Weird Autocomplete Combobox Error

Okay, this is a really weird error in my windows form.

I have included my test vb class which is a test app. Load it up into Visual Studio.Net and run the app. What is does is populate a combo box with some test data and then it implements some autocomplete code so that as the user types is autocompletes the selection.

Now here's the weird scenario that breaks the form:

- User clicks combo arrow to drop down list of items
- User types the letter 'M'
- Combo Selects 'Martin' option
- User hits enter key
- Combo Selection is still 'Martin'

- User clicks combo arrow to drop down list of items
- User types the letter 'A'
- Combo Selects 'Alan' options
- User hits enter key
- Combo Selection is still 'Alan'

- User clicks submit button
- Combo Selection says that it is 'Martin' What's up with that

Any help that you can offer would be greatly appreciated.

Thanks,

Martin

Imports test
Public Class test
Inherits System.Windows.Forms.Form

#Region " Windows Form Designer generated code "

Public Sub New()
MyBase.New()

'This call is required by the Windows Form Designer.
InitializeComponent()

        'Add any initialization after the InitializeComponent() call
        Dim dtOptions As New DataTable("Options")
        dtOptions.Columns.Add("ID", GetType(String))
        dtOptions.Columns.Add("Text", GetType(String))

        Dim drOption As DataRow
        Dim aText() As String = {"", "Martin", "Scott", "Alan"}
        Dim aIDs() As String = {"", "MartinB", "ScottS", "AlanK"}

Dim i As Integer = 0

        For i = 0 To aIDs.GetLength(0) - 1
            drOption = dtOptions.NewRow()
            drOption(0) = aIDs(i)
            drOption(1) = aText(i)
            dtOptions.Rows.Add(drOption)
        Next

        cmbTest.DataSource = dtOptions
        cmbTest.ValueMember = "ID"
        cmbTest.DisplayMember = "Text"
    End Sub

    'Form overrides dispose to clean up the component list.
    Protected Overloads Overrides Sub Dispose(ByVal disposing As Boolean)
        If disposing Then
            If Not (components Is Nothing) Then
                components.Dispose()
            End If
        End If
        MyBase.Dispose(disposing)
    End Sub

'Required by the Windows Form Designer
Private components As System.ComponentModel.IContainer

    'NOTE: The following procedure is required by the Windows Form Designer
    'It can be modified using the Windows Form Designer.
    'Do not modify it using the code editor.
    Friend WithEvents btnSubmit As System.Windows.Forms.Button
    Friend WithEvents cmbTest As System.Windows.Forms.ComboBox

    <System.Diagnostics.DebuggerStepThrough()> Private Sub InitializeComponent()
        Me.btnSubmit = New System.Windows.Forms.Button
        Me.cmbTest = New System.Windows.Forms.ComboBox
        Me.SuspendLayout()
        '
        'btnSubmit
        '
        Me.btnSubmit.Location = New System.Drawing.Point(8, 40)
        Me.btnSubmit.Name = "btnSubmit"
        Me.btnSubmit.TabIndex = 6
        Me.btnSubmit.Text = "Submit"
        '
        'cmbTest
        '
        Me.cmbTest.Location = New System.Drawing.Point(8, 8)
        Me.cmbTest.Name = "cmbTest"
        Me.cmbTest.Size = New System.Drawing.Size(121, 21)
        Me.cmbTest.TabIndex = 9
        '
        'test
        '
        Me.AutoScaleBaseSize = New System.Drawing.Size(5, 13)
        Me.ClientSize = New System.Drawing.Size(144, 70)
        Me.Controls.Add(Me.cmbTest)
        Me.Controls.Add(Me.btnSubmit)
        Me.Name = "test"
        Me.Text = "Form1"
        Me.ResumeLayout(False)

End Sub

#End Region

    Private Sub btnSubmit_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnSubmit.Click
        MessageBox.Show("Selection on Submit: " & cmbTest.SelectedValue)
    End Sub

    Private Sub cmbTest_Leave(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles cmbTest.Leave
        Trace.WriteLine("**Leave")
        Trace.WriteLine("Selection after leave routine: " & Me.cmbTest.SelectedValue)
    End Sub

    Private Sub cmbTest_KeyPress(ByVal sender As System.Object, ByVal e As System.Windows.Forms.KeyPressEventArgs) Handles cmbTest.KeyPress
        Trace.WriteLine("**Key Press - " & e.KeyChar.ToString())
        Trace.WriteLine("Current Selection: " & cmbTest.SelectedValue)

        If Char.IsControl(e.KeyChar) Then
            Trace.WriteLine("Control Key Pressed")
            Return
        End If

        With Me.cmbTest
            Dim ToFind As String = .Text.Substring(0, .SelectionStart) & e.KeyChar
            Dim Index As Integer = .FindStringExact(ToFind)
            If Index = -1 Then Index = .FindString(ToFind)
            If Index = -1 Then Return
            .SelectedIndex = Index
            .Select(ToFind.Length, .Text.Length - .SelectionStart)
            e.Handled = True
        End With

        Trace.WriteLine("New Selection: " & cmbTest.SelectedValue)
        Trace.WriteLine("")
    End Sub
End Class

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Weird Autocomplete Combobox Error

Ast123

If you're using VS 2005, you can use the new AutoCompleteMode to enable system AutoComplete. I'm assuming you're not...

The Windows Forms ComboBox is a wrapper on the Windows OS provided COMBOBOX control (see http://msdn.microsoft.com/library/default.asp url=/library/en-us/shellcc/platform/commctls/comboboxes/aboutcomboboxes.asp). You've turned up a subtle issue with the way the system ComboBox handles the interaction between it's list and edit controls (it doesn't appear to expect the index to be changed while the drop down is popped up). The only work-around I've found is to sync the SelectedIndex when the ComboBox DropDown is popped up. In VS 2005, we've added an event for this but in VS 2003, you'll need to sub-class the ComboBox to get this behavior. The following sample shows how to sub-class the ComboBox to add a DropDownCloseUp event. You can then use this event to sync the SelectedIndex (shown further below).

Public Class ACComboBox
Inherits System.Windows.Forms.ComboBox

Private Const WM_REFLECT_COMMAND As Integer = &H2000 + &H111
Private Const CBN_CLOSEUP As Integer = &H8

Public Event DropDownCloseUp As EventHandler

    Private Function HiWord(ByVal val As Integer) As Integer
        Return (val >> 16) And &HFFFF
    End Function

    Protected Overrides Sub WndProc(ByRef m As System.Windows.Forms.Message)
        If (m.Msg = WM_REFLECT_COMMAND) And (HiWord(m.WParam) = CBN_CLOSEUP) Then
            RaiseEvent DropDownCloseUp(Me, EventArgs.Empty)
        End If

MyBase.WndProc(m)
End Sub
End Class

You're code when need to be modified as follows:

        If Char.IsControl(e.KeyChar) Then
            Trace.WriteLine("Control Key Pressed")
            Return
        End If

        Trace.WriteLine("New Selection: " & cmbTest.SelectedValue)
        Trace.WriteLine("")
    End Sub

Private newIndex As Integer

    Private Sub cmbTest_DropDownCloseUp(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles cmbTest.DropDownCloseUp
        If (newIndex <> cmbTest.SelectedIndex) Then
            cmbTest.SelectedIndex = newIndex
        End If
    End Sub

Joe Stegman
The Windows Forms Team
Microsoft Corp.

This posting is provided "AS IS" with no warranties, and confers no rights.

Weird Autocomplete Combobox Error

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