empty record in combo box

[MTmace]

I need to add an empty record at the begining of a databound combo box.

MTmace

[kleinma]

is it just to have the combobox be blank when the user gets to it?

[jmcilhinney]

A data-bound control displays whatever its DataSource contains. If there is no blank entry in your DataSource then there is no blank entry in your ComboBox, period. This sounds like a hack anyway. As klienma intimated, I'm guessing that you don't actually want yuor user to select a blank entry, but rather you want to have the ability to selct no entry at all. To do this you need to set the SelectedIndex to -1. There is a bug that requires you to do this twice when the ComboBox is data-bound. If you want the user to be able to clear their selection, what I always do is to handle the KeyDown event for the ComboBox and test for the Delete key. When the user depresses the Delete key I set the SelectedIndex to -1 (twice).

[MTmace]

I tried this and it did not work.

With Me.cboBox
.Items.Clear()
.DataSource = dvView
.DisplayMember = "field1"
.ValueMember = "field1"
.SelectedIndex = -1
.SelectedIndex = -1
End With

Is this what you were talking about?

MTmace

[jmcilhinney]

It works for me. In fact, despite the fact that I've read that you have to set the SelectedIndex twice when your ComboBox is data-bound, the following code produced an empty ComoBox for me:

Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        Dim dt As New DataTable
 
        dt.Columns.Add("Col1", GetType(Integer))
        dt.Columns.Add("Col2", GetType(String))
 
        For i As Integer = 1 To 10
            Dim dr As DataRow = dt.NewRow()
 
            dr(0) = i
            dr(1) = "Row " & i.ToString()
            dt.Rows.Add(dr)
        Next
 
        Dim dv As New DataView(dt)
 
        Me.ComboBox1.DataSource = dv
        Me.ComboBox1.DisplayMember = "Col2"
        Me.ComboBox1.ValueMember = "Col1"
        Me.ComboBox1.SelectedIndex = -1
    End Sub

By the way, your calling Items.Clear is redundant because setting the DataSorce excludes any other items anyway. Also, it may be unnecessary to us a DataView as well, which I assume dvView is. All DataTables are associated with a DataView already through their DefaultView property. You would only have to specifically create a DataView if you wanted to be able to filter or sort it without affecting the DefaultView. Also, when you bind a DataTable to a control it is actually the contents of the DefaultView that is displayed. This means that if you filter or sort the DefaultView, the changes will be reflected in the control. All this means that you would usually just bind the DataTable itself to your control, except in very specific circumstances.

[MTmace]

That is interesting. I copy and pasted your code into my form and the default value was Row1.

I am using a dataview because I use the same datatable through out my form in other combo boxes. If I do not use dataviews then when I make a change to one combo box all the other values will change to what ever was last selected.

MTmace

[jmcilhinney]

That is interesting. I copy and pasted your code into my form and the default value was Row1.

I am using a dataview because I use the same datatable through out my form in other combo boxes. If I do not use dataviews then when I make a change to one combo box all the other values will change to what ever was last selected.

MTmace

That sounds like legitimate use of DataViews. I just thought I'd mention it because I've seen many people use them unnecessarily. As for why the code I used works for me and not you, . I tested it in VS.NET 2003 on .NET 1.1 SP1.

[MTmace]

I am still having problems with this issue. I tried this with a new project without a problem. But still will not work with my main project.


Weird

More info:
The combobox that I am having trouble with is in a tab control. If I move the combo box off of the tab control the above code does work.
Why would the location of the combo box matter and how can I fix this problem?
It is not that it is in a tab control that causes the problem. The combo box can not be in on the first tab.

MTmace

[MTmace]

This codes works:

Me.TabControl1.SelectedTab = TabPage2

Dim dt As New DataTable

dt.Columns.Add("Col1", GetType(Integer))
dt.Columns.Add("Col2", GetType(String))

For i As Integer = 1 To 10
Dim dr As DataRow = dt.NewRow()

dr(0) = i
dr(1) = "Row " & i.ToString()
dt.Rows.Add(dr)
Next

Dim dv As New DataView(dt)

Me.ComboBox1.DataSource = dv
Me.ComboBox1.DisplayMember = "Col2"
Me.ComboBox1.ValueMember = "Col1"
Me.ComboBox1.SelectedIndex = -1



I guess you need to have the page selected before you can access or modify certain methods.
Another problem arises when I try to add

Me.TabControl1.SelectedTab = TabPage1

at the end of the code to have the initial page show.

Is there a better way to handle this problem?

MTmace