If..Then..Else

' Single-line If..Then (condition and action on one line)
If score >= 50 Then lblResult.Text = "Pass"

' Block If..Then (multiple statements, requires End If)
If score >= 50 Then
    lblResult.Text = "Pass"
    lblResult.BackColor = Color.LightGreen
    btnNext.Enabled = True
End If

' Checking Nothing (null) before using an object
If lstStudents.SelectedItem IsNot Nothing Then
    lblSelected.Text = lstStudents.SelectedItem.ToString()
End If

' Validating user input before processing
Private Sub btnSubmit_Click(sender As Object, e As EventArgs) Handles btnSubmit.Click
    If String.IsNullOrWhiteSpace(txtName.Text) Then
        lblError.Text = "Name is required."
        txtName.Focus()
        Return                ' early exit -- nothing else runs
    End If
    ' ... rest of processing
End Sub

' Grade classification using ElseIf chain
Dim mark As Integer
If Not Integer.TryParse(txtMark.Text, mark) Then
    lblGrade.Text = "Invalid input"
    Return
End If

Dim grade As String
If mark >= 90 Then
    grade = "A+"
ElseIf mark >= 80 Then
    grade = "A"
ElseIf mark >= 70 Then
    grade = "B"
ElseIf mark >= 60 Then
    grade = "C"
ElseIf mark >= 50 Then
    grade = "D"
Else
    grade = "F"
End If

lblGrade.Text = $"Mark: {mark}  |  Grade: {grade}"

' Discount calculator with ElseIf
Dim amount As Decimal
Decimal.TryParse(txtAmount.Text, amount)

Dim discount As Double
If amount > 500 Then
    discount = 0.2   ' 20%
ElseIf amount > 200 Then
    discount = 0.1   ' 10%
ElseIf amount > 100 Then
    discount = 0.05  ' 5%
Else
    discount = 0     ' no discount
End If

Dim savings = amount * discount
Dim total   = amount - savings
lblDiscount.Text = $"Discount: {discount:P0}  |  Save: {savings:C}  |  Pay: {total:C}"

' Role-based access control with nested If
If txtUsername.Text <> "" Then
    If txtPassword.Text = GetPassword(txtUsername.Text) Then
        If txtUsername.Text = "admin" Then
            lblStatus.Text = "Admin access granted"
            btnDeleteAll.Visible = True
        Else
            lblStatus.Text = "User access granted"
            btnDeleteAll.Visible = False
        End If
    Else
        lblStatus.Text = "Wrong password"
    End If
Else
    lblStatus.Text = "Username required"
End If

' BETTER: flatten with AndAlso to reduce nesting
If String.IsNullOrWhiteSpace(txtUsername.Text) Then
    lblStatus.Text = "Username required"
    Return
End If

If txtPassword.Text <> GetPassword(txtUsername.Text) Then
    lblStatus.Text = "Wrong password"
    Return
End If

' Reached here: login valid
If txtUsername.Text = "admin" Then
    lblStatus.Text = "Admin access granted"
    btnDeleteAll.Visible = True
Else
    lblStatus.Text = "User access granted"
End If

' Syntax: If(condition, valueIfTrue, valueIfFalse)
Dim result   = If(score >= 50, "Pass", "Fail")
Dim absValue = If(x >= 0, x, -x)
Dim status   = If(isLoggedIn, "Welcome!", "Please log in")

' Assign in one line
lblResult.Text = If(score >= 50, "Pass", "Fail")

' Use inside string interpolation
lblInfo.Text = $"Status: {If(age >= 18, "Adult", "Minor")}"

' Null-coalescing form: If(obj, fallback)
Dim name = If(txtName.Text, "(unnamed)")
' Returns txtName.Text if non-Nothing, else "(unnamed)"

' Multi-line IIf alternative (old VB6 style -- avoid)
' lblResult.Text = IIf(score >= 50, "Pass", "Fail")  -- evaluates BOTH sides always
' Use If() instead -- short-circuits like AndAlso

' Good use: set a property based on a condition
btnDelete.Enabled = If(lstItems.SelectedIndex >= 0, True, False)
' Simplified:
btnDelete.Enabled = (lstItems.SelectedIndex >= 0)  ' Boolean expression directly

' AndAlso -- ALL conditions must be True
If age >= 18 AndAlso age <= 65 AndAlso hasLicence Then
    lblEligible.Text = "Eligible to drive"
End If

' OrElse -- at least ONE condition must be True
If role = "admin" OrElse role = "manager" Then
    btnApprove.Enabled = True
End If

' Not -- inverts Boolean
If Not String.IsNullOrWhiteSpace(txtEmail.Text) Then
    lblEmailOK.Text = "Email provided"
End If

' Complex: range check + nullable check
If temperature IsNot Nothing AndAlso
   temperature >= 36.0 AndAlso
   temperature <= 37.5 Then
    lblTemp.Text = "Normal body temperature"
ElseIf temperature > 37.5 Then
    lblTemp.Text = "Fever detected"
Else
    lblTemp.Text = "Below normal"
End If

' Boolean flag patterns
Dim isValid = Not String.IsNullOrWhiteSpace(txtName.Text) AndAlso
              Not String.IsNullOrWhiteSpace(txtEmail.Text) AndAlso
              txtEmail.Text.Contains("@")
btnSubmit.Enabled = isValid

Private Sub txtPassword_TextChanged(sender As Object, e As EventArgs) Handles txtPassword.TextChanged
    Dim pwd = txtPassword.Text
    Dim score = 0

    If pwd.Length >= 8                                     Then score += 1
    If pwd.Any(Function(c) Char.IsUpper(c))                Then score += 1
    If pwd.Any(Function(c) Char.IsLower(c))                Then score += 1
    If pwd.Any(Function(c) Char.IsDigit(c))                Then score += 1
    If pwd.Any(Function(c) Not Char.IsLetterOrDigit(c))   Then score += 1

    If score <= 1 Then
        lblStrength.Text = "Weak" : lblStrength.ForeColor = Color.Red
    ElseIf score <= 3 Then
        lblStrength.Text = "Fair" : lblStrength.ForeColor = Color.Orange
    ElseIf score = 4 Then
        lblStrength.Text = "Strong" : lblStrength.ForeColor = Color.Blue
    Else
        lblStrength.Text = "Very Strong" : lblStrength.ForeColor = Color.Green
    End If
End Sub

Private Sub btnConvert_Click(sender As Object, e As EventArgs) Handles btnConvert.Click
    Dim temp As Double
    If Not Double.TryParse(txtTemp.Text, temp) Then
        lblResult.Text = "Enter a valid temperature."
        Return
    End If

    If rdbCtoF.Checked Then
        Dim fahrenheit = temp * 9 / 5 + 32
        lblResult.Text = $"{temp}°C = {fahrenheit:F1}°F"

        If fahrenheit > 100 Then
            lblNote.Text = "Above boiling point!"
        ElseIf fahrenheit <= 32 Then
            lblNote.Text = "At or below freezing point."
        Else
            lblNote.Text = "Normal temperature range."
        End If
    Else
        Dim celsius = (temp - 32) * 5 / 9
        lblResult.Text = $"{temp}°F = {celsius:F1}°C"
    End If
End Sub