ComboBox items.count doesn't match DataSource

Question

I have a ComboBox that is bound to a DataSource. I want to dynamically add items to the ComboBox based on certain conditions. So what I've done is add the options to a new list, and then change the DataSource of the ComboBox like so:

cbo.DataSource = null;
cbo.DataSource = cbos;
cbo.DisplayMember = "Title";
cbo.ValueMember = "Value";

Then, I check cbo.Items.Count, and it has not incremented - it does not equal the count of the DataSource. Any ideas what I can do here?

Note this is WinForms and not ASP.NET.

Solution 1

If anyone experiences this problem on a dynamically added combobox, the answer is to ensure that you add the combobox to the controls of a container in the form.

By adding "this.Controls.Add(cbo);" to the code before setting the datasource, the problem goes away.

Solution 2

Did you check the Count immediately or at a later time? There is the possibility that the ComboBox does not actually update it's contents until there is an operation such as a UI refresh and hence the count will be off until that time.

On case where this may happen is if you update the DataSource before the Handle is created for the ComboBox. I dug through the code a bit on reflector and it appears the items will not be updated in this case until the ComboBox is actually created and rendered.

Solution 3

I've found the cause...

I took out the cbo.Datasource = null line.. and added a cbo.Invalidate() at the end. This has solved the problem.

Thanks all for the advice.

Solution 4

cbo.DataSource = null; 
cbo.DataSource = cbos; 
cbo.DisplayMember = "Title"; 
cbo.ValueMember = "Value"; 

Now before setting cbo.SelectedValue, or relying on Items to be up-to-date, call

cbo.CreateControl ;

and Items will be re-calculated.

The problem is that SelectedValue/SelectedIndex, which are WinForms properties, only accept the values that are legal according to the Items list, but that one is built only after GUI interaction, i.e. after instantiating a "real" Windows GUI combo box, i.e. after obtaining a Windows handle for the combobox.

CreateControl forces the creation of the Windows handle, no matter what.

Solution 5

 ComboBox cbNew = new ComboBox();
    cbNew.Name = "cbLine" + (i+1);
    cbNew.Size = cbLine1.Size;
    cbNew.Location = new Point(cbLine1.Location.X, cbLine1.Location.Y + 26*i);
    cbNew.Enabled = false;
    cbNew.DropDownStyle = ComboBoxStyle.DropDownList;
    cbNew.DataSource = DBLayer.GetTeams(lineName).Tables[0];
    cbNew.DisplayMember = "teamdesc";
    cbNew.ValueMember = "id";
    Console.WriteLine("ComboBox {0}, itemcount={1}", cbNew.Name, cbNew.Items.Count);
        // The output displays itemcount = 0 for run-time created controls
        // and >0 for controls created at design-time
    gbLines.Controls.Add(cbNew);

TO

 ComboBox cbNew = new ComboBox();
    cbNew.Name = "cbLine" + (i+1);
    cbNew.Size = cbLine1.Size;
    cbNew.Location = new Point(cbLine1.Location.X, cbLine1.Location.Y + 26*i);
    cbNew.Enabled = false;
    cbNew.DropDownStyle = ComboBoxStyle.DropDownList;
    Console.WriteLine("ComboBox {0}, itemcount={1}", cbNew.Name, cbNew.Items.Count);
        // The output displays itemcount = 0 for run-time created controls
        // and >0 for controls created at design-time
    gbLines.Controls.Add(cbNew);
    cbNew.DataSource = DBLayer.GetTeams(lineName).Tables[0];
    cbNew.DisplayMember = "teamdesc";
    cbNew.ValueMember = "id";

The DataSource, DisplayMember and ValueMember property must be set after the control has been added to its container.

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Comments

[Madeleine]

I'm checking the Items.Count property immediately after in code, because I need to then do some more logic at that point.

[coder14]

Wouldn't it be simpler to check the number of items in your data source?

[Madeleine]

The problem is that I have to set the selected item, usually to the new one, and that index doesnt exist.

[Jose Basilio]

Found your own solution. Go ahead and accept your own answer. +1

[Madeleine]

Came across this issue again somewhere else in code. The issue is that the combo box is being added at run time and therefore as Jared mentioned, no handle has been created for the combo box. By adding "this.Controls.Add(cbo);" the problem disappears.

[Null Head]

You should have rather added this line before setting the datasource. cbo.Items.clear()

[TheBlastOne]

-1, untrue. It happens also if the ComboBox already is contained in a container that does not have a Windows handle yet.

[Madeleine]

I don't think that a -1 is fair here, just because you have found another reason that this occurs. This answer solved the problem in the stated example.

[TheBlastOne]

Unable to undo unless answer is edited. I keep wondering what kind of edititing rules are behind so's frontend...

[Brad]

CreateControl did not fix the problem for me. Adding the control to the collection did.