Combobox - DrawItem and dragging scroll bar problem

David Hovey

Hi there. I need some help with drawing items for a ComboBox Control. For the most part the ComboBox draws just fine. I can scroll up and down and everything is good. The problem is when I drag the scrollbar "thumb" or box to get through the list faster. The items do not display correctly. The ComboBox is set to DropDownList and OwnerDrawVariable. I understand the code below is not very meaningful as I could use the windows implementation to draw the items. The program I'm working on displays certain items in bold, so the code below just illustrates the problem I have. Thanks for your help!!

Public Class Form1

Private Sub Button1\_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click

    Dim i As Integer

    For i = 1 To 200
        ComboBox1.Items.Add(i)
    Next

End Sub

Private Sub ComboBox1\_DrawItem(ByVal sender As Object, ByVal e As System.Windows.Forms.DrawItemEventArgs) Handles ComboBox1.DrawItem

    If e.Index = -1 Then
        Exit Sub
    End If

    e.DrawBackground()

    e.Graphics.DrawString(ComboBox1.Items(e.Index).ToString(), e.Font, New SolidBrush(Color.Black), e.Bounds.X, e.Bounds.Y)

End Sub

End Class

Luc Pattyn

Hi, not sure this will fix it, however it might as your current code is too expensive: for every item drawn you: - create a new SolidBrush - and you don't call Dispose on it, which you should as the class offers such method. It would be better to create a brush once, and keep it handy in a class member so you can reuse it for all items. And actually, you don't even need this as there is a class offering standard brushes, so all you need is Brushes.Black :)

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David Hovey

Thanks for the comment. You remind me to think about that more. New code is below, but issue still exists. I was thinking that perhaps I needed a custom class, that overrides the OnPaint method. This doesn't seem correct as the method would draw the part of the control that doesn't drop down. I searched for quite a while and it doesn't seem anyone else has had this problem.

Public Class Form1

Private Sub Button1\_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click

    Dim i As Integer

    For i = 1 To 200
        ComboBox1.Items.Add(i)
    Next

End Sub

Private Sub ComboBox1\_DrawItem(ByVal sender As Object, ByVal e As System.Windows.Forms.DrawItemEventArgs) Handles ComboBox1.DrawItem

    If e.Index = -1 Then
        Exit Sub
    End If

    e.DrawBackground()

    e.Graphics.DrawString(ComboBox1.Items(e.Index).ToString(), e.Font, Brushes.Black, e.Bounds.X, e.Bounds.Y)

End Sub

End Class

Luc Pattyn

Hi, I haven't done any ComboBox.DrawItem myself, however I use ListBopx.DrawItem a lot, and they are documented very similarly. Never had any problems. Is your CB double-buffered? is your Form? Mine normally are. You haven't told in any detail what is wrong with the result. here are some suggestions: - if the comboBox.Items are strings, no need to call ToString() - use the entire e.Bounds in DrawString, not just X and Y - if the visual result does not provide a clue, I would add logging code to figure for which items DrawItem gets called/doesn't get called assuming some of the items are not repainted, hence show previous content. BTW: the MSDN example on CB.DrawItem creates and doesn't dispose of its SolidBrush and Font! :)

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David Hovey

Ok. Well here is the info I have. I changed the code quite a bit. Created a custom combobox at runtime. Form and combobox are doublebuffered. It also creates a log file showing index, bounds, and time of DrawItem method being executed. To describe the problem. It would appear that the Background is not being drawn correctly (but this only describes the appearance-I don't think this is the cause). When I quickly scroll the entire list, to the end, the majority of the items appear as a "blob" of black as if all of the items were drawn over top of eachother. The odd thing is this is intermittent. Out of the 20 items shown in the drop down maybe 3 or 4 will be displayed correctly. The log file shows that when quickly dragging through a list, the items are only being drawn on either the last or second to the last visible item bounds. When dragging up the items are only drawn on either the top or second visible item bounds. Since this is the case, internally, either Windows or the .NET framework, would have to be storing the item graphics and then be moving the previous item graphics up or down. But if you click on the scroll bar to get the next or previous "page" all the items will be redrawn from top to bottom... I uploaded the log file to my website http://www.hoveytech.com/drawitemlog.txt Not sure where to go from here. Thanks for your quick answers and continued help!

Public Class Form1
Private m_fBold As Font
Private WithEvents m_cboCustom1 As CustomCombo

Private Sub m\_cboCustom1\_DrawItem(ByVal sender As Object, ByVal e As System.Windows.Forms.DrawItemEventArgs) Handles m\_cboCustom1.DrawItem

    If e.Index = -1 Then
        Exit Sub
    End If

    e.DrawBackground()

    If e.Index Mod 4 = 0 Then
        e.Graphics.DrawString(m\_cboCustom1.Items(e.Index).ToString(), m\_fBold, Brushes.Black, e.Bounds)
    Else
        e.Graphics.DrawString(m\_cboCustom1.Items(e.Index).ToString(), e.Font, Brushes.Black, e.Bounds)
    End If

    LogDrawItem(e.Index, e.State, e.Bounds)

End Sub

Private Sub Form1\_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load


    Me.DoubleBuffered = True

    m\_cboCustom1 = New CustomCombo
    m\_cboCustom1.Location = New Point(30, 30)
    m\_cboCustom1.DropDownStyle = ComboBoxStyle.DropDownList
    m\_cboCustom1.DrawMode = DrawMode.OwnerDrawVariable
    m\_cboCustom1.MaxDropDownIte

Luc Pattyn

Hi David, I noticed you have chosen OwnerDrawVariable, which means each item has its own height, and before DrawItem fires a MeasureItem event gets fired, asking you for the height of the item. Suggestion: - try OwnerDrawFixed; - or provide a MeasureItem handler. With this fixed it works fine for me (also remove the logging in DrawItem for good responsiveness). :)

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David Hovey

Thanks for your help! It works now!!