This is in reference to thread796-1499946.
Using Kliot's code to add a readonly attribute to the Combobox works great (see code below).
However being new to this I am trying to understand why the background color changes to grey when readonly and white when not. and How can I control this within the comboboxreadonly class.
Option Strict Off
Public Class ComboBoxReadOnly
Inherits System.Windows.Forms.ComboBox
Private _ReadOnly As Boolean
Private _DroppedDown As Boolean
Private _SelectedIndex As Integer = -1
Private _DropDownStyle As ComboBoxStyle = ComboBoxStyle.DropDown
Private Declare Auto Function GetWindow Lib "user32.dll" (ByVal hwnd As IntPtr, ByVal wCmd As Int32) As IntPtr
Private Declare Auto Function SendMessage Lib "user32.dll" (ByVal hwnd As IntPtr, ByVal wMsg As Int32, ByVal wParam As Boolean, ByVal lParam As Int32) As Int32
Private Const EM_SETREADONLY As Int32 = &HCF
Private Const EM_EMPTYUNDOBUFFER As Int32 = &HCD
Private Const CB_SHOWDROPDOWN As Int32 = &H14F
Private Const GW_CHILD As Int32 = 5
'<Browsable(True), DefaultValue(False)> _
Public Property [ReadOnly]() As Boolean
Get
Return _ReadOnly
End Get
Set(ByVal Value As Boolean)
' In design mode we don't want setting the read only property
' to alter the dropdown style.
If Not DesignMode Then
' If the DropDownStyle is anything other than DropDown then setting
' ReadOnly to true will have no affect. Therefore we'll force the style
' to DropDown as it goes to ReadOnly and restore it when it's turned off.
If Value Then
' If the value is changing then we want to save the dropdown style.
' In case the value gets set to true more than once we don't want
' to loose the saved drop down style.
If _ReadOnly <> Value Then
_DropDownStyle = MyBase.DropDownStyle
MyBase.DropDownStyle = ComboBoxStyle.DropDown
End If
Else ' restore the saved drop down style
MyBase.DropDownStyle = _DropDownStyle
End If
End If
_ReadOnly = Value
' If readonly then don't let the user tab to the field
MyBase.TabStop = Not Value
' Setting TabStop to false causes the text in the box to be selected if it matches
' an entry in the list. Setting selection length to zero removes the selection.
MyBase.SelectionLength = 0
' Send the textbox portion of the combo the readonly message.
' It will change the color and behavior.
SendMessage(GetWindow(Me.Handle, GW_CHILD), EM_SETREADONLY, Value, 0)
' If text was typed or pasted into the textbox, the context menu will
' have the undo activated. When the text box is in the readonly state
' the undo will still be active from the right click context menu
' allowing the user to restore the previous value. This sendmessage
' will clear the undo buffer which will clear the undo.
SendMessage(GetWindow(Me.Handle, GW_CHILD), EM_EMPTYUNDOBUFFER, Value, 0)
' the dropdown may have been dropped before the readonly is set
_DroppedDown = False
Me.Refresh()
End Set
End Property
'
' Saving and returning a local copy of the selected index keeps a
' changed value from being returned when the control is in the
' readonly state. The OnSelectedIndexChanged event captures the
' index value that is returned here. Must be shadows or else the
' value passed won't cause the OnSelectedIndexChanged method to fire
' and the text value to be displayed, won't.
'
Public Shadows Property SelectedIndex() As Integer
Get
Return _SelectedIndex
End Get
Set(ByVal Value As Integer)
_SelectedIndex = Value
MyBase.SelectedIndex = Value
If Value = -1 Then ' Set it twice to work around databound bug KB327244
_SelectedIndex = Value
MyBase.SelectedIndex = Value
End If
End Set
End Property
'
' Intercepting message 273 when readonly and the listbox is dropped
' keeps the user from selecting an item in the list and having it update
' the text value of the combo as well as firing the associated changed events.
' Since we intercept a windows message we will have to manually bring up
' the listbox.
'
' msg 305 (0x131) = an item was clicked from the dropdown list
' msg 273 (0x111) = (WM_COMMAND) follows dropdown list click and all other actions?
' msg 8465 (0x2111) = (WM_REFLECT + WM_COMMAND) subsequent command after the 273
'
Protected Overrides Sub WndProc(ByRef m As System.Windows.Forms.Message)
' Cannot use me.DroppedDown, it causes a System.StackOverflowException.
' Asking for it's value must produce windows messages for the combobox
' and thus create a recursive loop.
If _ReadOnly AndAlso _DroppedDown Then
If m.Msg = 273 Then
_DroppedDown = False
' bring up the dropdown
SendMessage(Me.Handle, CB_SHOWDROPDOWN, System.Convert.ToInt32(False), 0)
Exit Sub
End If
End If
MyBase.WndProc(m)
End Sub
'
' This event will not fire when msg 273 is intercepted in WndProc. When in the
' readonly state clicking on an item in the listbox appears to have no effect. It
' does in fact change the value of MyBase.SelectedIndex. Saving the last good index
' value locally allows the overriden Index property to supply the proper index value.
'
Protected Overrides Sub OnSelectedIndexChanged(ByVal e As System.EventArgs)
_SelectedIndex = MyBase.SelectedIndex
MyBase.OnSelectedIndexChanged(e)
End Sub
'
' We must manually track dropped state. Asking the control if it's
' dropped from within WndProc will cause a System.StackOverflowException.
'
Protected Overrides Sub OnDropDown(ByVal e As System.EventArgs)
_DroppedDown = True
MyBase.OnDropDown(e)
End Sub
'
' The up and down arrow keys cause the combobox to change selection to the next
' or previous in the list. The page up and page down keys change the selection
' by one page at a time as defined by the size of the dropdown list. Setting
' e.Handled to true if any of these keys is pressed stops the selection change
' when readonly. The alt down arrow combination is allowed since it drops the listbox.
'
Protected Overrides Sub OnKeyDown(ByVal e As System.Windows.Forms.KeyEventArgs)
If _ReadOnly Then
If e.KeyCode = Keys.Up OrElse e.KeyCode = Keys.PageUp OrElse _
e.KeyCode = Keys.PageDown OrElse _
(e.KeyCode = Keys.Down And ((Control.ModifierKeys And Keys.Alt) <> Keys.Alt)) Then
e.Handled = True
End If
End If
MyBase.OnKeyDown(e)
End Sub
'
' The combobox default behavior when pressing F4 is to drop the listbox.
' If F4 is immediately pressed a second time the OnSelectionChangeCommitted
' event fires regardless of whether a change has been made or not. When
' readonly we don't want a change event to fire. This code will stop it.
'
Protected Overrides Sub OnSelectionChangeCommitted(ByVal e As System.EventArgs)
If _ReadOnly Then
Else
MyBase.OnSelectionChangeCommitted(e)
End If
End Sub
End Class
Using Kliot's code to add a readonly attribute to the Combobox works great (see code below).
However being new to this I am trying to understand why the background color changes to grey when readonly and white when not. and How can I control this within the comboboxreadonly class.
Option Strict Off
Public Class ComboBoxReadOnly
Inherits System.Windows.Forms.ComboBox
Private _ReadOnly As Boolean
Private _DroppedDown As Boolean
Private _SelectedIndex As Integer = -1
Private _DropDownStyle As ComboBoxStyle = ComboBoxStyle.DropDown
Private Declare Auto Function GetWindow Lib "user32.dll" (ByVal hwnd As IntPtr, ByVal wCmd As Int32) As IntPtr
Private Declare Auto Function SendMessage Lib "user32.dll" (ByVal hwnd As IntPtr, ByVal wMsg As Int32, ByVal wParam As Boolean, ByVal lParam As Int32) As Int32
Private Const EM_SETREADONLY As Int32 = &HCF
Private Const EM_EMPTYUNDOBUFFER As Int32 = &HCD
Private Const CB_SHOWDROPDOWN As Int32 = &H14F
Private Const GW_CHILD As Int32 = 5
'<Browsable(True), DefaultValue(False)> _
Public Property [ReadOnly]() As Boolean
Get
Return _ReadOnly
End Get
Set(ByVal Value As Boolean)
' In design mode we don't want setting the read only property
' to alter the dropdown style.
If Not DesignMode Then
' If the DropDownStyle is anything other than DropDown then setting
' ReadOnly to true will have no affect. Therefore we'll force the style
' to DropDown as it goes to ReadOnly and restore it when it's turned off.
If Value Then
' If the value is changing then we want to save the dropdown style.
' In case the value gets set to true more than once we don't want
' to loose the saved drop down style.
If _ReadOnly <> Value Then
_DropDownStyle = MyBase.DropDownStyle
MyBase.DropDownStyle = ComboBoxStyle.DropDown
End If
Else ' restore the saved drop down style
MyBase.DropDownStyle = _DropDownStyle
End If
End If
_ReadOnly = Value
' If readonly then don't let the user tab to the field
MyBase.TabStop = Not Value
' Setting TabStop to false causes the text in the box to be selected if it matches
' an entry in the list. Setting selection length to zero removes the selection.
MyBase.SelectionLength = 0
' Send the textbox portion of the combo the readonly message.
' It will change the color and behavior.
SendMessage(GetWindow(Me.Handle, GW_CHILD), EM_SETREADONLY, Value, 0)
' If text was typed or pasted into the textbox, the context menu will
' have the undo activated. When the text box is in the readonly state
' the undo will still be active from the right click context menu
' allowing the user to restore the previous value. This sendmessage
' will clear the undo buffer which will clear the undo.
SendMessage(GetWindow(Me.Handle, GW_CHILD), EM_EMPTYUNDOBUFFER, Value, 0)
' the dropdown may have been dropped before the readonly is set
_DroppedDown = False
Me.Refresh()
End Set
End Property
'
' Saving and returning a local copy of the selected index keeps a
' changed value from being returned when the control is in the
' readonly state. The OnSelectedIndexChanged event captures the
' index value that is returned here. Must be shadows or else the
' value passed won't cause the OnSelectedIndexChanged method to fire
' and the text value to be displayed, won't.
'
Public Shadows Property SelectedIndex() As Integer
Get
Return _SelectedIndex
End Get
Set(ByVal Value As Integer)
_SelectedIndex = Value
MyBase.SelectedIndex = Value
If Value = -1 Then ' Set it twice to work around databound bug KB327244
_SelectedIndex = Value
MyBase.SelectedIndex = Value
End If
End Set
End Property
'
' Intercepting message 273 when readonly and the listbox is dropped
' keeps the user from selecting an item in the list and having it update
' the text value of the combo as well as firing the associated changed events.
' Since we intercept a windows message we will have to manually bring up
' the listbox.
'
' msg 305 (0x131) = an item was clicked from the dropdown list
' msg 273 (0x111) = (WM_COMMAND) follows dropdown list click and all other actions?
' msg 8465 (0x2111) = (WM_REFLECT + WM_COMMAND) subsequent command after the 273
'
Protected Overrides Sub WndProc(ByRef m As System.Windows.Forms.Message)
' Cannot use me.DroppedDown, it causes a System.StackOverflowException.
' Asking for it's value must produce windows messages for the combobox
' and thus create a recursive loop.
If _ReadOnly AndAlso _DroppedDown Then
If m.Msg = 273 Then
_DroppedDown = False
' bring up the dropdown
SendMessage(Me.Handle, CB_SHOWDROPDOWN, System.Convert.ToInt32(False), 0)
Exit Sub
End If
End If
MyBase.WndProc(m)
End Sub
'
' This event will not fire when msg 273 is intercepted in WndProc. When in the
' readonly state clicking on an item in the listbox appears to have no effect. It
' does in fact change the value of MyBase.SelectedIndex. Saving the last good index
' value locally allows the overriden Index property to supply the proper index value.
'
Protected Overrides Sub OnSelectedIndexChanged(ByVal e As System.EventArgs)
_SelectedIndex = MyBase.SelectedIndex
MyBase.OnSelectedIndexChanged(e)
End Sub
'
' We must manually track dropped state. Asking the control if it's
' dropped from within WndProc will cause a System.StackOverflowException.
'
Protected Overrides Sub OnDropDown(ByVal e As System.EventArgs)
_DroppedDown = True
MyBase.OnDropDown(e)
End Sub
'
' The up and down arrow keys cause the combobox to change selection to the next
' or previous in the list. The page up and page down keys change the selection
' by one page at a time as defined by the size of the dropdown list. Setting
' e.Handled to true if any of these keys is pressed stops the selection change
' when readonly. The alt down arrow combination is allowed since it drops the listbox.
'
Protected Overrides Sub OnKeyDown(ByVal e As System.Windows.Forms.KeyEventArgs)
If _ReadOnly Then
If e.KeyCode = Keys.Up OrElse e.KeyCode = Keys.PageUp OrElse _
e.KeyCode = Keys.PageDown OrElse _
(e.KeyCode = Keys.Down And ((Control.ModifierKeys And Keys.Alt) <> Keys.Alt)) Then
e.Handled = True
End If
End If
MyBase.OnKeyDown(e)
End Sub
'
' The combobox default behavior when pressing F4 is to drop the listbox.
' If F4 is immediately pressed a second time the OnSelectionChangeCommitted
' event fires regardless of whether a change has been made or not. When
' readonly we don't want a change event to fire. This code will stop it.
'
Protected Overrides Sub OnSelectionChangeCommitted(ByVal e As System.EventArgs)
If _ReadOnly Then
Else
MyBase.OnSelectionChangeCommitted(e)
End If
End Sub
End Class
