The CheckBox Control

' --- Reading Checked in a Button click ---
Private Sub btnOrder_Click(...) Handles btnOrder.Click
    Dim msg = ""
    If chkExtraShot.Checked  Then msg &= "Extra shot, "
    If chkOatMilk.Checked    Then msg &= "Oat milk, "
    If chkDecaf.Checked      Then msg &= "Decaf, "
    If msg = "" Then msg = "Standard espresso"
    lblOrder.Text = msg.TrimEnd(","c, " "c)
End Sub

' --- Setting Checked in code ---
Private Sub Form1_Load(...) Handles MyBase.Load
    chkNewsletter.Checked = True   ' pre-tick on startup
    chkTerms.Checked      = False  ' must be ticked by user
End Sub

' --- Reacting immediately with CheckedChanged ---
Private Sub chkDarkMode_CheckedChanged(...) Handles chkDarkMode.CheckedChanged
    If chkDarkMode.Checked Then
        Me.BackColor = Color.FromArgb(30, 30, 30)
        lblStatus.Text = "Dark mode ON"
    Else
        Me.BackColor = SystemColors.Control
        lblStatus.Text = "Dark mode OFF"
    End If
End Sub

' --- Toggling with Not ---
chkRememberMe.Checked = Not chkRememberMe.Checked   ' flip state in code

' --- Using If/Else vs IIf for short assignments ---
Dim fee = If(chkExpressShipping.Checked, 15.0, 0.0)
lblShipping.Text = fee.ToString("C2")

' --- Loop through all CheckBoxes on a Panel ---
Private Function GetCheckedItems(panel As Panel) As List(Of String)
    Dim result As New List(Of String)
    For Each ctrl As Control In panel.Controls
        If TypeOf ctrl Is CheckBox Then
            Dim chk = DirectCast(ctrl, CheckBox)
            If chk.Checked Then result.Add(chk.Text)
        End If
    Next
    Return result
End Function

' --- Select All / Clear All helpers ---
Private Sub SetAllCheckBoxes(panel As Panel, state As Boolean)
    For Each ctrl As Control In panel.Controls
        If TypeOf ctrl Is CheckBox Then
            DirectCast(ctrl, CheckBox).Checked = state
        End If
    Next
End Sub

Private Sub btnSelectAll_Click(...) Handles btnSelectAll.Click
    SetAllCheckBoxes(pnlOptions, True)
End Sub

Private Sub btnClearAll_Click(...) Handles btnClearAll.Click
    SetAllCheckBoxes(pnlOptions, False)
End Sub

' --- Count how many are checked ---
Private Function CountChecked(panel As Panel) As Integer
    Return panel.Controls.OfType(Of CheckBox)().Count(Function(c) c.Checked)
End Function

' --- Validate: at least one must be checked ---
Private Function ValidateAtLeastOne(panel As Panel) As Boolean
    If CountChecked(panel) = 0 Then
        MessageBox.Show("Please select at least one option.", "Validation",
                        MessageBoxButtons.OK, MessageBoxIcon.Warning)
        Return False
    End If
    Return True
End Function

' --- Persist state to settings (save/load ticked boxes) ---
Private Sub SaveCheckBoxStates()
    My.Settings.ShowNews      = chkNews.Checked
    My.Settings.ShowAds       = chkAds.Checked
    My.Settings.ShowAnalytics = chkAnalytics.Checked
    My.Settings.Save()
End Sub

' --- ThreeState property enables Indeterminate state ---
' CheckState enum values: Checked, Unchecked, Indeterminate

Private Sub chkSelectAll_CheckedChanged(...) Handles chkSelectAll.CheckedChanged
    ' Ignore the Indeterminate state when the user clicks chkSelectAll
    If chkSelectAll.CheckState = CheckState.Indeterminate Then Return

    Dim state = chkSelectAll.Checked
    chkItem1.Checked = state
    chkItem2.Checked = state
    chkItem3.Checked = state
    chkItem4.Checked = state
End Sub

' --- Each child updates the parent "Select All" state ---
Private Sub AnyChild_CheckedChanged(...) Handles
        chkItem1.CheckedChanged, chkItem2.CheckedChanged,
        chkItem3.CheckedChanged, chkItem4.CheckedChanged

    Dim checkedCount = {chkItem1, chkItem2, chkItem3, chkItem4}.Count(
                            Function(c) c.Checked)
    Dim total        = 4

    ' Suppress re-entry while we update the parent
    RemoveHandler chkSelectAll.CheckedChanged, AddressOf chkSelectAll_CheckedChanged

    chkSelectAll.CheckState = If(checkedCount = 0,   CheckState.Unchecked,
                               If(checkedCount = total, CheckState.Checked,
                                                         CheckState.Indeterminate))
    AddHandler chkSelectAll.CheckedChanged, AddressOf chkSelectAll_CheckedChanged
End Sub

' --- Reading CheckState ---
Select Case chkOption.CheckState
    Case CheckState.Checked       : lblStatus.Text = "Yes"
    Case CheckState.Unchecked     : lblStatus.Text = "No"
    Case CheckState.Indeterminate : lblStatus.Text = "Some"
End Select

' In the Designer, set Appearance = Button and FlatStyle = Flat
' The checkbox behaves identically — only the look changes.

Private Sub chkBold_CheckedChanged(...) Handles chkBold.CheckedChanged
    Dim style = txtEditor.Font.Style
    If chkBold.Checked Then
        txtEditor.Font = New Font(txtEditor.Font, style Or FontStyle.Bold)
    Else
        txtEditor.Font = New Font(txtEditor.Font, style And Not FontStyle.Bold)
    End If
End Sub

Private Sub chkItalic_CheckedChanged(...) Handles chkItalic.CheckedChanged
    Dim style = txtEditor.Font.Style
    txtEditor.Font = New Font(txtEditor.Font,
        If(chkItalic.Checked, style Or FontStyle.Italic,
                               style And Not FontStyle.Italic))
End Sub

' Toggle panel visibility — common UI pattern
Private Sub chkShowFilters_CheckedChanged(...) Handles chkShowFilters.CheckedChanged
    pnlFilters.Visible = chkShowFilters.Checked
    chkShowFilters.Text = If(chkShowFilters.Checked, "Hide Filters ▲", "Show Filters ▼")
End Sub

' Real-world settings form pattern:
' 1. Load saved settings into checkboxes on Form_Load
' 2. Enable/disable dependent controls based on Checked state
' 3. Save back when OK is clicked

Private Sub SettingsForm_Load(...) Handles MyBase.Load
    ' Restore from app settings
    chkAutoSave.Checked        = My.Settings.AutoSave
    chkAutoSaveInterval.Enabled = chkAutoSave.Checked
    nudInterval.Enabled         = chkAutoSave.Checked

    chkNotifications.Checked   = My.Settings.Notifications
    chkSoundAlerts.Enabled      = chkNotifications.Checked
    chkPopupAlerts.Enabled      = chkNotifications.Checked

    chkStartWithWindows.Checked = My.Settings.StartWithWindows
    chkMinimizeToTray.Checked   = My.Settings.MinimizeToTray
End Sub

' Enable/disable sub-options based on parent checkbox
Private Sub chkAutoSave_CheckedChanged(...) Handles chkAutoSave.CheckedChanged
    chkAutoSaveInterval.Enabled = chkAutoSave.Checked
    nudInterval.Enabled          = chkAutoSave.Checked
End Sub

Private Sub chkNotifications_CheckedChanged(...) Handles chkNotifications.CheckedChanged
    chkSoundAlerts.Enabled = chkNotifications.Checked
    chkPopupAlerts.Enabled = chkNotifications.Checked
    If Not chkNotifications.Checked Then
        chkSoundAlerts.Checked = False
        chkPopupAlerts.Checked = False
    End If
End Sub

' Save on OK
Private Sub btnOK_Click(...) Handles btnOK.Click
    My.Settings.AutoSave          = chkAutoSave.Checked
    My.Settings.Notifications     = chkNotifications.Checked
    My.Settings.SoundAlerts       = chkSoundAlerts.Checked
    My.Settings.PopupAlerts       = chkPopupAlerts.Checked
    My.Settings.StartWithWindows  = chkStartWithWindows.Checked
    My.Settings.MinimizeToTray    = chkMinimizeToTray.Checked
    My.Settings.Save()
    Me.DialogResult = DialogResult.OK
    Me.Close()
End Sub