Combobox SelectedItem does not work

Question

I have the code below

FooCB.DisplayMember = "FooNome";
FooCB.ValueMember = "Foo";
FooCB.DataSource = FooRepository.Instance.All();
FooCB.DataBindings.Add("SelectedItem", Bar, "Foo");

but when I display the form the SelectedItem is always the first. What am I doing wrong?

Check whenever datasource is updated after you have form shown. — Sinatr, Mar 13 '13 at 12:44
@JensKloster If I am not mistake, in line `FooCB.DataBindings.Add("SelectedItem", Bar, "Foo");` I am setting the selected item. `Bar` is Form property. Do I need to initialize it previously? I am using WinForms — Cristhian Boujon, Mar 13 '13 at 12:55
@JensKloster no, but I tried and the issue keep it. See my edit. — Cristhian Boujon, Mar 13 '13 at 13:11

score 1 · Answer 1 · answered Mar 29 '13 at 00:18

I have been struggling a little with the behaviour of Winforms comboboxes and databinding recently and these are my observations (.Net4) when binding the ComboBox.DataSource to a list of items and also binding an object property to ComboBox.SelectedItem.

When binding a list of objects (in your case List<Foo>) to ComboBox.DataSource, the first object in the list is always shown in the combobox.
If you bind an object property to ComboBox.SelectedItem (in your case Bar.Foo) and that object property matches one of the combobox list objects then that object is displayed in the combobox. If the object property is null (Bar.Foo == null) or the object property is not in the combobox list then the first object is shown in the combobox.
Setting ComboBox.SelectedItem = null or ComboBox.SelectedIndex = -1 clears the displayed item on the combobox even though this seems to warn against it. And will set your bound object property to null.
If a user clears the combobox selection when using ComboBox.DropDownStyle == DropDown (with backspace) then the bound object property is set to null.
If you have implemented INotifyPropertyChanged on the object whose property is bound to Combobox.SelectedItem (Bar.Foo) and you programatically set the bound property to a value and that value appears in the combobox list then the changed value will be displayed. If you set the property to null or a value not in the list then the combobox displayed value will not change.

So what can you do about it? The only real issue I have is having no value displayed when my bound property is null so I have just been explicitly setting Combobox.SelectedItem = null as in point #3. You may be able to extend ComboBox and override the default behaviour but so far I have been content with an extra line of code here and there combined with using default values on non-nullable properties.

score 0 · Answer 2 · answered Feb 26 '16 at 00:33

Probably you are missing some decleration. If you created the Combobox from the Tool Box, -I had the similar problem- you might want to add name of the Combobox's Name as a tag on XAML.

Other than that, if you created it dynamically by code, check if you are missing any decleration for class.

B H · Answer 3 · 2018-07-29T01:28:14.223

I can't tell from the OP's code whether I'm answering their question, but maybe this will help somebody reading this question. The ComboBox has four ways to set the current value:

SelectedIndex
SelectedItem
SelectedText
SelectedValue

You need to be consistent about what you're setting (and about which event handler you're using). You'll get an error if you set SelectedIndex to something dumb (less than -1 or longer than the list). However, you don't get errors setting the other three to something that doesn't exist for that type of selection.

Suppose you use a Dictionary (that's pseudo code) as a binding source and set DisplayMember = "Value" and ValueMember = "Key", the mapping would look like:

SelectedIndex - -1 to index of last item
SelectedItem - KeyValuePair<Key, Value>
SelectedText - Dictionary value
SelectedValue - Dictionary key

Supplying either value or key to SelectedItem will not generate an error, it will simply act like the OP has described. And that's why I thought this answer might help somebody.

I might also note that, if you're swapping out the contents of the ComboBox, it's not always safe to use SelectedIndex. Suppose the same basic kind of data is in the ComboBox, but selections are limited in some cases compared to others. Using SelectedIndex to persist a previous selection that was still valid in the new list of options is only going work if that previous selection held the exact same place in the list. You'd almost think this was the voice of very, very recent experience speaking...

Combobox SelectedItem does not work

3 Answers3