Sub Procedures

' Define a Sub with no parameters
Private Sub DisplayWelcome()
    lblWelcome.Text = "Welcome to VB 2026!"
    lblWelcome.ForeColor = Color.DarkBlue
    lblWelcome.Font = New Font("Segoe UI", 14, FontStyle.Bold)
End Sub

' Call the Sub -- use the name followed by parentheses
Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
    DisplayWelcome()        ' called when form opens
End Sub

Private Sub btnReset_Click(sender As Object, e As EventArgs) Handles btnReset.Click
    DisplayWelcome()        ' called again from button
End Sub

' A Sub can call other Subs -- building a pipeline
Private Sub ProcessOrder()
    ValidateOrder()
    CalculateTotals()
    UpdateInventory()
    SendConfirmation()
End Sub

' Early exit with Return (like Break for loops)
Private Sub SaveRecord()
    If String.IsNullOrWhiteSpace(txtName.Text) Then
        lblError.Text = "Name is required."
        Return               ' exit the Sub early
    End If
    ' ... rest of save logic only runs if name is filled in
End Sub

' Sub with one parameter
Private Sub ShowMessage(message As String)
    lblStatus.Text = message
End Sub

ShowMessage("File saved successfully.")
ShowMessage("Error: connection failed.")

' Sub with multiple parameters
Private Sub DisplayStudentInfo(name As String, score As Integer, grade As String)
    lstResults.Items.Add($"{name,-20} {score,3}   {grade}")
End Sub

DisplayStudentInfo("Alice Lim",    92, "A")
DisplayStudentInfo("Bob Tan",      74, "B")
DisplayStudentInfo("Carol Wong",   85, "A")

' Parameters vs Arguments
' PARAMETER = variable declared in the Sub definition
' ARGUMENT  = actual value passed when calling the Sub
Private Sub SetLabelColour(lbl As Label, colour As Color)
    lbl.BackColor = colour
End Sub

SetLabelColour(lblName,  Color.LightYellow)     ' lbl=lblName, colour=LightYellow
SetLabelColour(lblEmail, Color.LightPink)        ' lbl=lblEmail, colour=LightPink

' Named arguments (VB 2026 supports named arguments for clarity)
DisplayStudentInfo(name:="Diana", score:=88, grade:="A")   ' named args

' ByVal -- caller's variable is NOT changed
Private Sub TryDouble(ByVal n As Integer)
    n = n * 2             ' only changes the local copy
End Sub

Dim x = 5
TryDouble(x)
' x is still 5 -- ByVal protected it
lblResult.Text = x.ToString()   ' "5"

' ByRef -- caller's variable IS changed
Private Sub ActuallyDouble(ByRef n As Integer)
    n = n * 2             ' modifies the original variable
End Sub

Dim y = 5
ActuallyDouble(y)
lblResult.Text = y.ToString()   ' "10" -- y was modified

' Practical ByRef: swap two values
Private Sub Swap(ByRef a As Integer, ByRef b As Integer)
    Dim temp = a
    a = b
    b = temp
End Sub

Dim p = 10, q = 20
Swap(p, q)
lblResult.Text = $"p={p}  q={q}"   ' "p=20  q=10"

' Practical ByRef: multiple "outputs" when you can't use a Function
Private Sub ParseName(fullName As String, ByRef firstName As String, ByRef lastName As String)
    Dim parts = fullName.Split(" "c)
    firstName = If(parts.Length > 0, parts(0), "")
    lastName  = If(parts.Length > 1, parts(1), "")
End Sub

Dim fn = "", ln = ""
ParseName("Alice Lim", fn, ln)
lblFirst.Text = fn    ' "Alice"
lblLast.Text  = ln    ' "Lim"

' Optional parameter with a default value
Private Sub ShowAlert(message As String, Optional title As String = "Alert", Optional beep As Boolean = False)
    If beep Then Console.Beep()
    MsgBox(message, MsgBoxStyle.Information, title)
End Sub

' All three ways to call it:
ShowAlert("File saved.")                            ' title="Alert", beep=False
ShowAlert("Saved.", "Success")                    ' beep=False
ShowAlert("Low disk space!", "Warning", True)      ' all three provided

' More practical: formatted display sub
Private Sub AddListItem(lst As ListBox, text As String,
                         Optional prefix As String = "",
                         Optional upperCase As Boolean = False)
    Dim item = If(prefix <> "", prefix & " ", "") & text
    lst.Items.Add(If(upperCase, item.ToUpper(), item))
End Sub

AddListItem(lstLog, "Server started")                          ' "Server started"
AddListItem(lstLog, "Connection failed", "[ERROR]")          ' "[ERROR] Connection failed"
AddListItem(lstLog, "disk full",           "[WARN]", True)   ' "[WARN] DISK FULL"

' ParamArray -- accepts any number of Double values
Private Sub ShowStats(ParamArray values() As Double)
    If values.Length = 0 Then
        lblStats.Text = "No values provided." : Return
    End If
    Dim total = values.Sum()
    Dim avg   = total / values.Length
    Dim min   = values.Min()
    Dim max   = values.Max()
    lblStats.Text = $"Count={values.Length}  Sum={total:F1}  Avg={avg:F2}  Min={min}  Max={max}"
End Sub

ShowStats(10, 20, 30)                      ' 3 values
ShowStats(5, 15, 25, 35, 45)               ' 5 values
ShowStats(100)                             ' 1 value
ShowStats()                                ' 0 values -- handled by If check

' Mixed: required param + ParamArray
Private Sub LogMessage(category As String, ParamArray parts() As String)
    Dim combined = String.Join(" | ", parts)
    lstLog.Items.Add($"[{category}] {combined}")
End Sub

LogMessage("INFO",  "App started")
LogMessage("ERROR", "File not found", "C:\data.txt", "Errno 2")
LogMessage("AUDIT", "Login", "admin", "192.168.1.1", "success")

' Private -- only Form1 can call this
Private Sub UpdateStatusBar(message As String)
    lblStatus.Text = message
    lblStatus.ForeColor = Color.DarkGreen
End Sub

' Public -- any form in the project can call this
Public Sub ClearAllControls(ParamArray controls() As Control)
    For Each ctrl As Control In controls
        If TypeOf ctrl Is TextBox Then CType(ctrl, TextBox).Clear()
        If TypeOf ctrl Is Label   Then CType(ctrl, Label).Text = ""
    Next
End Sub

' Shared -- called on class, not on an instance
Public Shared Sub LogError(source As String, message As String)
    System.IO.File.AppendAllText("errors.log",
        $"[{DateTime.Now:yyyy-MM-dd HH:mm:ss}] {source}: {message}{Environment.NewLine}")
End Sub

' Call the Shared sub without creating an instance:
FormHelper.LogError("SaveRecord", "Database timeout")

' Standard event handler signature
Private Sub btnSave_Click(sender As Object, e As EventArgs) Handles btnSave.Click
    SaveRecord()            ' delegate to a helper Sub
End Sub

' One Sub handles multiple events (Handles clause with commas)
Private Sub TextBox_TextChanged(sender As Object, e As EventArgs) _
    Handles txtName.TextChanged, txtEmail.TextChanged, txtPhone.TextChanged
    ValidateForm()          ' validate whenever any text box changes
End Sub

' Shared validation logic -- called from multiple events
Private Sub ValidateForm()
    Dim nameOK  = Not String.IsNullOrWhiteSpace(txtName.Text)
    Dim emailOK = txtEmail.Text.Contains("@")
    Dim phoneOK = txtPhone.Text.Length >= 8
    btnSubmit.Enabled = nameOK AndAlso emailOK AndAlso phoneOK
    lblStatus.Text = If(btnSubmit.Enabled, "Ready to submit", "Please complete all fields")
End Sub

' sender parameter tells you WHICH control raised the event
Private Sub Button_Click(sender As Object, e As EventArgs) Handles btn1.Click, btn2.Click, btn3.Click
    Dim btn = CType(sender, Button)
    lblClicked.Text = $"You clicked: {btn.Text}"
End Sub

' Wire event handler at runtime (without Handles clause)
AddHandler btnDynamic.Click, AddressOf btnDynamic_Click
RemoveHandler btnDynamic.Click, AddressOf btnDynamic_Click   ' remove when no longer needed

Public Class RegistrationForm

    Private Sub HighlightField(txt As TextBox, lbl As Label, errorMsg As String)
        txt.BackColor = Color.LightPink
        lbl.Text      = errorMsg
        lbl.ForeColor = Color.Red
        lbl.Visible   = True
    End Sub

    Private Sub ClearFieldError(txt As TextBox, lbl As Label)
        txt.BackColor = Color.White
        lbl.Text      = ""
        lbl.Visible   = False
    End Sub

    Private Sub ValidateForm(ByRef isValid As Boolean)
        isValid = True

        If String.IsNullOrWhiteSpace(txtName.Text) Then
            HighlightField(txtName, lblNameErr, "Name is required.")
            isValid = False
        Else
            ClearFieldError(txtName, lblNameErr)
        End If

        If Not txtEmail.Text.Contains("@") OrElse Not txtEmail.Text.Contains(".") Then
            HighlightField(txtEmail, lblEmailErr, "Enter a valid email.")
            isValid = False
        Else
            ClearFieldError(txtEmail, lblEmailErr)
        End If
    End Sub

    Private Sub btnRegister_Click(sender As Object, e As EventArgs) Handles btnRegister.Click
        Dim valid As Boolean
        ValidateForm(valid)         ' ByRef: valid gets updated inside sub
        If Not valid Then Return
        MsgBox("Registration successful!")
    End Sub

End Class