Lesson 13: Mastering If..Then..Else in VB2022

Private Sub btnLogin_Click(sender As Object, e As EventArgs) Handles btnLogin.Click
    Dim username As String = txtUsername.Text.Trim()
    Dim password As String = txtPassword.Text
    
    ' Check for empty fields
    If String.IsNullOrEmpty(username) Or String.IsNullOrEmpty(password) Then
        MessageBox.Show("Please enter both username and password", "Input Error")
        Return
    End If
    
    ' Validate credentials
    If username = "admin" And password = "Secure123!" Then
        MessageBox.Show("Admin access granted!", "Login Successful")
        OpenAdminDashboard()
    ElseIf username = "user" And password = "Pass2022" Then
        MessageBox.Show("User access granted!", "Login Successful")
        OpenUserDashboard()
    Else
        MessageBox.Show("Invalid username or password", "Login Failed")
        attempts += 1
        
        If attempts >= 3 Then
            MessageBox.Show("Account locked! Too many failed attempts")
            btnLogin.Enabled = False
        End If
    End If
End Sub

Private Sub btnCalculate_Click(sender As Object, e As EventArgs) Handles btnCalculate.Click
    Dim score As Integer
    
    ' Validate input
    If Not Integer.TryParse(txtScore.Text, score) Then
        MessageBox.Show("Please enter a valid numeric score")
        Return
    End If
    
    If score < 0 Or score > 100 Then
        MessageBox.Show("Score must be between 0 and 100")
        Return
    End If
    
    ' Determine grade
    Dim grade As String
    
    If score >= 90 Then
        grade = "A"
    ElseIf score >= 80 Then
        grade = "B"
    ElseIf score >= 70 Then
        grade = "C"
    ElseIf score >= 60 Then
        grade = "D"
    Else
        grade = "F"
    End If
    
    lblResult.Text = $"Grade: {grade}"
    
    ' Additional feedback
    If grade = "A" Then
        lblFeedback.Text = "Excellent work!"
        lblFeedback.ForeColor = Color.DarkGreen
    ElseIf grade = "F" Then
        lblFeedback.Text = "Please see your instructor"
        lblFeedback.ForeColor = Color.DarkRed
    Else
        lblFeedback.Text = "Good effort"
        lblFeedback.ForeColor = Color.Blue
    End If
End Sub

Private Sub btnCheck_Click(sender As Object, e As EventArgs) Handles btnCheck.Click
    Dim temp As Double
    
    ' Validate input
    If Not Double.TryParse(txtTemperature.Text, temp) Then
        MessageBox.Show("Please enter a valid temperature")
        Return
    End If
    
    ' Determine alert level
    Dim alert As String
    Dim color As Color
    
    If temp > 40 Then
        alert = "EXTREME HEAT WARNING! Avoid outdoor activities"
        color = Color.DarkRed
    ElseIf temp > 35 Then
        alert = "Heat Warning: Stay hydrated"
        color = Color.OrangeRed
    ElseIf temp > 30 Then
        alert = "Hot day: Limit sun exposure"
        color = Color.Orange
    ElseIf temp > 20 Then
        alert = "Pleasant weather"
        color = Color.Green
    ElseIf temp > 10 Then
        alert = "Cool: Bring a jacket"
        color = Color.Blue
    ElseIf temp > 0 Then
        alert = "Cold: Dress warmly"
        color = Color.DarkBlue
    Else
        alert = "FREEZING: Avoid prolonged exposure"
        color = Color.Purple
    End If
    
    lblAlert.Text = alert
    lblAlert.ForeColor = color
End Sub