Windows Form Combobox.Items.Clear() leaves empty slots - winforms

Question

I am working on a Windows Form that has multiple comboboxes. Depending on what is chosen in the first combobox determines what items are filled into the second combobox. The issue I am running into is if I choose ChoiceA in ComboBox1, ComboBox2 is clear()ed, then is filled with ChoiceX, ChoiceY, and ChoiceZ. I then choose ChoiceB in ComboBox1, ComboBox2 is clear()ed, but there are no choices to add to ComboBox2, so it should remain empty. The issue is, after choosing ChoiceB, there's a big white box with three empty slots in ComboBox2. So, basically, however many items are cleared N, that's how many empty slots show up after choosing ChoiceB.
This might be a tad confusing, I hope I explained it well enough.
-- EDIT Adding Code, hope it helps clear things up. BTW, mainItemInfo is another "viewmodel" type class. It interfaces back into the form to make updates.
private void cmbType_SelectedIndexChanged(object sender, EventArgs e)
{
DropDownItem item = (DropDownItem)cmbType.SelectedItem;
if (!String.IsNullOrWhiteSpace(item.Text))
{
cmbBrand.Enabled = true;
btnAddBrand.Enabled = true;
mainItemInfo.FillBrands(new Dictionary<string, string> { { "Type", item.Text } });
mainItemInfo.SyncBrands(this);
}
}
public void FillBrands(Dictionary<string, string> columnsWhere)
{
// Clear list
Brands.Clear();
// Get data
StorageData storage = new StorageData(File.ReadAllLines(ItemsFilePath));
// Fill Brands
foreach (string type in storage.GetDistinctWhere(columnsWhere, "Brand"))
{
Brands.Add(type, new DropDownItem(type, type));
}
}
public void SyncBrands(IPopupItemInfo form)
{
form.ClearcmbBrand();
var brands = from brand in Brands.Keys
orderby Brands[brand].Text ascending
select brand;
foreach (var brand in brands)
{
form.AddTocmbBrand(Brands[brand]);
}
}
public void AddTocmbBrand(DropDownItem brand)
{
cmbBrand.Items.Add(brand);
}
public void ClearcmbBrand()
{
cmbBrand.Items.Clear();
}

Answer 1

Simply, you can add an item then clear the combobox again:
cmbBrand.Items.Clear();
cmbBrand.Items.Add(DBNull.Value);
cmbBrand.Items.Clear();

Answer 2

You should able to set the datasource of listbox2 to null to clear it, then set it again with the new data.
So, in pseudo-code, something like:
ItemSelectedInListBox1()
{
List futureListbox2Items = LoadOptionsBaseOnSelectedItem(item)
Listbox2.Datasource = null
Listbox2.Datasource = futureListBox2Items
}
That should refresh the list of items displayed in Listbox2 with no white spaces.

Answer 3

I was able to fix the extra space. I changed the Add and Clear methods to:
public void AddTocmbModel(DropDownItem model)
{
cmbModel.Items.Add(model);
cmbModel.DropDownHeight = cmbModel.ItemHeight * (cmbModel.Items.Count + 1);
}
public void ClearcmbModel()
{
cmbModel.Items.Clear();
cmbModel.DropDownHeight = cmbModel.ItemHeight;
}