The ColorPicker WinForms Control Revisited

Refactoring the original ColorPicker control by employing the Adapter design pattern to support plug-in display adapters for ComboBox-like appearance and more.

1. The CheckBox covers the entire ColorPicker client area.
2. The CheckBox.BackColor property holds the value exposed by the ColorPicker.Color property.
3. The ColorPicker sets the CheckBox.Text property to either the current color name or to an empty string (with respect to the value of the ColorPicker.TextDisplayed property).
4. The CheckBox.CheckStateChanged event is handled by the ColorPicker control to show or hide the drop-down color editor according to the value of the CheckBox.CheckState property.
5. Finally, the CheckBox.CheckState value is set to 'unchecked' after the user finishes selection in the dropped-down color selector.

Public Interface IDropDownDisplayAdapter
  ReadOnly Property Adaptee() As Control
  Property Color() As System.Drawing.Color
  Property Text() As String
  Property HasDropDownAppearance() As Boolean
  Event DropDownAppearanceChanged As EventHandler
End Interface

Terminology note: I'll frequently use phrases like "the adapter displays this or that". In reality, however, the adapter doesn't display anything; it's the task of the adaptee - the control used to actually render the display and interact with the user. Nevertheless, from the point of view of the ColorPicker control (the client of the adapter) that communicates with the adapter, the use of the word "adapter" seemed more natural than "adaptee".

CheckBoxDisplayAdapter.vb:

Imports System.Drawing

' Implements the IDropDownDisplayAdapter interface by using 
' the standard CheckBox control as the adaptee.
Friend NotInheritable Class CheckBoxDisplayAdapter
  Implements IDropDownDisplayAdapter

  Public Event DropDownAppearanceChanged As EventHandler _
    Implements IDropDownDisplayAdapter.DropDownAppearanceChanged

  Public Sub New(ByVal checkBox As CheckBox)
    Debug.Assert(Not checkBox Is Nothing)
    Me._CheckBox = checkBox
    checkBox.Appearance = Appearance.Button
    checkBox.TextAlign = ContentAlignment.MiddleCenter
  End Sub


  Public Property Color() As System.Drawing.Color _
    Implements IDropDownDisplayAdapter.Color
    Get
      Return Me._CheckBox.BackColor
    End Get
    Set(ByVal Value As System.Drawing.Color)
      ' Store the color in the BackColor property and set the ForeColor
      ' to a value that will make the displayed text readable.
      Me._CheckBox.BackColor = Value
      Me._CheckBox.ForeColor = ColorPicker.GetInvertedColor(Value)
    End Set
  End Property


  Public Property HasDropDownAppearance() As Boolean _
    Implements IDropDownDisplayAdapter.HasDropDownAppearance
    Get
      Return Me._CheckBox.Checked
    End Get
    Set(ByVal Value As Boolean)
      Me._CheckBox.Checked = Value
    End Set
  End Property


  Public Property Text() As String Implements IDropDownDisplayAdapter.Text
    Get
      Return Me._CheckBox.Text
    End Get
    Set(ByVal Value As String)
      Me._CheckBox.Text = Value
    End Set
  End Property


  Public ReadOnly Property Adaptee() As System.Windows.Forms.Control _
    Implements IDropDownDisplayAdapter.Adaptee
    Get
      Return Me._CheckBox
    End Get
  End Property


  Private Sub _CheckBox_CheckStateChanged( _
    ByVal sender As Object, _
    ByVal e As System.EventArgs) Handles _CheckBox.CheckStateChanged
    
    RaiseEvent DropDownAppearanceChanged(Me, EventArgs.Empty)
  End Sub

  Private WithEvents _CheckBox As CheckBox ' the adaptee

End Class

Public Class ComboBoxDisplay
  Inherits Control

  Public Event DropDownAppearanceChanged As EventHandler
  Public Overridable Property Color() As Color
  Public Overridable Property HasDropDownAppearance() As Boolean
...

ComboBoxDisplayAdapter.vb

' Implements the IDropDownDisplayAdapter by using the ComboBoxDisplay
' as the adaptee control.
Public NotInheritable Class ComboBoxDisplayAdapter
  Implements IDropDownDisplayAdapter

  ' The ComboBoxDisplay control - our adaptee.
  Private WithEvents _Display As ComboBoxDisplay


  Public Sub New(ByVal display As ComboBoxDisplay)
    If display Is Nothing Then
      Throw New ArgumentNullException("display")
    End If
    Me._Display = display
  End Sub


  Public ReadOnly Property Adaptee() As System.Windows.Forms.Control _
    Implements IDropDownDisplayAdapter.Adaptee
    Get
      Return Me._Display
    End Get
  End Property


  Public Property Color() As System.Drawing.Color _
    Implements IDropDownDisplayAdapter.Color
    Get
      Return Me._Display.Color
    End Get
    Set(ByVal Value As System.Drawing.Color)
      Me._Display.Color = Value
    End Set
  End Property


  Public Event DropDownAppearanceChanged As EventHandler _
    Implements IDropDownDisplayAdapter.DropDownAppearanceChanged

  Public Property HasDropDownAppearance() As Boolean _
    Implements IDropDownDisplayAdapter.HasDropDownAppearance
    Get
      Return Me._Display.HasDropDownAppearance
    End Get
    Set(ByVal Value As Boolean)
      Me._Display.HasDropDownAppearance = Value
    End Set
  End Property


  Public Property Text() As String Implements IDropDownDisplayAdapter.Text
    Get
      Return Me._Display.Text
    End Get
    Set(ByVal Value As String)
      Me._Display.Text = Value
    End Set
  End Property


  Private Sub _Display_DropDownAppearanceChanged( _
    ByVal sender As Object, _
    ByVal e As System.EventArgs) Handles _Display.DropDownAppearanceChanged
    RaiseEvent DropDownAppearanceChanged(Me, e)
  End Sub

End Class

Namespace LaMarvin.Windows.Forms

Public Class ColorPicker
  Inherits Control
  Public Event ColorChanged As EventHandler
  Public Property Color() As Color
  Public Property Appearance() As ColorPickerAppearance
  Public Property DisplayAdapter() As IDropDownDisplayAdapter

ColorPicker1.DisplayAdapter = New LabelDisplayAdapter(New Label)

• Extract the files with the 'use folder names' option.
• Rebuild the solution (Build | Rebuild Solution) after you open it for the first time (this step will ensure the ColorPicker controls will display correctly at design-time).