What is the best way to manage a parent/child relationship between two comboboxes on a form? Both comboboxes have collections as their datasources. When a selection is made in the parent combobox, I would like the child combobox item list to be limited to those items related to the item selected in the parent.

I have tried the following, but the child combobox is never populated:

        private void cboManufacturer_SelectedIndexChanged(object sender, EventArgs e)
        {
            if (cboManufacturer.SelectedIndex != -1)
            {
                vwAircraftModelGroupsCollection.Query.Where
                    (
                        vwAircraftModelGroupsCollection.Query.MftShortMake.Equal(cboManufacturer.SelectedValue)
                    );
                vwAircraftModelGroupsCollection.Query.Load();
                cboAircraftGroup.Refresh();
            }
        }

The vwAircraftModelGroupsCollection (child) isn't loaded in this approach until an item is selected in the parent combobox (cboManufacturer). Is there a better way to do this?

The relation context for this sample snippet is Division -> Department. So Division is the parent and Department is the child.

/// <summary>
/// Binds the divisions.
/// </summary>
private void BindDivisions()
{
	try
	{
		DivisionsCollection colDivisions = new DivisionsCollection();
		colDivisions.Query.OrderBy(colDivisions.Query.Name.Ascending);

		if (colDivisions.Query.Load())
		{
			this.cmbDivision.DataSource = null;
			this.cmbDivision.DataSource = colDivisions;
			this.cmbDivision.DisplayMember = DivisionsMetadata.PropertyNames.DisplayName;
			this.cmbDivision.ValueMember = DivisionsMetadata.PropertyNames.DivisionId;
		}
	}

	catch (Exception exc)
	{
		frmErrorDialog FriendlyError = new frmErrorDialog(exc);
		FriendlyError.Show();
	}
}

/// <summary>
/// Handles the SelectedIndexChanged event of the cmbDivision control.
/// </summary>
/// <param name="sender">The source of the event.</param>
/// <param name="e">The <see cref="T:System.EventArgs"/> instance containing the event data.</param>
private void cmbDivision_SelectedIndexChanged(object sender, EventArgs e)
{
	try
	{
		if (this.cmbDivision.SelectedValue.ToString() != "BusinessObjects.Divisions")
		{
			BindDepartment(Convert.ToInt32(this.cmbDivision.SelectedValue));
		}
	}

	catch (Exception exc)
	{
		frmErrorDialog FriendlyError = new frmErrorDialog(exc);
		FriendlyError.Show();
	}
}

/// <summary>
/// Binds the department.
/// </summary>
/// <param name="divisionId">The division id.</param>
private void BindDepartment(int divisionId)
{
	try
	{
		DepartmentsCollection colDepartments = new DepartmentsCollection();
		colDepartments.Query.Where
		(
			colDepartments.Query.DivisionId.Equal(divisionId)
		)
		.OrderBy(colDepartments.Query.Name.Ascending);

		if (colDepartments.Query.Load())
		{
			this.cmbDepartment.DataSource = null;
			this.cmbDepartment.DataSource = colDepartments;
			this.cmbDepartment.DisplayMember = DepartmentsMetadata.PropertyNames.DisplayName;
			this.cmbDepartment.ValueMember = DepartmentsMetadata.PropertyNames.DepartmentId;
		}
	}

	catch (Exception exc)
	{
		frmErrorDialog FriendlyError = new frmErrorDialog(exc);
		FriendlyError.Show();
	}
}

The key is that the ComboBox items do not change until the DataSource changes. Calling Refresh only redraws the screen. Once you have loaded a new collection, you need to set the ComboBox DataSource to it.

cboAircraftGroup.DataSource = vwAircraftModelGroupsCollection;

Thanks, guys. You've answered both my questions: 1) it appears I was on the right track from a "best approach" standpoint, and 2) I now understand the problems I was having with the combobox control.