Visual c sharp

Visual C#
1. What is C#?
C# (pronounced C sharp) is a new programming
language designed for building a wide range of
enterprise applications that run on the .NET
Framework. An evolution of Microsoft C and
Microsoft C++, C# is simple, modern, type safe,
and object oriented. C# code is compiled as
managed code, which means it benefits from the
services of the common language runtime.
These services include language interoperability,
garbage collection, enhanced security, and
improved versioning support.

2. Get Started
Warning: C# is case-sensitive.

Example 2-1. A Simple Welcome Program: Welcome.cs
// Namespace Declaration
using System;

// Program start class
class WelcomeCSS
{

// Main begins program
execution.
public static void
Main()
{
// Write to console
Console.WriteLine("Welcome to the C#
Station Tutorial!");
}
}

Note:
Example 2-1 has 4 primary elements, a
namespace declaration, a class, a Main method,
and a program statement. You can compile:
csc.exe Welcome.cs
This produces a file named Welcome.exe, which
can then be executed.
Namespaces contain groups of code that can be
called upon by C# programs.

Example 2-2. Getting Command-Line Input: NamedWelcome.cs

using System;


class NamedWelcome
{
// Main begins program execution.
public static void Main(string[] args)
{
// Write to console
Console.WriteLine("Hello, {0}!", args[0]);
Console.WriteLine("Welcome to the C# Station Tutorial!");
}
}

The argument list within the first statement is
different than before. It has a formatted string
with a "{0}" parameter embedded in it. The first
parameter in a formatted string begins at number
0, the second is 1, and so on. The "{0}"
parameter means that the next argument
following the end quote will determine what goes
in that position.
3. Operators, Types, and Variables
Variables and Types
C# is a strongly "Typed" language. Thus all
operations on variables are performed with
consideration of what the variable's "Type" is.
The C# simple types consist of the Boolean type
and three numeric types - Integrals, Floating
Point, Decimal and String. The term "Integrals"

refers to the classification of types that include
sbyte, byte, short, ushort, int, uint, long, ulong,
and char.
The Boolean Type
Boolean types are declared using the keyword,
bool. They have two values: true or false. In other
languages, such as C and C++, boolean
conditions can be satisfied where 0 means false
and anything else means true. However, in C# the
only values that satisfy a Boolean condition is true
and false, which are official keywords.

Example3-1. Displaying Boolean Values: Boolean.cs
using System;

class Booleans
{
public static void Main()
{
bool content = true;
bool noContent = false;

Console.WriteLine("It is {0} that C#
Station provides C# programming language
content.", content);
Console.WriteLine("The statement

above is not {0}.", noContent);
}
}

Integral Types
In C#, an integral is a category of types. They are
whole numbers, either signed or unsigned, and
the char type. The char type is a Unicode
character, as defined by the Unicode Standard.
For more information, visit The Unicode Home
Page. Table 2-1 shows the integral types, their
size, and range.
Table 3-1. The Size and Range of C# Integral Types
Type Size (in bits) Range
sbyte 8 -128 to 127
byte 8 0 to 255
short 16 -32768 to 32767
ushort 16 0 to 65535
int 32 -2147483648 to 2147483647
uint 32 0 to 4294967295
-9223372036854775808 to
long 64
9223372036854775807
ulong 64 0 to 18446744073709551615
char 16 0 to 65535

As you can see from the table above, you have a
wide range of options to choose from, depending
on your requirements.
Floating Point and Decimal Types
A C# floating point type is either a float or double.
They are used any time you need to represent a
real number. Decimal types should be used when
representing financial or money values. Table 2-2
shows the floating point and decimal types, their
size, precision, and range.
Table 3-2. The Floating Point and Decimal Types with Size,
Precision, and Range
Size (in
Type Precision Range
bits)
float 32 7 digits 1.5 x 10-45 to 3.4 x 1038
5.0 x 10-324 to 1.7 x
double 64 15-16 digits
10308
28-29 decimal
decimal 128 1.0 x 10-28 to 7.9 x 1028
places

The string Type
A string is a string of text characters. You typically
create a string with a string literal, enclosed in
quotes: "This is an example of a string."
Some characters aren't printable, but you still
need to use them in strings.

Table 3-3. C# Character Escape Sequences
Meaning
Escape Sequence

Single Quote
'

Double Quote
"

Backslash

Null, not the same as the C# null value
0

Bell
a

Backspace
b

Form Feed
f

Newline
n

Carriage Return
r

Horizontal Tab
t

Vertical Tab
v

Another useful feature of C# strings is the
verbatim literal, which is a string with a @ symbol
prefix, as in @"Some string". Verbatim literals
make escape sequences translate as normal
characters to enhance readability. To appreciate
the value of verbatim literals, consider a path
statement such as
"c:topdirsubdirsubdirmyapp.exe". You can
improve readability of the string with a verbatim

literal, like this:
@"c:topdirsubdirsubdirmyapp.exe".
That is fine, but now you have the problem where
quoting text is not as easy. In that case, you
would specify double double quotes. For example,
the string "copy "c:source file name with
spaces.txt" c:newfilename.txt" would be written
as the verbatim literal @"copy ""c:source file
name with spaces.txt"" c:newfilename.txt".
C# Operators
Results are computed by building expressions.
These expressions are built by combining
variables and operators together into statements.
The following table describes the allowable
operators, their precedence, and associativity.
Table 3-4. Operators with their Precedence and Associativity
Category
(by Associativit
Operator(s)
precedence y
)
(x) x.y f(x) a[x] x++ x-- new
Primary left
typeof sizeof checked unchecked
Unary + - ! ~ ++x --x (T)x left
Multiplicativ
* / % left
e
Additive + - left

Shift << >> left
Relational < > <= >= is left
Equality == != right
Logical AND & left
Logical XOR ^ left
Logical OR | left
Conditional
&& left
AND
Conditional
|| left
OR
Conditional> ?: right
= *= /= %= += -= <<= >>=
Assignment right
&= ^= |=

The Array Type
Another data type is the Array, which can be
thought of as a container that has a list of storage
locations for a specified type. When declaring an
Array, specify the type, name, dimensions, and
size.

Example 3-2. Array Operations: Array.cs
using System;

class Array
{
{
int[] myInts = { 5, 10, 15 };
bool[][] myBools = new bool[2][];

myBools[0] = new bool[2];
myBools[1] = new bool[1];
double[,] myDoubles = new double[2, 2];
string[] myStrings = new string[3];

Console.WriteLine("myInts[0]: {0}, myInts[1]: {1},
myInts[2]: {2}", myInts[0], myInts[1], myInts[2]);

myBools[0][0] = true;
myBools[0][1] = false;
myBools[1][0] = true;
Console.WriteLine("myBools[0][0]: {0}, myBools[1]
[0]: {1}", myBools[0][0], myBools[1][0]);

myDoubles[0, 0] = 3.147;
myDoubles[0, 1] = 7.157;
myDoubles[1, 1] = 2.117;
myDoubles[1, 0] = 56.00138917;
Console.WriteLine("myDoubles[0, 0]: {0},
myDoubles[1, 0]: {1}", myDoubles[0, 0], myDoubles[1,
0]);

myStrings[0] = "Joe";
myStrings[1] = "Matt";
myStrings[2] = "Robert";
Console.WriteLine("myStrings[0]: {0},
myStrings[1]: {1}, myStrings[2]: {2}", myStrings[0],
myStrings[1], myStrings[2]);

}
}

And here's the output:
myInts[0]: 5, myInts[1]: 10, myInts[2]: 15
myBools[0][0]: True, myBools[1][0]: True
myDoubles[0, 0]: 3.147, myDoubles[1, 0]:
56.00138917
myStrings[0]: Joe, myStrings[1]: Matt,
myStrings[2]: Robert

The second array is a jagged array. It is
essentially an array of arrays. We needed to use
the new operator to instantiate the size of the
primary array and then use the new operator
again for each sub-array.
The third example is a two dimensional array.
Arrays can be multi-dimensional, with each
dimension separated by a comma. It must also be
instantiated with the new operator.
Arrays sizes can be any int type value. Their
indexes begin at 0.

4. Control Statements - Selection
The if Statement
When you have multiple expressions to evaluate,
you can use the if/else form of the if statement.

In both the regular OR (|) operator and the
conditional OR (||) operator, the boolean
expression will evaluate to true if either of the two
sub-expressions on either side of the operator
evaluate to true. The primary difference between
the two OR forms are that the regular OR operator
will evaluate both sub-expressions every time.
However, the conditional OR will evaluate the
second sub-expression only if the first sub-
expression evaluates to false.
Both the regular AND (&) operator and the
conditional AND (&&) operator will return true
when both of the sub-expressions on either side of
the operator evaluate to true. The difference
between the two is that the regular AND operator
will evaluate both expressions every time.
However, the conditional AND operator will
evaluate the second sub-expression only when the
first sub-expression evaluates to true.
The conditional operators (&& and ||) are
commonly called short-circuit operators because
they do not always evaluate the entire expression.
Thus, they are also used to produce more efficient
code by ignoring unnecessary logic.

The Switch Statement
Another form of selection statement is the switch
statement, which executes a set of logic
depending on the value of a given parameter. The
types of the values a switch statement operates
on can be booleans, enums, integral types, and
strings. Example 4-1 shows how to use the switch
statement with both int and string types.

Example 4-1. Switch Statements: SwitchSelection.cs

using System;

class SwitchSelect
{
{
string myInput;
int myInt;

begin:

Console.Write("Please enter a number between 1 and 3:
");
myInput = Console.ReadLine();
myInt = Int32.Parse(myInput);

// switch with integer
type
switch (myInt)
{
case 1:
Console.WriteLine("Your number is {0}.", myInt);

break;
case 2:
break;
case 3:
break;
default:
Console.WriteLine("Your number {0} is not
between 1 and 3.", myInt);
break;
}

decide:

Console.Write("Type "continue" to go on or "quit" to
stop: ");
myInput = Console.ReadLine();

// switch with string type
switch (myInput)
{
case "continue":
goto begin;
case "quit":
Console.WriteLine("Bye.");
break;
default:
Console.WriteLine("Your input {0} is incorrect.",
myInput);
goto decide;
}
}

}

Table 4-1. C# Branching Statements
Branching
Description
statement
break Leaves the switch block
Leaves the switch block, skips remaining logic
in enclosing loop, and goes back to loop
continue condition to determine if loop should be
executed again from the beginning. Works
only if switch statement is in a loop.
Leaves the switch block and jumps directly to
goto
a label of the form "<labelname>:"
return Leaves the current method.
throw Throws an exception.

You may also include a default choice following all
other choices. If none of the other choices match,
then the default choice is taken and its statements
are executed.
Each case label must end with a branching
statement, as described in Table 4-1, which is
normally the break statement. Here's an example
that shows how to combine case statements:
switch (myInt)
{
case 1:

case 2:
case 3:
break;
default:
Console.WriteLine("Your number {0} is not
between 1 and 3.", myInt);
break;
}

By placing case statements together, with no code
in-between, you create a single case for multiple
values. A case without any code will automatically
fall through to the next case. The example above
shows how the three cases for myInt equal to 1,
2, or 3, where case 1 and case 2 will fall through
and execute code for case 3.
Warning: Don't create loops like this. It is *bad*
programming style. The only reason it is here is
because I wanted to show you the syntax of the
goto statement. Instead, use one of the
structured looping statements.
5. Control Statements - Loops
The while Loop
Its syntax is as follows:
while (<boolean expression>) { <statements> }.

The statements can be any valid C# statements.
The do Loop
A do loop is similar to the while loop, except that
it checks its condition at the end of the loop.
The for Loop
A for loop works like a while loop, except that the
syntax of the for loop includes initialization and
condition modification. for loops are good for
when you know exactly how many times you want
to perform the statements within the loop. The
contents within the for loop parenthesis holds
three sections separated by semicolons
(<initializer list>; <boolean expression>;
<iterator list>) { <statements> }.
The foreach Loop
A foreach loop is used to iterate through the items
in a list. It operates on arrays or collections such
as ArrayList, which can be found in the
System.Collections namespace. The syntax of a
foreach loop is
foreach (<type> <item name> in <list>)
{ <statements> }.

The type is the type of item contained in the list.
For example, if the type of the list was int[] then
the type would be int.
The item name is an identifier that you choose,
which could be anything but should be
meaningful.
The in keyword is required.
The list could be either an array or a collection, as
defined by types found in the System.Collections
namespace. While iterating through the items of a
list with a foreach loop, the list is read-only. This
means that you can't change the value of any
items in the list within a foreach loop.
Example 5-1 demonstrates how to use a foreach
loop.

Program 5-1. The ForEach Loop: ForEachLoop.cs

using System;

class ForEachLoop
{
{
string[] names = {"Cheryl", "Joe", "Matt", "Robert"};

foreach (string person in names)
{

Console.WriteLine("{0} ", person);
}
}

}

6. Methods
Method Structure
Here's a description of the syntax required for creating a
method:
attributes modifiers return-type method-
name( parameters ) { statements }
We defer discussion of attributes and modifiers to a
later lesson. The return-type can be any C# type. It
can be assigned to a variable for use later in the
program.
7. Namespaces
Namespaces are C# program elements designed to help
you organize your programs. They also provide
assistance in avoiding name clashes between two sets of
code. Implementing Namespaces in your own code is a
good habit because it is likely to save you from problems
later when you want to reuse some of your code.

Namespaces don't correspond to file or directory
names. If naming directories and files to correspond to
namespaces helps you organize your code, then you may
do so, but it is not required.

Example 7-1. The C# Station Namespace: NamespaceCSS.cs

using System;

// The C# Station Namespace
namespace csharp_station
{
class NamespaceCSS
{
{
// Write to console
Console.WriteLine("This is the new C# Station Namespace.");
}
}
}

Above example shows how to create a namespace. We
declare the new namespace by putting the word
namespace in front of csharp_station. Curly braces
surround the members inside the csharp_station
namespace.

Example 7-2. Nested Namespace 1: NestedNamespace1.cs

using System;

// The C# Station Tutorial Namespace
{
namespace tutorial
{
class NamespaceCSS
{
{
// Write to console
Console.WriteLine("This is the new C# Station Tutorial
Namespace.");
}
}
}
}

Example 7-3. Nested Namespace 2: NestedNamespace2.cs

using System;

// The C# Station Tutorial Namespace
namespace csharp_station.tutorial
{
class NamespaceCSS

{
{
// Write to console
Console.WriteLine("This is the new C# Station Tutorial
Namespace.");
}
}
}

Example 7-3 shows another way of writing nested
namespaces. It specifies the nested namespace with the
dot operator between csharp_station and tutorial. The
result is exactly the same as Listing 6-2. However,
Listing 7-3 is easier to write.

Example 7-4. Calling Namespace Members: NamespaceCall.cs

using System;

{
// nested namespace
namespace tutorial
{
class myExample1
{
public static void myPrint1()
{
Console.WriteLine("First Example of calling another

namespace member.");
}
}
}

class NamespaceCalling
{
{
// Write to console
tutorial.myExample1.myPrint1();
tutorial.myExample2.myPrint2();
}
}
}

// same namespace as nested namespace above
{
class myExample2
{
public static void myPrint2()
{
Console.WriteLine("Second Example of calling another
namespace member.");
}
}
}

Notice that I used different names for the two classes
myExample1 and myExample2. This was necessary

because every namespace member of the same type
must have a unique name. Remember, they are both in
the same namespace and you wouldn't want any
ambiguity about which class to use.
Elements in different namespaces can have the same
name. The methods myPrint1() and myPrint2() have
different names only because it would make the lesson a
little easier to follow. They could have had the same
name with no effect, because their classes are
different, thus avoiding any ambiguity.
If you would like to call methods without typing their
fully qualified name, you can implement the using
directive. In Listing 6-5, we show two using directives.

Example 7-5. The using Directive: UsingDirective.cs

using System;
using csharp_station.tutorial;

class UsingDirective
{
{
// Call namespace member
myExample.myPrint();
}
}

// C# Station Tutorial Namespace
{
class myExample
{
public static void myPrint()
{
Console.WriteLine("Example of using a using directive.");
}
}
}

Sometimes you may encounter a long namespace and wish to
have it shorter. This could improve readability and still avoid
name clashes with similarly named methods. Example 7-6
shows how to create an alias with the alias directive using
csTut = csharp_station.tutorial.myExample. Now the
expression csTut can be used anywhere, in this file, in place of
csharp_station.tutorial.myExample. We use it in Main().

Example 7-6. The Alias Directive: AliasDirective.cs

using System;
using csTut = csharp_station.tutorial.myExample; // alias

class AliasDirective
{
{

// Call namespace member
csTut.myPrint();
myPrint();
}

// Potentially ambiguous method.
static void myPrint()
{
Console.WriteLine("Not a member of
csharp_station.tutorial.myExample.");
}
}

// C# Station Tutorial Namespace
{
class myExample
{
public static void myPrint()
{
Console.WriteLine("This is a member of
csharp_station.tutorial.myExample.");
}
}
}

So far, all we've shown in our namespaces are classes.
However, namespaces can hold other types as follows:
Classes
Structures
Interfaces

Enumerations
Delegates

8. Introduction to Classes
Classes are declared by using the keyword class followed
by the class name and a set of class members
surrounded by curly braces. Every class has a
constructor, which is called automatically any time an
instance of a class is created. The purpose of
constructors is to initialize class members when the
class is created. Constructors do not have return values
and always have the same name as the class.

Example 8-1. Example C# Classes: Classes.cs

using System;

// helper class
class OutputClass
{
string myString;

// Constructor
public OutputClass(string inputString)
{
myString = inputString;
}

// Instance Method
public void printString()
{

Console.WriteLine("{0}", myString);
}

// Destructor
~OutputClass()
{
// Some resource cleanup routines
}
}

class ExampleClass
{
{
// Instance of OutputClass
OutputClass outCl = new OutputClass("This is printed by the
output class.");

// Call Output class' method
outCl.printString();
}
}

Constructors are not mandatory, as indicated by the
implementation of ExampleClass. In this case, a default
constructor is provided. A default constructor is simply
a constructor with no arguments. However, a
constructor with no arguments is not always useful. To
make default constructors more useful, you can
implement them with initializers. Here is an example:

public OutputClass() : this("Default Constructor
String") { }
This default constructor is followed by an initializer.
The colon, ":", marks the beginning of the initializer,
followed by the this keyword. The this keyword refers
to this particular object. It effectively makes a call to
the constructor of the same object it is defined in.
After the this keyword is a parameter list with a string.
The action taken by the initializer above is to invoke the
OutputClass constructor that takes a string type as an
argument. The initializer helps you to ensure your class
fields are initialized when a class is instantiated.
A class can have multiple constructors. The specific
constructor called depends on the number of
parameters and the type of each parameter.
In C#, there are two types of class members, instance
and static. Instance class members belong to a specific
occurrence of a class. For example, if you create two
OutputClass instances as follows:
OutputClass oc1 = new OutputClass("OutputClass1");
OutputClass oc2 = new OutputClass("OutputClass2");
On the other hand, if a class member is static, you can
access it simply by using the syntax <classname>.<static
class member>.
Suppose OutputClass had the following static method:

public static void staticPrinter()
{
Console.WriteLine("There is only one of me.");
}
Then you could call that function from Main() like this:
OutputClass.staticPrinter();
You must call static class members through their class
name and not their instance name. This means that you
don't need to instantiate a class to use its static
members. There is only ever one copy of a static class
member. A good use of static members is when there is
a function to be performed and no intermediate state is
required, such as math calculations. Matter of fact, the
.NET Frameworks Base Class Library includes a Math
class that makes extensive use of static members.
Use static constructor to initialize static fields in a
class. You declare a static constructor by using the
keyword static just in front of the constructor name. A
static constructor is called before an instance of a class
is created, before a static member is called, and before
the static constructor of a derived class (covered in a
later chapter). They are called only once.
Destructors look just like constructors, except they have
a tilde, "~", in front of them. They don't take any
parameters and do not return a value. Destructors are
places where you could put code to release any

resources your class was holding during its lifetime.
They are normally called when the C# garbage collector
decides to clean your object from memory.
So far, the only class members you've seen are Fields,
Methods, Constructors, and Destructors. Here is a
complete list of the types of members you can have in
your classes:
• Constructors
• Destructors
• Fields
• Methods
• Properties
• Indexers
• Delegates
• Events
• Nested Classes
9. Class Inheritance
Example 9-1. Inheritance: BaseClass.cs

using System;

public class ParentClass
{
public ParentClass()
{
Console.WriteLine("Parent Constructor.");
}

public void print()

{
Console.WriteLine("I'm a Parent Class.");
}
}

public class ChildClass : ParentClass
{
public ChildClass()
{
Console.WriteLine("Child Constructor.");
}

{
ChildClass child = new ChildClass();

child.print();
}
}

Output:
Parent Constructor.
Child Constructor.
I'm a Parent Class.
First we must declare our intention to use ParentClass
as the base class of ChildClass. This is accomplished
through the ChildClass declaration public class
ChildClass : ParentClass. The base class is specified by

adding a colon, ":", after the derived class identifier and
then specifying the base class name.
C# supports single class inheritance only. Therefore,
you can specify only one base class to inherit from.
However, it does allow multiple interface inheritance, a
subject covered later.
Base classes are automatically instantiated before
derived classes. Notice the output from example 9-1.
The ParentClass constructor executed before the
ChildClass constructor.

Listing 9-2. Derived Class Communicating with Base Class: BaseTalk.cs
using System;

public class Parent
{
string parentString;
public Parent()
{
Console.WriteLine("Parent Constructor.");
}
public Parent(string myString)
{
parentString = myString;
Console.WriteLine(parentString);
}
public void print()
{
Console.WriteLine("I'm a Parent Class.");
}
}

public class Child : Parent
{
public Child() : base("From Derived")
{
Console.WriteLine("Child Constructor.");
}
public new void print()
{
base.print();
Console.WriteLine("I'm a Child Class.");
}
{
Child child = new Child();
child.print();
((Parent)child).print();
}
}

Output:
From Derived

Child Constructor.

I'm a Parent Class.

I'm a Child Class.

I'm a Parent Class.

Derived classes can communicate with base classes
during instantiation. Example 9-2 shows how this is
done at the child constructor declaration. The colon,
":", and keyword base call the base class constructor
with the matching parameter list. The first line of
output shows the base class constructor being called
with the string "From Derived".
Another way to access base class members is through an
explicit cast. This is done in the last statement of the
Child class Main() method. Remember that a derived
class is a specialization of its base class. This fact
allows us to perform a cast on the derived class, making
it an instance of its base class.
Notice the new modifier on the Child class print()
method. This enables this method to hide the Parent
class print() method, thus explicitly preventing
polymorphism. Without the new modifier, the compiler
will produce a warning to draw your attention to this.

10. Polymorphism
Another primary concept of object-oriented
programming is Polymorphism. It allows you to
implement derived class methods through a base
class pointer during run-time.

Example 10-1. A Base Class With a Virtual Method:
DrawingObject.cs

using System;

public class DrawingObject
{
public virtual void Draw()
{
Console.WriteLine("I'm just a generic drawing object.");
}
}

The Draw() method has a virtual modifier. The
virtual modifier indicates to derived classes that
they can override this method.

Example 10-2. Derived Classes With Override
Methods: Line.cs, Circle.cs, and
Square.cs
using System;

public class Line : DrawingObject
{
public override void Draw()
{
Console.WriteLine("I'm a Line.");
}
}

public class Circle : DrawingObject
{
{
Console.WriteLine("I'm a Circle.");
}
}

public class Square : DrawingObject
{
{
Console.WriteLine("I'm a Square.");
}
}

Example 10-2 shows three classes. These classes
inherit the DrawingObject class. Each class has a
Draw() method and each Draw() method has an
override modifier. The override modifier allows a
method to override the virtual method of its base
class at run-time. The override will happen only if
the class is referenced through a base class
reference. Overriding methods must have the

same signature, name and parameters, as the
virtual base class method it is overriding.

Listing 10-3. Program Implementing
Polymorphism: DrawDemo.cs
using System;

public class DrawDemo
{
public static int Main( )
{
DrawingObject[] dObj = new
DrawingObject[4];

dObj[0] = new Line();
dObj[1] = new Circle();
dObj[2] = new Square();
dObj[3] = new DrawingObject();

foreach (DrawingObject drawObj in dObj)
{
drawObj.Draw();
}

return 0;
}

}

Output:
I'm a Line.
I'm a Circle.
I'm a Square.
I'm just a generic drawing object.
The code in this example can be compiled with the
following command line:
csc DrawDemo.cs DrawingObject.cs Circle.cs
Line.cs Square.cs
It will create the file DrawDemo.exe, which
defaulted to the name of the first file on the
command line.
11. Properties
Properties provide the opportunity to protect a
field in a class by reading and writing to it through
the property. In other languages, this is often
accomplished by programs implementing
specialized getter and setter methods. C#
properties enable this type of protection while also
letting you access the property just like it was a

field. To get an appreciation for what properties
accomplish, let's take a look at how to provide
field encapsulation by traditional methods.

Example 11-1. An Example of Traditional Class Field
Access: Accessors.cs

using System;

public class PropertyHolder
{
private int someProperty = 0;

public int getSomeProperty()
{
return someProperty;
}

public void setSomeProperty(int propValue)
{
someProperty = propValue;
}

}

public class PropertyTester
{
public static int Main(string[] args)
{
PropertyHolder propHold = new PropertyHolder();

propHold.setSomeProperty(5);

Console.WriteLine("Property Value: {0}",

propHold.getSomeProperty());

return 0;
}
}

This method of accessing information in a field has
been good because it supports the object-oriented
concept of encapsulation. If the implementation
of someProperty changed from an int type to a
byte type, this would still work. Now the same
thing can be accomplished much smoother with
properties.

Example 11-2. Accessing Class Fields With Properties:
Properties.cs

using System;

{

public int SomeProperty
{
get
{
}
set

{
someProperty = value;
}
}
}

{
{

propHold.SomeProperty = 5;

propHold.SomeProperty);

return 0;
}
}

Above code shows how to create and use a
property. The PropertyHolder class has the
SomeProperty property implementation. Notice
that the first letter of the first word is capitalized.
That's the only difference between the names of
the property SomeProperty and the field
someProperty. The property has two accessors,
get and set. get and set are reserved words in
C#.

Properties can be made read-only. This is
accomplished by having only a get accessor in the
property implementation.

Example 11-3. Read-Only Property:
ReadOnlyProperty.cs

using System;

{

public PropertyHolder(int propVal)
{
someProperty = propVal;
}

{
get
{
}
}
}

{
{
PropertyHolder propHold = new PropertyHolder(5);


propHold.SomeProperty);

return 0;
}
}

Example 11-4. Write-Only Property:
WriteOnlyProperty.cs

using System;

{

{
set
{
someProperty = value;

Console.WriteLine("someProperty is equal to {0}",
someProperty);
}
}
}

{

{

propHold.SomeProperty = 5;

return 0;
}
}

12. Indexers
Indexers are real easy. They allow your class to
be used just like an array. On the inside of a
class, you manage a collection of values any way
you want. These objects could be a finite set of
class members, another array, or some complex
data structure. Regardless of the internal
implementation of the class, its data can be
obtained consistently through the use of
indexers. Here's an example.

Example 12-1. An Example of An Indexer:
IntIndexer.cs

using System;

/// <summary>
/// A simple indexer example.
/// </summary>

class IntIndexer
{
private string[] myData;

public IntIndexer(int size)
{
myData = new string[size];

for (int i=0; i < size; i++)
{
myData[i] = "empty";
}
}

public string this[int pos]
{
get
{
return myData[pos];
}
set
{
myData[pos] = value;
}
}

static void Main(string[] args)
{
int size = 10;

IntIndexer myInd = new IntIndexer(size);

myInd[9] = "Some Value";
myInd[3] = "Another Value";

myInd[5] = "Any Value";

Console.WriteLine("nIndexer Outputn");

for (int i=0; i < size;i++)
{
Console.WriteLine("myInd[{0}]: {1}", i, myInd[i]);
}
}
}

The Indexer is identified by the this keyword and square
brackets, this[int pos]. It accepts a single position
parameter, pos. As you may have already guessed, the
implementation of an Indexer is the same as a
Property. It has get and set accessors that are used
exactly like those in a Property. This indexer returns a
string, as indicated by the string return value in the
Indexer declaration.
The Main() method simply instantiates a new IntIndexer
object, adds some values, and prints the results. Here's
the output:
Indexer Output

myInd[0]: empty
myInd[1]: empty
myInd[2]: empty

myInd[3]: Another Value
myInd[4]: empty
myInd[5]: Any Value
myInd[6]: empty
myInd[7]: empty
myInd[8]: empty
myInd[9]: Some Value
Using an integer is a common means of accessing arrays
in many languages, but the C# Indexer goes beyond
this. Indexers can be declared with multiple parameters
and each parameter may be a different type. Additional
parameters are separated by commas, the same as a
method parameter list. Valid parameter types for
Indexers include integers, enums, and strings.
Additionally, Indexers can be overloaded. In the
following program, we modify the previous program to
accept overloaded Indexers that accept different types.

Listing 11-2. Overloaded Indexers: OvrIndexer.cs

using System;

/// <summary>
/// Implements overloaded indexers.
/// </summary>
class OvrIndexer
{
private string[] myData;
private int arrSize;

public OvrIndexer(int size)

{
arrSize = size;
myData = new string[size];

{
myData[i] = "empty";
}
}

public string this[int pos]
{
get
{
return myData[pos];
}
set
{
myData[pos] = value;
}
}

public string this[string data]
{
get
{
int count = 0;

for (int i=0; i < arrSize; i++)
{
if (myData[i] == data)
{
count++;
}
}

return count.ToString();
}
set
{
for (int i=0; i < arrSize; i++)
{
if (myData[i] == data)
{
myData[i] = value;
}
}
}
}

{
int size = 10;
OvrIndexer myInd = new OvrIndexer(size);

myInd[9] = "Some Value";
myInd[3] = "Another Value";
myInd[5] = "Any Value";

myInd["empty"] = "no value";

Console.WriteLine("nIndexer Outputn");

{
Console.WriteLine("myInd[{0}]: {1}", i, myInd[i]);
}

Console.WriteLine("nNumber of "no value" entries: {0}",
myInd["no value"]);
}

}

Here's the output:
Indexer Output

myInd[0]: no value
myInd[1]: no value
myInd[2]: no value
myInd[3]: Another Value
myInd[4]: no value
myInd[5]: Any Value
myInd[6]: no value
myInd[7]: no value
myInd[8]: no value
myInd[9]: Some Value

Number of "no value" entries: 7
The reason both Indexers in above code can coexist in
the same class is because they have different
signatures. An Indexer signature is specified by the
number and type of parameters in an Indexers
parameter list. The class will be smart enough to figure
out which Indexer to invoke, based on the number and
type of arguments in the Indexer call. An indexer with
multiple parameters would be implemented something
like this:

public object this[int param1, ..., int paramN]
{
get
{
// process and return some class data
}
set
{
// process and assign some class data
}
}

13. Structs
A struct allows you to create new value-type objects
that are similar to the built-in types (int, float, bool,
etc.). When would you use a struct instead of a class?
Think about how the built-in types are used. They have
values and distinct operations to manipulate those
values. If you have a need to create an object that
behaves in this manner, consider implementing it as a
struct. Later in this article, I'll explain a couple rules
for using structs, which will give you a better idea of
when to use them. In the meantime, here's an example.

Example 13-1. Example of a struct: StructExample.cs

using System;

struct Point

{
public int x;
public int y;

public Point(int x, int y)
{
this.x = x;
this.y = y;
}

public Point Add(Point pt)
{
Point newPt;

newPt.x = x + pt.x;
newPt.y = y + pt.y;

return newPt;
}
}

/// <summary>
/// Example of declaring and using a struct
/// </summary>
class StructExample
{
{
Point pt1 = new Point(1, 1);
Point pt2 = new Point(2, 2);
Point pt3;

pt3 = pt1.Add(pt2);

Console.WriteLine("pt3: {0}:{1}", pt3.x, pt3.y);

}
}

It is easy to tell that a type is a struct because of the
keyword struct used in its definition. The basic layout
of a struct is much like a class, but with differences,
which will be explained in following paragraphs.
Notice that there is a Point struct declared in the Add()
method. It does not need to be instantiated with a new
operator, like a class. When this occurs, the struct is
implicitly instantiated with its default (or
parameterless) constructor. The parameterless
constructor initializes all struct fields to default values.
i.e. integrals are 0, floating points are 0.0, and booleans
are false. It's illegal to define a parameterless
constructor for a struct.
Although not required, a struct may be instantiated with
a new operator. In Example 13-1 the pt1 and pt2 Point
structs are initialized with values by using the
constructor defined in the Point struct. A third Point
struct, pt3 is declared and defaults to using the
parameterless (default) constructor, because it doesn't
matter what it's value is at this point. The Add()
method of the pt1 struct is then invoked, passing the
pt2 struct as a parameter. The result is assigned to pt3,

showing that a struct may be used just like any other
value type. Here's the output from Example 13-1:
pt3: 3:3
Another difference between structs and classes is that
structs can not have destructors. Also, structs cannot
inherit another class or struct or be inherited from.
However, a struct may inherit multiple interfaces. An
interface is a C# reference type with members that do
not have implementations. Any class or struct inheriting
an interface must implement every one of that
interface's methods. Interfaces are a subject for a later
lesson.
14. Interfaces
An interface looks like a class, but has no
implementation. The only things it contains are
definitions of events, indexers, methods and/or
properties. The reason interfaces only provide
definitions is because they are inherited by classes and
structs, which must provide an implementation for each
interface member defined.
So, what are interfaces good for if they don't implement
functionality? They're great for putting together plug-n-
play like architectures where components can be
interchanged at will. Since all interchangeable
components implement the same interface, they can be
used without any extra programming. The interface

forces each component to expose specific public
members that will be used in a certain way.

Example 14-1. Defining an Interface: MyInterface.cs

interface IMyInterface
{
void MethodToImplement();
}

Example 14-2. Using an Interface: InterfaceImplementer.cs

class InterfaceImplementer : IMyInterface
{
static void Main()
{
InterfaceImplementer iImp = new InterfaceImplementer();
iImp.MethodToImplement();
}

public void MethodToImplement()
{
Console.WriteLine("MethodToImplement() called.");
}
}

Notice that this method implementation has the exact
same signature, parameters and method name, as
defined in the IMyInterface interface. Any difference
will cause a compiler error. Interfaces may also inherit

other interfaces. Example 14-3 shows how inherited
interfaces are implemented.

Listing 14-3. Interface Inheritance: InterfaceInheritance.cs

using System;

interface IParentInterface
{
void ParentInterfaceMethod();
}

interface IMyInterface : IParentInterface
{
void MethodToImplement();
}

class InterfaceImplementer : IMyInterface
{
static void Main()
{
InterfaceImplementer iImp = new InterfaceImplementer();
iImp.MethodToImplement();
iImp.ParentInterfaceMethod();
}

public void MethodToImplement()
{
Console.WriteLine("MethodToImplement() called.");
}

public void ParentInterfaceMethod()
{
Console.WriteLine("ParentInterfaceMethod() called.");
}

}

When one interface inherits another, any implementing
class or struct must implement every interface member
in the entire inheritance chain. Since the
InterfaceImplementer class in Listing 13-3 inherits from
IMyInterface, it also inherits IParentInterface.
Therefore, the InterfaceImplementer class must
implement the MethodToImplement() method specified
in the IMyInterface interface and the
ParentInterfaceMethod() method specified in the
IParentInterface interface.
15. Introduction to Delegates and Events
Delegates
A delegate is a C# language element that allows
you to reference a method. If you were a C or C+
+ programmer, this would sound familiar because
a delegate is basically a function pointer.
However, developers who have used other
languages are probably wondering, "Why do I
need a reference to a method?" The answer boils
down to giving you maximum flexibility to
implement any functionality you want at runtime.

Example 15-1. Declaring and Implementing a Delegate: SimpleDelegate.cs

using System;

// this is the delegate declaration
public delegate int Comparer(object obj1, object obj2);

public class Name
{
public string FirstName = null;
public string LastName = null;

public Name(string first, string last)
{
FirstName = first;
LastName = last;
}

// this is the delegate method handler
public static int CompareFirstNames(object name1, object
name2)
{
string n1 = ((Name)name1).FirstName;
string n2 = ((Name)name2).FirstName;

if (String.Compare(n1, n2) > 0)
{
return 1;
}
else if (String.Compare(n1, n2) < 0)
{
return -1;
}
else
{
return 0;

}
}

public override string ToString()
{
return FirstName + " " + LastName;
}
}

class SimpleDelegate
{
Name[] names = new Name[5];

public SimpleDelegate()
{
names[0] = new Name("Joe", "Mayo");
names[1] = new Name("John", "Hancock");
names[2] = new Name("Jane", "Doe");
names[3] = new Name("John", "Doe");
names[4] = new Name("Jack", "Smith");
}

{
SimpleDelegate sd = new SimpleDelegate();

// this is the delegate instantiation
Comparer cmp = new
Comparer(Name.CompareFirstNames);

Console.WriteLine("nBefore Sort: n");

sd.PrintNames();

// observe the delegate argument
sd.Sort(cmp);

Console.WriteLine("nAfter Sort: n");

sd.PrintNames();
}

// observe the delegate parameter
public void Sort(Comparer compare)
{
object temp;

for (int i=0; i < names.Length;
i++)
{
for (int j=i; j < names.Length; j++)
{
// using delegate "compare" just like
// a normal method
if ( compare(names[i], names[j]) > 0 )
{
temp = names[i];
names[i] = names[j];
names[j] = (Name)temp;
}
}
}
}

public void PrintNames()
{
Console.WriteLine("Names: n");

foreach (Name name in names)

{
Console.WriteLine(name.ToString());
}
}
}

Delegate declarations look somewhat like
methods, except they have the delegate modifier,
are terminated with a semi-colon (;), and have no
implementation. Below, is the delegate
declaration from Listing 15-1.
public delegate int Comparer(object obj1, object
obj2);
This delegate declaration defines the signature of
a delegate handler method that this delegate can
refer to. The delegate handler method, for the
Comparer delegate, can have any name, but must
have a first parameter of type object, a second
parameter of type object, and return an int type.
The following method from Listing 15-1 shows a
delegate handler method that conforms to the
signature of the Comparer delegate.
public static int CompareFirstNames(object
name1, object name2)
{

...
}
To use a delegate, you must create an instance of
it. The instance is created, similar to a class
instance, with a single parameter identifying the
appropriate delegate handler method, as shown
below.
Comparer cmp = new
Comparer(Name.CompareFirstNames);
The delegate, cmp, is then used as a parameter to
the Sort() method, which uses it just like a normal
method. Observe the way the delegate is passed
to the Sort() method as a parameter in the code
below.
sd.Sort(cmp);
Using this technique, any delegate handler
method may be passed to the Sort() method at
run-time. i.e. You could define a method handler
named CompareLastNames(), instantiate a new
Comparer delegate instance with it, and pass the
new delegate to the Sort() method.

Events
Modern GUI programs operate on an event-based
model. That is, some event in the system occurs
and interested modules are notified so they can
react appropriately. With Windows Forms, there
is not a polling mechanism taking up resources
and you don't have to code a loop that sits waiting
for input. It is all built into the system with
events.
A C# event is a class member that is activated
whenever the event it was designed for occurs.
Anyone interested in the event can register and
be notified as soon as the event fires. At the time
an event fires, registered methods will be invoked.
Events and delegates work hand-in-hand to
provide a program's functionality. It starts with a
class that declares an event. Any class, including
the same class that the event is declared in, may
register one of its methods for the event. This
occurs through a delegate, which specifies the
signature of the method that is registered for the
event. The delegate may be one of the pre-
defined .NET delegates or one you declare
yourself. Whichever is appropriate, you assign
the delegate to the event, which effectively
registers the method that will be called when the

event fires. Example 15-2 shows a couple
different ways to implement events.

Listing 15-2. Declaring and Implementing Events: EventDemo.cs

using System;
using System.Drawing;
using System.Windows.Forms;

// custom delegate
public delegate void StartDelegate();

class EventDemo : Form
{
// custom event
public event StartDelegate StartEvent;

public EventDemo()
{
Button clickMe = new Button();

clickMe.Parent = this;
clickMe.Text = "Click Me";
clickMe.Location = new Point(
(ClientSize.Width - clickMe.Width) /2,
(ClientSize.Height - clickMe.Height)/2);

// an EventHandler delegate is assigned
// to the button's Click event
clickMe.Click += new EventHandler(OnClickMeClicked);

// our custom "StartDelegate" delegate is assigned
// to our custom "StartEvent" event.
StartEvent += new StartDelegate(OnStartEvent);

// fire our custom event
StartEvent();
}

// this method is called when the "clickMe" button is pressed
public void OnClickMeClicked(object sender, EventArgs ea)
{
MessageBox.Show("You Clicked My Button!");
}

// this method is called when the "StartEvent" Event is fired
public void OnStartEvent()
{
MessageBox.Show("I Just Started!");
}

{
Application.Run(new EventDemo());
}
}

The EventDemo class inherits Form, which
essentially makes it a Windows Form. This
automatically gives you all the functionality of a
Windows Form, including Title Bar,
Minimize/Maximize/Close buttons, System Menu,
and Borders. A lot of power, that inheritance
thing, eh?

The way a Windows Form's application is started
is by calling the Run() method of the static
Application object with a reference to the Form
object as its parameter. This starts up all the
underlying Windows plumbing, displays the GUI,
and ensures that events are fired as appropriate.
Let's look at the custom event first. Below is the
event declaration, which is a member of the
EventDemo class. It is declared with the event
keyword, a delegate type, and an event name.
public event StartDelegate StartEvent;
Anyone interested in an event can register by
hooking up a delegate for that event. On the next
line, we have a delegate of type StartDelegate,
which the event was declared to accept, hooked
up to the StartEvent event. The += syntax
registers a delegate with an event. To unregister
with an event, use the -= with the same syntax.
StartEvent += new
StartDelegate(OnStartEvent);
Firing an event looks just like a method call, as
shown below:
StartEvent();

This was how to implement events from scratch,
declaring the event and delegate yourself.
However, much of the event programming you'll
do will be with pre-defined events and delegates.
This leads us to the other event code you see in
Listing 15-2, where we hook up an EventHandler
delegate to a Button Click event.
clickMe.Click += new
EventHandler(OnClickMeClicked);
The Click event already belongs to the Button
class and all we have to do is reference it when
registering a delegate. Similarly, the
EventHandler delegate already exists in the
System namespace of the .NET Frameworks Class
Library. All you really need to do is define your
callback method (delegate handler method) that is
invoked when someone presses the clickMe
button. The OnClickMeClicked() method, shown
below, conforms to the signature of the
EventHander delegate, which you can look up in
the .NET Framework Class Library reference.
public void OnClickMeClicked(object sender,
EventArgs ea)
{
MessageBox.Show("You Clicked My

Button!");
}
Any time the clickMe button is pressed with a
mouse, it will fire the Click event, which will
invoke the OnClickMeClicked() method. The
Button class takes care of firing the Click event
and there's nothing more you have to do.
Because it is so easy to use pre-defined events
and delegates, it would be a good idea to check if
some exist already that will do what you need,
before creating your own.
16. Introduction to Exception Handling
Exceptions
Exceptions are unforeseen errors that happen in
your programs. Most of the time, you can, and
should, detect and handle program errors in your
code. For example, validating user input,
checking for null objects, and verifying the values
returned from methods are what you expect, are
all examples of good standard error handling that
you should be doing all the time.
However, there are times when you don't know if
an error will occur. For example, you can't predict
when you'll receive a file I/O error, run out of
system memory, or encounter a database error.

These things are generally unlikely, but they could
still happen and you want to be able to deal with
them when they do occur. This is where
exception handling comes in.
When exceptions occur, they are said to be
"thrown". What is actually thrown is an object
that is derived from the System.Exception class.
The System.Exception class provides several
methods and properties for obtaining information
on what went wrong. For example, the Message
property provides summary information about
what the error was, the StackTrace property
provides information from the stack for where the
problem occurred, and the ToString() method is
overridden to reveal a verbose description of the
entire exception.
Identifying the exceptions you'll need to handle
depends on the routine you're writing. For
example, if the routine opened a file with the
System.IO.File.OpenRead() method, it could
throw any of the following exceptions:
• SecurityException
• ArgumentException
• ArgumentNullException
• PathTooLongException

• DirectoryNotFoundException
• UnauthorizedAccessException
• FileNotFoundException
• NotSupportedException
It's easy to find out what exceptions a method can
raise by looking in the .NET Frameworks SDK
Documentation. Just go to the Reference/Class
Library section and look in the
Namespace/Class/Method documentation for the
methods you use. The exception in the list above
were found by looking at the OpenRead() method
definition of the File class in the System.IO
namespace. Each exception identified has a
hyperlink to its class definition that you can use to
find out what that exception is about. Once
you've figured out what exceptions can be
generated in your code, you need to put the
mechanisms in place to handle the exceptions,
should they occur.
try/catch Blocks
When exceptions are thrown, you need to be able
to handle them. This is done by implementing a
try/catch block. Code that could throw an
exception is put in the try block an exception
handling code goes in the catch block

Note: The programs in here cause exceptions on purpose. The exception that you see is generated
intentionally to show you what the exception message looks like before you see it yourself in your
own programs.

Example 16-1. Using try/catch Blocks: TryCatchDemo.cs

using System;
using System.IO;

class TryCatchDemo
{
{
try
{
File.OpenRead("NonExistentFile");
}
catch(Exception ex)
{
Console.WriteLine(ex.ToString());
}
}
}

In exception handling, more specific exceptions
will be caught before their more general parent
exceptions. For example, the following snippet
shows how to place multiple catch blocks:
catch(FileNotFoundException fnfex)
{
Console.WriteLine(fnfex.ToString());

}
catch(Exception ex)
{
}
Exceptions that are not handled will normally
bubble up the stack until a calling routine in the
call chain handles them. If you forget to include
try/catch blocks in a part of your code and there
aren't any try/catch blocks earlier in the call
chain, your program will abort with a message
describing the exception. To your users this
would be very cryptic and uncomfortable. It is
good practice to provide exception handling in
your programs.
Finally Blocks
An exception can leave your program in an
inconsistent state by not releasing resources or
doing some other type of cleanup. A catch block
is a good place to figure out what may have went
wrong and try to recover, however it can't account
for all scenarios. Sometimes you need to perform
clean up actions whether or not your program
succeeds. These situations are good candidates
for using a finally block.

Listing 16-2 illustrates the usefulness of a finally
block

Listing 16-2. Implementing a finally Block: FinallyDemo.cs

using System;
using System.IO;

class FinallyDemo
{
{
FileStream outStream = null;
FileStream inStream = null;

try
{
outStream = File.OpenWrite("DestinationFile.txt");
inStream = File.OpenRead("BogusInputFile.txt");
}
catch(Exception ex)
{
}
finally
{
if (outStream != null)
{
outStream.Close();
Console.WriteLine("outStream closed.");
}
if (inStream != null)
{
inStream.Close();
Console.WriteLine("inStream closed.");
}

}
}
}

A finally block is not required and you may ask what happens
if you just put code after the catch block. True, under normal
circumstances, if the exception is caught, all code following the
catch will be executed. However, try/catch/finally is for
exceptional circumstances and it is better to plan for the worst
to make your program more robust.

References:

1. http://msdn.microsoft.com/vcsharp/
2. http://www.csharp-
station.com/Tutorials/Lesson01.aspx

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