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Structure of a C# Program
// Specify namespaces we use classes from here
using System;
using System.Threading; // Specify more specific namespaces
namespace AppNamespace
{
// Comments that start with /// used for
// creating online documentation, like javadoc
/// <summary>
/// Summary description for Class1.
/// </summary>
class Class1
{
// .. Code for class goes here
}
}

Defining a Class
class Class1
{
static void Main(string[] args)
{
// Your code would go here, e.g.
Console.WriteLine("hi");
}/
* We can define other methods and vars for the class */
// Constructor
Class1()
{
// Code
}/
/ Some method, use public, private, protected
// Use static as well just like Java
public void foo()
{
// Code
}/
/ Instance, Static Variables
private int m_number;
public static double m_stuff;
}

C# Basics
• C# code normally uses the file extension of “.cs”.
• Note similarities to Java
– A few annoying differences, e.g. “Main” instead of “main”.
• If a namespace is left out, your code is placed into the
default, global, namespace.
• The “using” directive tells C# what methods you would like
to use from that namespace.
– If we left out the “using System” statement, then we would have had
to write “System.Console.WriteLine” instead of just
“Console.WriteLine”.
• It is normal for each class to be defined in a separate file,
but you could put all the classes in one file if you wish.
– Using Visual Studio .NET’s “P)roject, Add C)lass” menu option
will create separate files for your classes by default.

Getting Help
• If MSDN is installed
– Online help resource built into Visual Studio .NET.
– Help Menu, look up C# programming language reference
– Dynamic Help
• If MSDN is not installed, you can go online to access the
references. It is accessible from:
– http://msdn.microsoft.com/library/default.asp
– You will have to drill down to VS.NET, Documentation, VB and
C#, and then to the C# reference.
• Both include numerous tutorials, or search on keywords

Basics: Output with WriteLine
• System.Console.WriteLine() will output a string to the
console. You can use this just like Java’s
System.out.println():
System.Console.WriteLine(“hello world “ + 10/2);
will output:
hello world 5
• We can also use {0}, {1}, {2}, … etc. to indicate arguments
in the WriteLine statement to print. For example:
Console.WriteLine(“hi {0} you are {0} and your age is {1}”,
“Kenrick”, 23);
will output:
hi Kenrick you are Kenrick and your age is 23

WriteLine Options
• There are also options to control things such as the number
of columns to use for each variable, the number of
decimals places to print, etc. For example, we could
use :C to specify the value should be displayed as
currency:
Console.WriteLine(“you have {0:C} dollars.”, 1.3);
outputs as:
you have $1.30 dollars.
• See the online help or the text for more formatting options.

Data Types
• C# supports value
types and reference
types.
– Value types are
essentially the
primitive types found
in most languages, and
are stored directly on
the stack.
– Reference types are
objects and are created
on the heap.
Built-In Types
C# Type .NET Framework type
bool System.Boolean
byte System.Byte
sbyte System.SByte
char System.Char
decimal System.Decimal
double System.Double
float System.Single
int System.Int32
uint System.UInt32
long System.Int64
ulong System.UInt64
object System.Object
short System.Int16
ushort System.UInt16
string System.String
Ref
type

Automatic Boxing/Unboxing
• Automatic boxing and unboxing allows value types can be
treated like objects.
• For example, the following public methods are defined for
Object:
Equals Overloaded. Determines whether two
Object instances are equal.
GetHashCode Serves as a hash function for a particular
type, suitable for use in hashing algorithms
and data structures like a hash table.
GetType Gets the Type of the current instance.
ToString Returns a String that represents the current
Object.
We can then write code such as:
int i;
Console.WriteLine(i.ToString());
int hash = i.GetHashCode();
This is equivalent to performing:
z = new Object(i);
Console.WriteLine(z.ToString());
First version more efficient
due to automatic
boxing at VM level

Structures
• struct is another value type
– A struct can contain constructors, constants, fields, methods,
properties, indexers, operators, and nested types.
– Declaration of a struct looks just like a declaration of a class,
except we use the keyword struct instead of class. For
example:
public struct Point {
public int x, y;
public Point(int p1, int p2) { x = p1; y = p2; }
}
• So what is the difference between a class and struct?
Unlike classes, structs can be created on the stack
without using the keyword new, e.g.:
Point p1, p2;
p1.x = 3; p1.y = 5;
• We also cannot use inheritance with structs.

Enumeration Type
• Example:
// Enum goes outside in the class definition
enum Days {Sat, Sun, Mon, Tue, Wed, Thu, Fri};
// Inside some method
Days day1, day2;
int day3;
day1 = Days.Sat;
day2 = Days.Tue;
day3 = (int) Days.Fri;
Console.WriteLine(day1);
Output: Sat
Tue
6
Enumeration really
maps to Int as the
underlying data type

Strings
• The built-in string type is much like Java’s string
type.
– Note lowercase string, not String
– Concatenate using the + operator
– Just like Java, there are a variety of methods available
to:
• find the index Of matching strings or characters
• generate substrings
• compare for equality (if we use == on strings we are
comparing if the references are equal, just like Java)
• generate clones, trim, etc.
• See the reference for more details.

Classes
• Basic class definition already covered
• To specify inheritance use a colon after the
class name and then the base class.
– To invoke the constructor for the base class in a
derived class, we must use the keyword “base”
after the constructor in the derived class.
– We must also be explicit with virtual methods,
methods are not virtual by default as with Java

Class Example
public class BankAccount
{
public double m_amount;
BankAccount(double d) {
m_amount = d;
}
public virtual string GetInfo() {
return “Basic Account”;
}
}
public class SavingsAccount : BankAccount
{
// Savings Account derived from Bank Account
// usual inheritance of methods, variables
public double m_interest_rate;
SavingsAccount(double d) : base(100) { // $100 bonus for
signup
m_interest_rate = 0.025;
}
public override string GetInfo() {
string s = base.GetInfo();
return s + “ and Savings Account”;
}
}

Sample Class Usage
SavingsAccount a = new SavingsAccount(0.05);
Console.WriteLine(a.m_amount);
Console.WriteLine(a.m_interest_rate);
Console.WriteLine(a.GetInfo());
Then the output is:
100
0.05
Basic Account and Savings Account

Class Notes
• We must explicitly state that a method is virtual if
we want to override it
– By default, non-virtual methods cannot be overridden
• We also have to explicitly state that we are
overriding a method with the override keyword
• To invoke a base method, use
base.methodName().

Interfaces
• An interface in C# is much like an interface
in Java
• An interface states what an object can do,
but not how it is done.
– It looks like a class definition but we cannot
implement any methods in the interface nor
include any variables.
• Here is a sample interface:

Sample Interface
public interface IDrivable {
void Start();
void Stop();
void Turn();
}
public class SportsCar : IDriveable {
void Start() {
// Code here to implement start
}
void Stop() {
// Code here to implement stop
}
void Turn() {
// Code here to implement turn
}
}
Method that uses the Interface:
void GoForward(IDrivable d)
{
d.Start();
// wait
d.Stop();
}

Reading Input
• To input data, we must read it as a string and then
convert it to the desired type.
– Console.ReadLine() will return a line of input text as a string.
• We can then use type.Parse(string) to convert the string
to the desired type. For example:
string s;
int i;
s = Console.ReadLine();
i = int.Parse(s);
• we can also use double.Parse(s); float.Parse(s); etc.
• There is also a useful Convert class, with methods such
as Convert.ToDouble(val); Convert.ToBoolean(val);
Convert.ToDateTime(val); etc.

Procedural Stuff
• We also have our familiar procedural constructs:
– Arithmetic, relational, Boolean operators: all the same as Java/C+
+
– For, While, Do, If : all the same as Java/C++
– Switch statements: Like Java, except forces a break after a case.
Code is not allowed to “fall through” to the next case, but several
case labels may mark the same location.
– Math class: Math.Sin(), Math.Cos(), etc.
• Random class:
Random r = new Random();
r.NextDouble(); // Returns random double between 0-1
r.Next(10,20); // Random int, 10 £ int < 20

Passing Parameters
• Passing a value variable by default refers to
the Pass by Value behavior as in Java
public static void foo(int a)
{
a=1;
}
{
int x=3;
foo(x);
Console.WriteLine(x);
}
This outputs the value of 3 because x is passed by value to
method foo, which gets a copy of x’s value under the variable
name of a.

Passing by Reference
• C# allows a ref keyword to pass value types
by reference:
public static void foo(int ref a)
{
a=1;
}
{
int x=3;
foo(ref x);
Console.WriteLine(x);
}
The ref keyword must be used in both the parameter declaration
of the method and also when invoked, so it is clear what
parameters are passed by reference and may be changed.
Outputs the value of 1 since variable a in foo is really a reference
to where x is stored in Main.

Passing Reference Variables
• If we pass a reference variable (Objects,
strings, etc. ) to a method, we get the same
behavior as in Java.
• Changes to the contents of the object are
reflected in the caller, since there is only
one copy of the actual object in memory
and merely multiple references to that
object.

Passing a Reference Variable
• Consider the following:
public static void foo(string s)
{
s = "cow";
}
{
string str = "moo";
foo(str);
Console.WriteLine(str);
}
• Output is “moo” since inside method foo, the local
reference parameter s is set to a new object in
memory with the value “cow”. The original
reference in str remains untouched.

Passing Reference Var by Reference
• The following will change the string in the caller
public static void foo(string ref s)
{
s = "cow";
}
{
string str = "moo";
foo(ref str);
Console.WriteLine(str);
}
• Output = “cow” since foo is passed a reference to str

Arrays
• Arrays in C# are quite similar to Java arrays.
Arrays are always created off the heap and we
have a reference to the array data. The format is
just like Java:
Type arrayname = new Type[size];
• For example:
int arr = new int[100];
• This allocates a chunk of data off the heap large
enough to store the array, and arr references this
chunk of data.

More on Arrays
• The Length property tells us the size of an array dynamically
Console.WriteLine(arr.Length);
// Outputs 100 for above declaration
• If we want to declare a method parameter to be of type array we would
use:
public void foo(int[] data)
• To return an array we can use:
public int[] foo()
• Just like in Java, if we have two array variables and want to copy one
to the other we can’t do it with just an assignment.
– This would assign the reference, not make a copy of the array.
– To copy the array we must copy each element one at a time, or use the
Clone() method to make a copy of the data and set a new reference to it
(and garbage collect the old array values).

Multidimensional Arrays
• Two ways to declare multidimensional arrays.
• The following defines a 30 x 3 array:
int[,] arr = new int[30][3];
• Here we put a comma inside the [] to indicate two dimensions.
– This allocates a single chunk of memory of size 30*3*sizeof(int) and
creates a reference to it. We use the formulas for row major order to
access each element of the array.
• The following defines a 30 x 3 array using an array of arrays:
int[][] arr = new int[30][3];
• To an end user this looks much like the previous declaration, but
it creates an array of 30 elements, where each element is an
array of 3 elements.
– This gives us the possibility of creating ragged arrays but is slower to
access since we must dereference each array index.
– Just like Java arrays

Related to Arrays
• Check out the ArrayList class defined in
System.Collections.
– ArrayList is a class that behaves like a Java vector in that it allows
dynamic allocation of elements that can be accessed like an array
or also by name using a key.
• Lastly, C# provides a new loop method, called foreach
– Foreach will loop through each element in an array or collection.
For example:
string[] arr = {"hello", "world", "foo", "abracadabra"};
foreach (string x in arr) Console.WriteLine(x);
• Will output each string in the array.

Delegates
• C# uses delegates where languages such as
C++ use function pointers.
• A delegate defines a class that describes one
or more methods.
– Another method can use this definition,
regardless of the actual code that implements it.
– C# uses this technique to pass the
EventHandlers to the system, where the event
may be handled in different ways.

Delegates Example
Compare1 uses alphabetic comparison, Compare2 uses length
// Two different methods for comparison
public static int compare1(string s1, string s2)
{
return (s1.CompareTo(s2));
}
public static int compare2(string s1, string s2)
{
if (s1.Length <= s2.Length) return -1;
else return 1;
}

Delegates Example
public delegate int CompareDelegate(string s1, string s2);
// A method that uses the delegate to find the minimum
public static string FindMin(string[] arr, CompareDelegate compare)
{
int i, minIndex=0;
for (i=1; i<arr.Length; i++)
{
if (compare(arr[minIndex],arr[i])>0) minIndex=i;
}
return arr[minIndex];
}
{
string[] arr = {"hello", "world", "foo", "abracadabra"};
string s;
Console.WriteLine(FindMin(arr, new CompareDelegate(compare1)));
Console.WriteLine(FindMin(arr, new CompareDelegate(compare2)));
}
The output of this code is:
abracadabra (using compare1, alphabetic compare)
foo (using compare2, length of string compare)

Next Lecture
• Here we have covered all of the basic
constructs that exist in the C# language
under the Common Language Runtime!
• Next we will see how to use various
Windows.Forms features to create
Windows applications with graphical
interfaces.

Csharp_mahesh

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