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Object-Oriented Application Development Using VB .NET 1

VB .NET Programming

with Supplied Classes

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Objectives

In this chapter, you will: Use the namespaces and classes supplied with VB .NET

Use the String class

Create a String array

Use the ArrayList class

Work with dates

Format numeric output

Use the MessageBox class

Display a Form

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Using the Namespaces and ClassesSupplied with VB .NET

Namespace : It is a containerto group related

classes.

The Namespace is a method for organizingthe

.NET class library into tree form.

It looks like the Folder conceptthat is used for

organizing the files existed on disks in tree form.

Keyword Imports: gives compiler access to

classes contained in specific namespaces

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Using the String Class

String: collection of characters

VB .NET stores string data in instances of String class

String class is a member of Systemnamespace

Code to declare a string:

Dim s1 As String = "Hello Again"

The following steps will be done to execute the abovestatement

1. Create variable s1 with data type String.2. Create String instance.

3. Populate the instance with "Hello Again value.

4. Assigns the location of this instance to variable s1.

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String values in VB .NET are immutablethey cannot bechanged by the methods appeared as changing methods.

Methods that appear to change a string value, such as

Insert, Replace, and ToUpperactually create and return a

new Stringinstance

Those methods create and return a new String instance.

You can change its value directly like primitive

variable.

Example: Dim s1 As String = "Hello"

S1 = "Hello Again"

Index:

Each character in a String instance has an index

Index values in VB .NET begin with zero

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Using String Methods

Copymethod returns a second String that is a copy of theString sent as an argument

Dim s1 As String = "Hello Again"

' create a copy of s1

Dim s2 As String = String.Copy(s1)

Console.WriteLine("s2, a copy of s1, contains " & s2)

Charsmethod returns the character at a specified index

Note: Index will start from Zero value.Console.WriteLine("char at index 6 of s1 is " & s1.Chars(6))

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There are two options to compare two stringvalues:

Equalsmethod

Console.WriteLine("s1.Equals(s2) returns " & s1.Equals(s2))

Equalsign (=) , Compiler will invoke Equalsmethod on behalf of you.

If s1 = s2 Then Console.WriteLine("s1 = s2")

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SubStringmethod: Extracts one or more characters from a String

instance, and

Returns a new String instance containing extracted

characters.

Console.WriteLine("s1.Substring(0, 5) returns " & _

s1.Substring(0, 5))

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Replacemethod replaces one or more charactersin a string with one or more other characters

Console.WriteLine("s1.Replace(Hello, Hi) returns " & _

s1.Replace("Hello", "Hi"))Console.WriteLine("After Replace s1 contains " & s1)

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Insertmethod inserts one or more characters intoan existing string beginning at a specified index

Console.WriteLine("s1.Insert(6, There ) returns " & _

s1.Insert(6, "There "))

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StartsWithmethod Compares the string argument with beginning

characters of the String instance

Returns True or False depending on whether there

is a match

Console.WriteLine("s1.StartsWith(Hi) returns " & _s1.StartsWith("Hi"))

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EndsWithmethod compares the endingcharacters with the argument

Console.WriteLine("s1.EndsWith(Again) returns " & _

s1.EndsWith("Again"))

Use ToUppermethod to change the case of astring value to uppercase

Use ToLowermethod to change the case of astring value to lowercase

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IndexOfmethod Searches a String instance for a specific value

Returns index of the beginning of the value

Returns1 if no matching value was found

Console.WriteLine("s1.indexof(Again) returns " & _s1.IndexOf("Again"))

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Use ToStringmethod to convert numeric valuesto a string

' convert an integer to String

Dim i As Integer = 5

Console.WriteLine("value of Convert.ToString(i) is " & _Convert.ToString(i))

Use methods in Convertclass to convert Stringvalues containing numeric data to primitive datatypes & vice versa.

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Creating a String Array

Code to create a String array:

' declare a String array with 4 elements

Dim stringArray(3) As String

Elements of stringArray are reference variables ofdata type String

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Code to create four String instances and populate the arrayelementsstringArray(0) = "Hello"

stringArray(1) = "World"

stringArray(2) = "Wide"

stringArray(3) = "Web"

A specific element can be accessed using the index of thatelement

String methods can be invoked for String instances

referenced by array elements . For example: - console.writeline(Length of first element: & stringArray(0).length)

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Using the ArrayList Class

Major limitation of arrays: fixed size ArrayListclass

Member of System.Collectionsnamespace

Creates dynamically resizable arraythe number

of elements can change during runtime

Code to create an instance of ArrayList class:

' create an ArrayList instance

Dim dynArr As ArrayList = New ArrayList( ) ORDim dynArr As New ArrayList( )

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Code to create four instances of String class:' create String instances

Dim s1 As String = New String("Hello")

Dim s2 As String = New String("World")

Dim s3 As String = New String("Wide")

Dim s4 As String = New String("Web")

Add method is used to populate the elements

When a fourth element is added, the number of elementsincreases automatically

dynArr.Add(s1)dynArr.Add(s2)

dynArr.Add(s3)

dynArr.Add(s4)

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The following Methods/Properties are defined in theArrayList Class :

1. Contains(O) - > return True if O is in ArrayList object,and False if O is not in ArrayList object.

2. Capacity -> Gets/Sets No. of Elements.

3. Count -> return populated elements.

4. Remove(O) -> removes first occurrence of O object.

5. Reverse( ) -> reverses the element sequence.

6. IndexOf( O ) -> return the index of first occurrence of O,

and1 otherwise.7. Item( i ) -> sets/gets the object at index i . This property

can be used to loop through the array elements.

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Working with Dates

Systemnamespace contains DateTime class andTimeSpan class

Instance of DateTime class contains a date & time

value

Instance of TimeSpan class contains the differencebetween two dates

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Working with Dates

DateTimeproperties

Todayproperty gets system date and returns a

DateTime instance

Nowproperty captures the current time

DateTime methods to perform arithmetic on a

date value

AddMonthsadds a value to the month

AddDaysadds a value to the day

AddYearsadds a value to the year

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Working with Dates

To format a date: Invoke its ToStringmethod

Pass arguments that describe the desired format

For example: Code to return a date in the format

February 15, 2003:

DateTime.today.ToString("MMMM dd,yyyy")

Note: Look page 147 in boo k for form att ing date

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Working with Dates

Subtractmethod

Computes the number of days between two DateTimeinstances

Returns an instance of the TimeSpanclass

Comparemethod

Compares two DateTime instances

Returns

1 -> first DateTime instance is less than the second.

0 -> they are equal.

+1 -> first DateTime instance greater than the second.

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Formatting Numerical Output

Formatting means inserting commas, decimal

places, dollar signs, percent symbols,

parentheses, hyphens, etc.

Each primitive data type is represented by astruc ture, which has methods

When a primitive variable is declared, certain

methods associated with that variable can beinvoked

For example: ToStr ingmethod

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ToStr ingmethod is used to format numeric data

Format mask

A series of characters that describes the format to

be used.

Passed to ToString method to carry out formatting.

For example:

S = i.ToString("$#,###.00")

Console.WriteLine("Integer 1234 with $#,##0.00 format is " & s)

Output: $1,234.00

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Object-Oriented Application Development Using VB .NET29


Using ToString method, a number can beformatted

As currency -> argument C can be used

With or without commas -> argument N, and F

can be used respectively . As a percentage -> argument P can be used

As a telephone number

As a Social Security number

Dim sn as double = 1234556 S = sn.Tostring(###-##-###)

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Using the MessageBox Class

A message box can be used to display a message

and to get a response.

MessageBoxclass

Member of System.Windows.Formsnamespace

Has a single method named Show

Show method

Creates an instance of MessageBoxclass

Makes a message box appear

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Show method can receive up to four arguments Second argument :Caption to be displayed

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Show method can receive up to four arguments Third argument :Specifies the buttons to be displayed

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Show method can receive up to four arguments Fourth argument :Specifies the type of icon displayed

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Return value obtained from Show method

Indicates which button was clicked by user

Is of data type DialogResult

Can be compared to specific values using an If statement

Example: -

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Example

Dim res As DialogResult

res = MessageBox.Show("Hello", "Msg", _MessageBoxButtons.YesNoCancel, _

MessageBoxIcon.Question)

If res = DialogResult.Yes Then

MessageBox.Show("You have presses Yes")ElseIf res = DialogResult.No Then

MessageBox.Show("You have presses No")

Else

MessageBox.Show("You have presses Cancel")

End If

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Displaying a Form

Form A class in System.Windows.Formsnamespace

GuiModule.vb module contains instructions to

Instantiate GuiForm.vb Form Make it visible and invisible

GuiForm.vb

A subclass of Form

Inherits Formattributes and methods

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Example

Dim myForm As GuiForm

myForm = New GuiForm()

myForm.Text = "My GUI Form Demo"

myForm.BackColor = System.Drawing.Color.Brown

' use MessageBox to determine what to do

Dim result As DialogResult Do Until result = DialogResult.Cancel

result = MessageBox.Show("Press Yes to Show, No to Hide, Cancelto stop", "Form Demo", MessageBoxButtons.YesNoCancel)

If result = DialogResult.Yes Then myForm.Visible = True

If result = DialogResult.No Then myForm.Visible = False

Loop

myForm.Dispose()

clase 8 - 2 .ppt

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