Lecture 1

An Introduction
to C#
Lecture 1
Dr. Hakem Beitollahi
Computer Engineering Department
Soran University

Objectives of this lecture
 In this chapter you will learn:
 To write simple C# applications.
 To use input and output statements.
 C#’s primitive types.
 Basic memory concepts.
 To use arithmetic operators.
 The precedence of arithmetic operators.
 To write decision-making statements.
 To use relational and equality operators.
An introduction to C#— 2

First program in C#

First C# program: printing a line
Single-line comments.
Left brace { begins function
body.
Predefined namespace (System)– part of .NET FCL
User defined namespace
User defined class
A blank line
Function Main returns
nothing (void) and is static
Function Main appears exactly once in every C# program..
Corresponding right brace }
ends function body.
Statements end with a
semicolon ;.
Console

Anatomy of the first program (I)
// First program in C#.
 Comments start with: //
 Comments ignored during program execution
 Document and describe code
 Provides code readability
 Traditional comments: /* ... */
/* This is a traditional
comment. It can be
split over many lines */

Good Programming Practice Tip (1)
 Every program should begin with a
comment that explains the purpose of the
program, the author and the date and time
the program
Chapter 3: An introduction to C# — 7

using Directive
 Permits use of classes found in specific
namespaces without having to qualify them
 Framework class library
 Over 2,000 classes included
 Syntax
 using namespaceIdentifier;

namespace
 One of the great strengths of C# is that C# programmers
can use the rich set of namespaces provided by the
.NET
 These namespaces contain code that programmers can
reuse
 framework.Groups semantically related types under a
single umbrella
 System: most important and frequently used namespace
 An example of one of the features in namespace System is
Console
 We discuss many namespaces and their features
throughout the course.

Blank line
 Use blank lines and space characters
throughout a program to make the
program easier to read.
 Collectively, blank lines, space characters,
newline characters and tab characters are
known as whitespace
 The compiler ignores
 blank lines, tabs and extra spaces that
separate language elements.

 Our first class
 C# programs consist of pieces called classes, which are
logical groupings of members (e.g., methods) that
simplify program organization.
 These methods perform tasks and return information
when the tasks are completed.
 A C# program consists of classes and methods created
by the programmer and of preexisting classes found in
the Framework Class Library.
 Every program in C# consists of at least one class
definition that the programmer defines.
 The class keyword begins a class definition in C# and is
followed immediately by the class name (program)

Identifiers names
 By convention, each word in a class name begins with an uppercase
first letter and has an uppercase letter for each word in the class
name: SampleClassName
 Identifiers is a series of characters consisting of letters, digits,
underscores ( _ ) and “at” symbols (@).
 Identifiers cannot begin with a digit and cannot contain spaces.
 The “at” character (@) can be used only as the first character in an
identifier
 Examples of valid identifiers:
 Welcome1, _value, m_inputField1, button7, @value
 Examples of invalid identifiers
 5welcome, input field, val@, in$put
 C# is case sensitive

Common Programming Error (1)
C# is case sensitive. Not using the proper
case for an identifier, e.g.,writing Total
when the identifier is total, is a compiler
error.

 Always begin a class name with an
uppercase first letter. This practice makes
class names easier to identify.

Braces { }
 The left brace ({) begins the body of the
class definition.
 The corresponding right brace (}) ends the
class definition.
An introduction to C# — 15

Common Programming Error Tip (2)
 If braces do not occur in matching pairs, a
syntax error occurs.

 When typing an opening left brace ({) in a
program, immediately type the closing
right brace (}) then reposition the cursor
between the braces to begin typing the
body.

 Indent the entire body of each class definition
one “level” of indentation between the left brace
({) and the right brace (}) that delimit the class
body. This emphasizes the structure of the class
definition and helps make the class definition
easier to read.

static void Main(string[] args)
 present in all C# console and Windows applications.
 These applications begin executing at Main, which is
known as the entry point of the program.
 The parentheses after Main indicate that Main is a
program building block, called a method.
 C# class definitions normally contain one or more
methods and C# applications contain one or more
classes.
 For a C# console or Windows application, exactly one of
those methods must be called Main
 M in Main must be uppercase
Administrative — 19

The Console
 The Console class enables programs to output
information to the computer’s standard output.
 It provides methods to display strings and other types
of information in the Windows command prompt.
 Method Console.WriteLine displays (or prints) a line
of text in the console window.
 When Console.WriteLine completes its task, it
positions the output cursor at the beginning of the next
line in the console window.
 Statement: The entire line, including Console.WriteLine,
its argument in the parentheses ("Welcome to C#
Programming!") and the semicolon (;)
 Every statement must end with a semicolon.
Chapter 3: An introduction to C#— 20

 Omitting the semicolon at the end of a
statement is a syntax error.
 When the compiler reports a syntax error,
the error might not be on the line indicated
by the error message. First, check the line
where the error was reported. If that line
does not contain syntax errors, check the
lines that precede the one reported.
Chapter 3: An introduction to C#— 21

Administrative — 22
// Fig. 3.4: Welcome2.cs
// Printing a line with multiple statements.
using System;
class Welcome2
{
static void Main( string[] args )
{
Console.Write( "Welcome to " );
Console.WriteLine( "C# Programming!" );
}
}
Console.Write keeps the curse in the last position

Modifying the first program
(print same contents using different code)

// Printing multiple lines with a single statement.
using System;
class Welcome3
{
{
Console.WriteLine( "WelcomentonC#nProgramming!" );
}
}
Notice how a new line is output for
each n escape sequence.

Slash Operator (I)

Slash Operator (II)
Code Meaning
b Backspace
n Newline
r Carriage return
t Horizontal tab
v Vertical tab
” Double quote
backslash
a Beep
26/27

The Same Program in Windows Form Style

// Printing multiple lines in a dialog Box.
using System;
using System.Windows.Forms;
class Welcome4
{
{
Class MessageBox
is located in assembly
System.Windows.Forms
MessageBox.Show("WelcomentonC#nprogramming!");
}
}

 Including a namespace with the using
directive, but not adding a reference to
the proper assembly, results in a compiler
error.

Add a reference to your program (I)
 To begin, make sure you have an application
open. Select the Add Reference… option from
the Project menu, or right click the
References folder in the Solution Explorer
and select Add Reference… from the popup
menu that appears. This opens the Add
Reference dialog. Double click
System.Windows.Forms.dll to add this file to
the Selected Components list at the bottom of
the dialog, then click OK.
 See next slide

Add a reference to your program (II)

GUI EXample

Another Simple Program: Adding Integers

Addition Proggram
// Fig. 3.11: Addition.cs
// An addition program.
using System;
class Addition
{
{
string firstNumber, // first string entered by user
secondNumber; // second string entered by user
int number1, // first number to add
number2, // second number to add
sum; // sum of number1 and number2
// prompt for and read first number from user as string
Console.Write("Please enter the first integer: ");
firstNumber = Console.ReadLine();
// read second number from user as string
Console.Write("nPlease enter the second integer: ");
secondNumber = Console.ReadLine();
// convert numbers from type string to type int
number1 = Int32.Parse( firstNumber );
number2 = Int32.Parse( secondNumber );
// add numbers
sum = number1 + number2;
// display results
Console.WriteLine( "nThe sum is {0}.", sum );
} // end method Main
} //end class Addition
Define two string variables to enter
numbers
Define three integer variables
prompt for first number from user as
string
Read first number from user as string
usinf ReadLine method
convert numbers from type string to
type int using Int32 class and Parse
method
Add numbers
Display result in the screen

Variables
 A variable is a location in the computer’s memory where
a value can be stored for use by a program.
 All variables must be declared with a name and a data
type before they can be used in a program.
 firstNumber and secondNumber are data of type string,
which means that these variables store strings of
characters.
 Primitive data types: string, int, double and char
 int holds integer values (whole numbers): i.e., 0, -4, 97
 Types float and double can hold decimal numbers
 Type char can hold a single character: i.e., x, $, n, 7
 A variable name can be any valid identifier

 Choosing meaningful variable names helps a program to
be “self-documenting” (i.e., easier to understand simply
by reading it, rather than having to read manuals or use
excessive comments).

 By convention, variable-name identifiers
begin with a lowercase letter. As with class
names, every word in the name after the
first word should begin with a capital letter.
For example, identifier firstNumber has a
capital N in its second word, Number.

 Some programmers prefer to declare each
variable on a separate line. This format
allows for easy insertion of a comment that
describes each variable.

Prompt
Console.Write("Please enter the first integer: ");
firstNumber = Console.ReadLine();
 prompt the user to input an integer and read from the
user a string representing the first of the two integers
that the program will add.
 It is called a prompt, because it directs the user to take a
specific action.
 Method ReadLine causes the program to pause and wait
for user input.
 The user inputs characters from the keyboard, then
presses the Enter key to return the string to the program.

Convert string to int
// convert numbers from type string to type int
number1 = Int32.Parse( firstNumber );
number2 = Int32.Parse( secondNumber );
 Method Int32.Parse converts its string
argument to an integer.
 Class Int32 is part of the System
namespace.
 assign the integer that Int32.Parse
returns to variable number1.
 The = operator is a binary operator

Place spaces on either side of a binary
operator. This makes the operator stand
out and makes the program more
readable.

Sum
// add numbers
sum = number1 + number2;
 calculates the sum of the variables
number1 and number2 and assigns the
result to variable sum by using the
assignment operator =.

Display results (I)
// display results
Console.WriteLine( "nThe sum is {0}.", sum );
 It displays the result of the addition
 we want to output the value in a variable
using method WriteLine
 use {0} to indicate a placeholder for a
variable’s value.
 Suppose sum =117, the resulting string
will be “The sum is 117.”

Display results (II)
Console.WriteLine("The numbers entered are {0} and {1}", number1, number2);
 the value of number1 would replace {0} (because
it is the first variable) and the value of number2
would replace {1} (because it is the second
variable).
 The resulting string would be "The numbers
entered are 45 and 72".
 More formats can be used ({2}, {3} etc.) if there
are more variables to display in the string.

 Place a space after each comma in a
method’s argument list to make programs
more readable.

 Follow the closing right brace (}) of the
body of a method or class definition
with a singleline comment. This comment
should indicate the method or class that
the right brace terminates.

Memory Concepts

Memory Concepts
Number 1: 45
Number 2: 72
Sum: 117
MMememoroyr ylo lcoactaiotinosn sa fateftre sr tcoarilncgu lavatinluge sa nfodr sntourminbge trh1e a snudm n uomf nbuemr2b.er1 and
number2.
Memory location showing the name and value of variable number1.

Arithmetic

Arithmetic Operations (I)

Arithmetic Operations (II)
 Be careful!
 5 / 2 yields an integer 2
 5.0 / 2 yields a double value 2.5
 Remainder can be very useful
 Even numbers % 2 = 0
 Today is Saturday and you and your friends are
going to meet in 10 days. What day is in 10 days?
Saturday is the 6th day in a week
A week has 7 days
After 10 days
The 2nd day in a week is Tuesday
(6 + 10) % 7 is 2

Arithmetic (III)
 Operators have precedence
 Some arithmetic operators act before others
(i.e., multiplication before addition)
 Use parenthesis when needed
 Example: Find the average of three variables
a, b and c
 Do not use: a + b + c / 3
 Use: ( a + b + c ) / 3

Arithmetic (IV)

 Operators have precedence
1. Parentheses ()
2. *, /, %
3. +,-
4. …
3 + 4 * 4 + 5 * (4 + 3) – 1
3 + 4 * 4 + 5 * 7 – 1
3 + 16 + 5 * 7 – 1
3 + 16 + 35 – 1
19 + 35 – 1
54 – 1
53
in side parenthesis first
multiplication
multiplication
addition
addition
subtraction
Arithmetic (V)

Algebra: z = pr%q + w/x – y
C#: z = p * r % q + w / x - y;
1
2
3
4
5
6
Arithmetic (VI)

 Test yourself
 y = a * x * x + b * x + c;
 Y = a+b/(a%5)-a%a*3; (a= 7, b = 3)

Decision Making

Decision Making: Equality and Relational
Operators (I)
 Condition
 Expression can be either true or false
 if statement
 Simple version in this section, more detail later
 If a condition is true, then the body of the if
statement executed
 Control always resumes after the if statement
 Conditions in if statements can be formed
using equality or relational operators (next slide)

Decision Making: Equality and Relational
Operators (II)

It is a syntax error if the operators ==, !=, >=
and<= contain spaces between their
symbols (as in = =, ! =, > =, < =).

 Reversing the operators !=, >= and <= (as
in =!, => and =<) is a syntax error.

 Confusing the equality operator == with
the assignment operator = is a logic
error. The equality operator should be
read “is equal to,” and the assignment
operator should be read “gets” or “gets the
value of.” Some people prefer to read the
equality operator as “double equals” or
“equals equals.”

// Fig. 3.19: Comparison.cs
// Using if statements, relational operators and equality operators.
using System;
class Comparison
{
{
int number1, // first number to compare
number2; // second number to compare
Directly get string
and convert to int
// read in first number from user
Console.Write("Please enter first integer: ");
number1 = Int32.Parse(Console.ReadLine());
// read in second number from user
Console.Write("nPlease enter second integer: ");
number2 = Int32.Parse(Console.ReadLine());
if (number1 == number2)
Console.WriteLine(number1 + " == " + number2);
if (number1 != number2)
Console.WriteLine(number1 + " != " + number2);
if (number1 < number2)
Console.WriteLine(number1 + " < " + number2);
if (number1 > number2)
Console.WriteLine(number1 + " > " + number2);
if (number1 <= number2)
Console.WriteLine(number1 + " <= " + number2);
if ( number1 >= number2 )
If condition is true (i.e.,
values are equal), execute this
statement.
Console.WriteLine( number1 + " >= " + number2 );
}// end method Main
} // end class Comparison
if structure compares values
of num1 and num2 to test for
equality. if structure compares values
If condition is true (i.e.,
values are not equal), execute
this statement.
of num1 and num2 to test for
inequality.

 This expression uses the operator + to
“add” (or combine) numbers and strings.
 C# has a version of the + operator used
for string concatenation.
 Concatenation is the process that enables
a string and a value of another data type
(including another string) to be combined
to form a new string.

 Confusing the + operator used for string
concatenation with the + operator used for
addition can lead to strange results
 Assume y = 5;
 The expression "y + 2 = " + y + 2 results in
the string "y + 2 = 52“ and not "y + 2 = 7".
 The expression "y + 2 = " + (y + 2) results
in "y + 2 = 7“

 Replacing operator == in the condition
of an if structure, such as if ( x == 1 ),
with operator =, as in if ( x = 1 ), is a
logic error.
if (number1 = number2)

 Forgetting the left and right parentheses
for the condition in an if structure is a
syntax error. The parentheses are
required.
if number1 == number2
( )

 There is no semicolon (;) at the end of the
first line of each if structure. It is Syntax
error. If takes no action.
 Example:
if (number1 == number2);

 Place only one statement per line in a
program. This enhances program
readability.

 A lengthy statement may be spread over several lines.
 In case of splitting, choose breaking points that make
sense,
 after a comma in a comma separated list
 after an operator in a lengthy expression.
 Indent all subsequent lines with one level of indentation.
 Example:
int number1, // first number to add
number2, // second number to add
sum; // sum of number1 and number2

 Confirm that the operators in the expression are
performed in the expected order.
 If you are uncertain about the order of evaluation
in a complex expression, use parentheses to
force the order
 assignment (=), associate from right to left rather
than from left to right.
 Example
y = a * x * x + b * x + c; y = (a * x * x) + (b * x) + c;
y = x; it means that x is assigned into y

HomeWork
 First Homework
 Write a program in C# that print your name
with stars (*) in the screen
* *
* *
* * * *
* *
* *
* * * *
* *
* * * *
* *
* *
* *
* *
*
* *
* *
* * * * *
*
* * * * *
*
* * * * *
Deadline: Next week
* *
* * * *
* * * *
* * * *
* * *

Lecture 1

More Related Content

What's hot (20)

Viewers also liked (20)

Similar to Lecture 1 (20)

More from Soran University (7)

Recently uploaded (20)

Lecture 1