Explanation needed for strange behaviour of combobox

Question

I was just writing a windows application that populates three comboboxes from the same datasource. My datasource is a datatable.

The way i populate the comboboxes is by repeating the following code for each of the comboboxes:

'populate 1st combobox
cbx1.DataSource = table
cbx1.DisplayMember = "someColumn"
cbx1.ValueMember = "anotherColumn"
cbx1.SelectedIndex = Indx

'populate 2nd combobox
cbx2.DataSource = table
cbx2.DisplayMember = "someColumn"
cbx2.ValueMember = "anotherColumn"
cbx2.SelectedIndex = Indx

'populate 3rd combobox
cbx3.DataSource = table
cbx3.DisplayMember = "someColumn"
cbx3.ValueMember = "anotherColumn"
cbx3.SelectedIndex = Indx

When the application is run, and I select an item from the dropdown list of, say, cbx1, my choice is reflected in cbx2 and cbx3 as well. I find this behaviour strange and will be grateful if anybody could explain what is going on here behind the scenes.

On a side note, I've been able to circumvent this problem by modifying my code as shown below, but would still like to have an explanation for this seemingly strange behaviour.

'populate 1st combobox
Dim t1 as datatable = table.Copy
cbx1.DataSource = t1
cbx1.DisplayMember = "someColumn"
cbx1.ValueMember = "anotherColumn"
cbx1.SelectedIndex = Indx

'populate 2nd combobox
Dim t2 as datatable = table.Copy
cbx2.DataSource = t2
cbx2.DisplayMember = "someColumn"
cbx2.ValueMember = "anotherColumn"
cbx2.SelectedIndex = Indx

'populate 3rd combobox
Dim t3 as datatable = table.Copy
cbx3.DataSource = t3
cbx3.DisplayMember = "someColumn"
cbx3.ValueMember = "anotherColumn"
cbx3.SelectedIndex = Indx

@Jose: Yes, it is a windows application – Tracer Jan 21 '11 at 13:18 — Tracer, Jan 21 '11 at 13:18
The table instance is still the same – Novice Jan 21 '11 at 13:21 — Novice, Jan 21 '11 at 13:21

score 1 · Accepted Answer · edited May 23 '17 at 11:47

The behaviour is not so strange - you have three combo boxes all bound to the same data source, so when you select a value in the first combo box, you are changing the index of the current record in the underlying data source - since the other two combo boxes are bound to this, they will update also.

Edit: Behind the scenes, the reason for the behaviour is to do with how data binding is implemented in the .Net framework - see this question for a more detailed explanation.

As you have discovered, the solution is to use separate data sources for each combo box. There is a related question here which you may find interesting.

Thanks for your explanation and the links, the behaviour is now clear to me — Tracer, Jan 21 '11 at 13:56

score 0 · Answer 2 · answered Jan 21 '11 at 13:19

0

It is because you have assigned the same instance of the datatable to the comboboxes.

answered Jan 21 '11 at 13:19

Novice

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Yes, I suspected as much, that's why I was able to circumvent it the way I did. However, I'm more interested in what happens behind the scenes for it to exhibit such behaviour. I thought that selecting an item in the combobox shouldn't affect its datasource. – Tracer Jan 21 '11 at 13:21

Explanation needed for strange behaviour of combobox

2 Answers2