vb test 4 review

7/30/2019 Vb Test 4 Review

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CMSY 190


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Slide 7- 2

Form Names Each form has aName property

Programs refer to a form by this name

VB assigns name Form1

Name property allows usto change form name

Standard prefix is frm

Each form also has a file name (.vb extension)

Forms are stored on disk using this name

Right click in Solution Explorer, and select Rename tochange the file name


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Slide 7- 3

Adding a New Form to a Project ClickAdd New Item on the toolbar

Or Project on menu, thenAdd Windows Form

Add New Item dialog box appears Click on Windows Form

Change the default name

Click theAddbutton

New form now appears in:

Design window

Solution Explorer


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Slide 7- 4

Switching from Forms to Form

Code Design window has two tabs for each form

One for form design

One for the code associated with a form

If you have two formsfrmMain & frmError, youmay select these tabs: Error form design

Error form code

Main form design

Main form code


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Slide 7- 5

Creating an Instance of a Form

Dim statement creates an instance of a form

For example, to create an instance of frmError:

frmError is the form design name (the class)

NewfrmError creates an instance of the form

Variable errorFormrefers to the form in RAM

errorFormused to perform actions on the form The form is not yet visible, but it now exists

Show or ShowDialog makes the form visible

DimObjectVariableAs New ClassName()

Dim errorForm As New frmError()


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Slide 7- 6

Modal Forms & ShowDialog

MethodAmodal form prevents the user from changing focus

to another form in the application as long as it remainsopen

For example:

Variable errorFormrepresents an instance offrmError as shown in the previous slide

The ShowDialog method displays the form instancenamed errorFormas a modal form

Must close errorFormin order to change focus toanother form in the application

errorForm.ShowDialog()


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Closing a FormA form may close itself using the Close method and

referring to itself using the keyword "Me":

As inMe.Close()

Private Sub btnClose_Click(ByVal sender As System.Object, _

ByVal e As System.EventArgs) _Handles btnClose.Click

Me.Close()

End Sub


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IsNumeric Function This function accepts a string as an argument and returns

True if the string contains a number

Dim strNumber as String

strNumber = 576

If IsNumeric(strNumber)then returns true[statements]

End if

strNumber = 123abc

If IsNumeric(strNumber)then returns false

[statements]End if

Use IsNumeric function to determine if a given stringcontains numeric data


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Components of a Menu System

Each drop-down menu has a menu name Each drop-down menu has a list of actions or menu

commands that can be performed Some commands may lead to a submenu


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MenuStrip ControlAMenuStrip control adds a menu to a form

Double-click on theMenuStrip icon in theMenus &Toolbars section of the Toolbox

TheMenuStrip controlis displayed in the componenttray (bottom of Design window)

A MenuStrip can have manyToolStripMenuItemobjects: Each represents a single menu command

Name property - used by VB to identify it Text property text displayed to the user


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ToolStripMenuItem Object Names Should begin with mnu

Then by convention are named based on their textproperty and position in the menu hierarchy

mnuFile

mnuFileSave

mnuFilePrint

mnuFileExit


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Slide 7- 12

ToolStripMenuItem Text Properties The text property holds the menu item description that

is displayed to the user

If an access key is assigned, that letter must be precededwith an ampersand

Object Name Access Key Text Property

mnuFile F &File

mnuFileSave S &SavemnuFilePrint P &PrintmnuFileExit X E&xit


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Slide 7- 13

Menu Designer TheMenu Designerallows visual menu creation by

filling in boxes with the menu text:

Enter firstcommand inthe File menu

Enter thenext menuname


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Slide 7- 14

Components of a Menu System

Actions may be performed using a key or keycombination called a shortcut key


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Slide 7- 15

Shortcut Keys Keyboard based shortcuts that execute menu

commands without using the menu system

For example, ctrl-c to Copy to the clipboard

These are set via the Shortcut property of each menuitem

A shortcut is displayed to the user only if theShowShortcut property is set to true


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Slide 7- 16

ToolStripMenuItem Click Events

Menus and submenus require no codeCommands must have a click event procedure

Double click on the menu item Event procedure created in the code window Programmer supplies the code to execute

Double click the menu item objectmnuFileExit, then add a Me.Close command asshown below:

Private Sub mnuFileExit_Click(ByVal sender as System.Object, _ByVal e as System.EventArgs) Handles mnuFileExit.Click

Me.Close()

End Sub

Programmer supplied code

Click event procedure created by VB


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Slide 5- 17

ListBox Items.Add Method Items can be added to the end of a ListBox list in your

VB code using theAdd method

Format isListBox.Items.Add(Item)

ListBox is the name of the control

Item is a string value to add to the Items property

Example:lstStudents.Items.Add("Sharon")


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Slide 3- 18

String ConcatenationWe often need to combine two or more strings into a

longer one This operation is called Concatenation

Concatenation is signaled by the '&' operator in thesame way addition is signaled by a '+

Assume the user has entered their name into theTextBox txtName

Listbox lstGreeting can say, Hello to any name foundin the TextBox

lstGreeting.Items.Add("Hello " &txtName.Text)

Appends user name in txtName.Text to Hello andwrites the result in the lstGreeting listbox


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Slide 3- 19

A Full List of Conversion Functions There are conversion functions for each data type

CBool ( expr)

CByte ( expr)

CChar ( expr)

CDate ( expr)

CDbl ( expr)

CDec ( expr)

CInt ( expr)

CLng ( expr)

CObj ( expr)

CShort ( expr)

CSng ( expr)

CStr ( expr)


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Slide 9- 20

Creating Files with StreamWriter Objects

Add Imports System.IObefore class declared Makes StreamWriter classes available in code

AStreamWriterobject is used to create a sequential

text file in the following way: Declare an object variable of type StreamWriter

Object is assigned to a StreamWriter variable

VariablephoneFile now defines a stream of data thatcan be written tophonelist.txt

DimphoneFileAs System.IO.StreamWriter


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Slide 9- 21

Appending Text with StreamWriter

AStreamWriterobject is used to append data to asequential text file in the following way: Declare an object variable of type StreamWriter

CallAppendTextmethod passing the filename

Method returns a StreamWriter object

Object is assigned to a StreamWriter variable

VariablephoneFile now defines a stream of data thatcan be added to the end ofphonelist.txt

DimphoneFileAs StreamWriter

phoneFile = File.AppendText(phonelist.txt)


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Slide 9- 22

Appending to a FileWhen opening an existing file withAppendText

Existing contents are retained

New text adds on to the end of the old text

Note: If the file does not exist, a new file will be created

i.e., If adding a new number tophoneList, use:

phoneList = File.AppendText(PhoneNum.txt")


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Writing Data to a FileThe WriteLine method of the StreamWriter class writes

a line of data to a file. The following is the generalformat of the method: ObjectVar.WriteLine(Data)

ObjectVaris the name of a StreamWriter object variable.Data represents constants or variables whose contentswill be written to the file.

If writing a line of data to the phoneFile, use code similarto: phoneFile.WriteLine(PhoneNum.ToString)


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Slide 9- 24

Closing a StreamWriter Object

Should close files when finished with them

Avoids losing data

Data is initially written to a buffer

Writes unsaved data from the buffer to the file

The Close method of a StreamWriterobject clears thebuffer and closes the file

ObjectVar.Close() Streamwriter object identified byObjectVar


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Slide 9- 25

StreamReader Objects Use StreamReaderobjects to read from a file

Define and open similar to StreamWriter:

Sample code:

VariablephoneFile now defines a stream of data thatcan be read fromphonelist.txt

Must have Imports System.IObefore classdeclaration as was done with StreamWriter

DimObjectVarAs StreamReader

ObjectVar =File.OpenText(Filename)

Dim phoneFile As StreamReader

phoneFile = File.OpenText(phonelist.txt")


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Slide 9- 26

Detecting the End of a File The Peekmethod tests if youve reached end of file

(no more characters to read) Format is objectvar.Peek If no more characters, the value -1 is returned

Dim scoresFile As StreamReader

Dim strInput As String

scoresFile = File.OpenText("Scores.txt")

Do Until scoresFile.Peek = -1

strInput = scoresFile.ReadLine()

lstResults.Items.Add(strInput)

Loop

scoresFile.Close()


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Slide 9- 27

Determining Whether a File Exists

The File.OpenText method issues a runtime error ifthe file does not exist

Avoid this by using the File.Exists method

Format isFile.Exists(filename)

Returns a boolean result that can be tested:

If File.Exists(filename) Then

' Open the file.

inputFile = File.OpenText(filename)

Else

MessageBox.Show(filename & " does not exist.")

End If


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Slide 9 28

ReadLine MethodThe ReadLine in the StreamReader class reads a line of data from

a file. The general format of the method is as follows:

ObjectVar.ReadLine()

Dim inputFile As StreamReaderDim intAnswer as Integer

If File.Exists(Example.txt) Then

'Open the file

inputFile = File.OpenText(Example.txt)

Do While inputFile.Peek -1

intAnswer = CInt(intNum.ReadLine())lstOutput.Items.Add(The answer is & intAnswer.ToString)

Loop

'Close the file.inputfile.Close()

Else

MessageBox.Show("That file does not exist")

End If

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