User Defined Data Types

	IsNumeric ( string expression )
						

	Private Sub btnCalculate_Click(ByVal sender as System.Object, _
		ByVal e As System.EventArgs) Handles btnCalculate.Click

		If IsNumeric ( txtSubtotal.Text ) = false Then
			MessageBox.Show("Please enter a valid Number")
			Exit Sub
		End If

		' Code if Data was entered properly

	End Sub
						

• Data Validation: The process of verifying that all user entries are valid.
• One goal of this verification is to prevent runtime errors.
• The IsNumeric function tests whether the value of an expression is numeric.
  • The function returns a Boolean value, which can be tested in a conditional expression.
  • If the Boolean value is true, the expression is numeric.
• The IsNumeric function treats both positive and negative numbers as valid.
• The IsNumeric function treats both numbers with and without decimal places as valid.

Figure UDDT-2: General Syntax and Example for IsNumeric Function

Figure Structure-1: Student Data Form Design

Public Class Form1

    Private Structure StudentRecord
        Dim LastName As String
        Dim FirstName As String
        Dim ID As String
        Dim GPA As Double
    End Structure

    Dim Student() As StudentRecord
    Dim StudentIndex As Integer

	' AddEntry transfers the data in the TextBoxes on the Form
	' To the Next Student Record Structure

    Private Sub AddEntry(ByVal Index As Integer)
        Dim DataString As String
        Dim FullName As String

        FullName = Student(Index).LastName + ", " + _
            Student(Index).FirstName
        DataString = FullName.PadRight(20) + _
            Student(Index).ID.PadRight(14) + _
            Student(Index).GPA.ToString(".00")
        lstDisplay.Items.Add(DataString)
    End Sub

	'Sub to Validate proper Entry format on the form

    Private Sub ValidateData()
        Dim LastName As String
        LastName = txtLastName.Text
        If LastName = "" Then
            MessageBox.Show("Please Enter the Student's Last Name")
            txtLastName.Clear()
            txtLastName.Focus()
            Exit Sub
        End If

        Dim FirstName As String
        FirstName = txtFirstName.Text
        If FirstName = "" Then
            MessageBox.Show("Please Enter the Student's First Name")
            txtFirstName.Clear()
            txtFirstName.Focus()
            '            Me.ActiveControl = txtFirstName
            Exit Sub
        End If

        Dim StudentID As String
        Dim IDComponents() As String

        StudentID = txtStudentID.Text
        IDComponents = StudentID.Split("-")

        If IDComponents.Length <> 3 Or _
            IDComponents(0).Length <> 3 Or _
            IDComponents(1).Length <> 2 Or _
            IDComponents(2).Length <> 4 Or _
            IsNumeric(IDComponents(0)) = False Or _
            IsNumeric(IDComponents(1)) = False Or _
            IsNumeric(IDComponents(2)) = False Then
            MessageBox.Show("Please enter ID in proper Format: ###-##-####")
            txtStudentID.Clear()
            txtStudentID.Focus()
            Me.ActiveControl = txtStudentID
            Exit Sub

        End If

        Dim strGPA As String

        strGPA = txtStudentGPA.Text
        If IsNumeric(strGPA) = False Then
            MessageBox.Show("Please Enter a Grade Point Average Number between 0 and 4.0")
            txtStudentGPA.Clear()
            txtStudentGPA.Focus()
            Me.ActiveControl = txtStudentGPA
            Exit Sub
        End If

        Dim dblGPA As Double
        dblGPA = CDbl(strGPA)
        If dblGPA < 0 Or dblGPA > 4 Then
            MessageBox.Show("Please enter a Grade Point Average Number between 0 and 4.0")
            txtStudentGPA.Clear()
            txtStudentGPA.Focus()
            Me.ActiveControl = txtStudentGPA
            Exit Sub
        End If

        ReDim Preserve Student(StudentIndex)

        Student(StudentIndex).LastName = LastName
        Student(StudentIndex).FirstName = FirstName
        Student(StudentIndex).ID = StudentID
        Student(StudentIndex).GPA = dblGPA

        AddEntry(StudentIndex)

        StudentIndex += 1

    End Sub

	'Event Generated by Clicking on the ListBox

    Private Sub lstDisplay_SelectedIndexChanged(ByVal sender As System.Object, _
            ByVal e As System.EventArgs) Handles lstDisplay.SelectedIndexChanged
        Dim Index As Integer
        Index = lstDisplay.SelectedIndex
        DisplaySelectedStudent(Index)
    End Sub

	'Sub to transfer data from Structure element to Form TextBoxes

    Private Sub DisplaySelectedStudent(ByRef Index As Integer)
        txtFirstName.Text = Student(Index).FirstName
        txtLastName.Text = Student(Index).LastName
        txtStudentID.Text = Student(Index).ID
        txtStudentGPA.Text = Student(Index).GPA.ToString(".00")
    End Sub

	' Event Generated when Clear Button Is pressed

    Private Sub txtLastName_Click(ByVal sender As System.Object, _
            ByVal e As System.EventArgs) Handles txtLastName.Click
        ClearTextFields()
    End Sub

	'Sub to clear all TextBoxes at once.

    Private Sub ClearTextFields()
        txtFirstName.Clear()
        txtStudentID.Clear()
        txtLastName.Clear()
        txtStudentGPA.Clear()
        txtLastName.Focus()
    End Sub

	'Event Generated when Exit Button is pressed

    Private Sub btnExit_Click(ByVal sender As System.Object, _
            ByVal e As System.EventArgs) Handles btnExit.Click
        Me.Close()
    End Sub

	'Event Generated by Pressing Calculate Class GPA Button

    Private Sub btnAverageGPA_Click(ByVal sender As System.Object, _
            ByVal e As System.EventArgs) Handles btnAverageGPA.Click
        Dim Index As Integer
        Dim Sum As Double
        Dim Ave As Double

        For Index = 0 To StudentIndex - 1
            Sum += Student(Index).GPA
        Next

        If StudentIndex > 0 Then
            Ave = Sum / StudentIndex
        End If

        txtAverageGPA.Text = "Class Ave = " + Ave.ToString(".00")

    End Sub

	' Event Generated when Enter Button is pressed

    Private Sub btnEnter_Click(ByVal sender As System.Object, _
            ByVal e As System.EventArgs) Handles btnEnter.Click

        ValidateData()
        txtAverageGPA.Clear()
        ClearTextFields()
    End Sub

	' Event Generated when Search Button is pressed

    Private Sub btnSearch_Click(ByVal sender As System.Object, _
            ByVal e As System.EventArgs) Handles btnSearch.Click
        Dim ID As String
        Dim Index As Integer
        Dim MatchFound As Boolean = False
        ID = InputBox("Please Enter the Student's SSN (ID)", "Student Search")
        If ID <> "" Then
            For Index = 0 To StudentIndex - 1
                If ID = Student(Index).ID Then
                    DisplaySelectedStudent(Index)
                    MatchFound = True
                End If

            Next
        End If

        If MatchFound = False Then
            MessageBox.Show("No Match Found for " + ID)
            ClearTextFields()
        End If
    End Sub

	' Event Generated by Pressing the Display Button

    Private Sub btnDisplay_Click(ByVal sender As System.Object, _
            ByVal e As System.EventArgs) Handles btnDisplay.Click
        lstDisplay.Items.Clear()
        Dim Index As Integer

        For Index = 0 To StudentIndex - 1
            AddEntry(Index)
        Next

    End Sub
End Class

						

Figure Structure-2: Source Code for Example 1