0; i<args.Length; i++)
{
Console.WriteLine("{0}", args[i]);
}
}
}
Command line arguments can be passed to the
Main() method.
public static void Main(string[] args)
This
Main() method receives a string array of
command line arguments.
for (int i=0; i<args.Length; i++)
{
Console.WriteLine("{0}", args[i]);
}
We go through the array of these arguments
with a for loop and print them to the console.
The
Length property gives
the number of elements in the array. Loops and
arrays will be described in more detail later.
$ ./commandargs.exe 1 2 3
1
2
3
We provide three numbers as command line arguments and these are
printed to the console.
Variables
A
variable is a place to store data. A variable
has a name and a data type. A data type determines, what values can
be assigned to the variable. Integers, strings, boolean values etc.
Over the time of the program, variables can obtain various values
of the same data type. Variables are always initialized to the
default value of their type before any reference to the variable
can be made.
using System;
public class CSharpApp
{
static void Main()
{
string city = "Chennai";
string name = "Devi"; int age = 28;
string nationality = "Indian";
Console.WriteLine(city);
Console.WriteLine(name);
Console.WriteLine(age);
Console.WriteLine(nationality);
city = "Coimbatore";
Console.WriteLine(city);
}
}
In the above example, we work with four variables.
string city = "Chennai";
We declare a city variable of the string type and
initialize it to the "New York" value.
string name = "Devi"; int age = 28;
We declare and initialize two more variables.
We can put two statements into one line. But for
readability reasons, each statement should be on
a separate line.
Console.WriteLine(city);
Console.WriteLine(name);
Console.WriteLine(age);
Console.WriteLine(nationality);
We print the values of the variables to the terminal.
city = "Coimbatore";
We assign a new value to the city variable.
$ ./variables.exe
Chennai
Devi
28
Indian
Coimbatore
This is the output of the example.
Constants
Unlike variables,
constants retain their values. Once
initialized, they cannot be modified. Constants are created with
the
const keyword.
using System;
public class Constants
{
static void Main()
{
const int WIDTH = 100;
const int HEIGHT= 150;
int var = 40;
var = 50;
// WIDTH = 110;
}
}
In this example, we declare two constants and one variable.
const int WIDTH = 100;
const int HEIGHT= 150;
We use the
const keyword
to inform the compiler, that we declare a constant. It
is a convention to write constants in upper case letters.
int var = 40;
var = 50;
We declare and initialize a variable. Later, we assign a new value
to the variable. It is legal.
// WIDTH = 110;
This is not possible with a constant. If we uncomment this line,
we will get a compilation error.
String formatting
Building strings from variables is a very common task in programming.
C# has the string.Format() method to format strings.
Some dynamic languages like Perl, PHP or Ruby support variable interpolation.
Variable interpolation is replacing variables with their values inside string literals.
C# does not allow this. It has string formatting instead.
using System;
public class StringFormatting
{
static void Main()
{
int age = 28;
string name = "Ramya";
string output;
output = string.Format("{0} is {1} years old.",
name, age);
Console.WriteLine(output);
}
}
Strings are
immutable in C#. We cannot modify an existing string.
We must create a new string from existing strings and other types.
In the code example, we create a new string.
We also use values from two variables.
int age = 28;
string name = "Ramya";
string output;
Here we declare three variables.
output = string.Format("{0} is {1} years old.",
name, age);
We use the
Format() method of the
built-in string class. The {0}, {1} are the places where
the variables are evaluated. The numbers represent the
position of the variable. The {0} evaluates to the first
supplied variable and the {1} to the second etc.
$ ./stringformatting.exe
Ramya is 28 years old.
This is the output of the stringformatting.exe program.
This chapter covered some basics of the C# language.
