C#

private void cbArticle_CheckBoxCheckedChanged(object sender, EventArgs e)
{
    //If minimum ONE GroupCode is selected
    if (cbArticle.Text != "Artikel auswählen")
    {
        cbArticle.ForeColor = SystemColors.WindowText;

        GetQueryType(gbTimespan.Controls.OfType<RadioButton>().FirstOrDefault(n => n.Checked));

        //Gets all selected Methods, necessary for Checking the "Old selected Methods" after the Methods ComboBox Itemlist has realoaded
        string[] arrMethods = cbMethod.Text.Split(',');
        //Clear and reload the Methods
        cbMethod.Items.Clear();
        FillMethods();

        //Checks the "Old selected Methods"
        cbMethod.CheckBoxCheckedChanged -= cbMethod_CheckBoxCheckedChanged;
        for (int i = 0; i < arrMethods.Length; i++)
        {
            try
            {
                cbMethod.CheckBoxItems[arrMethods[i].Trim()].Checked = true;
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);
            }
        }
        cbMethod.CheckBoxCheckedChanged += cbMethod_CheckBoxCheckedChanged;
    }
    //If NO GroupCode was selected --> startup initializing
    else
    {
        cbArticle.ForeColor = SystemColors.GrayText;
        FillMethods();

        if (finished)
        {
            GetQueryType(gbTimespan.Controls.OfType<RadioButton>().FirstOrDefault(n => n.Checked));
        }
    }
}

private void cbMethod_CheckBoxCheckedChanged(object sender, EventArgs e)
{
    //If minimum ONE Method was selected
    if (cbMethod.Text != "Methode(n) auswählen")
    {
        cbMethod.ForeColor = SystemColors.WindowText;

        GetQueryType(gbTimespan.Controls.OfType<RadioButton>().FirstOrDefault(n => n.Checked));

        //Gets all selected GroupCodes, necessary for Checking the "Old selected GroupCodes" after the Article ComboBox Itemlist has realoaded
        string[] arrArticles = cbArticle.Text.Split(',');
        ////Clear and reload the GroupCodes
        cbArticle.Items.Clear();
        FillGroupCodes();

        //Checks the "Old selected GroupCodes"
        cbArticle.CheckBoxCheckedChanged -= cbArticle_CheckBoxCheckedChanged;
        for (int i = 0; i < arrArticles.Length; i++)
        {
            try
            {
                cbArticle.CheckBoxItems[arrArticles[i].Trim()].Checked = true;
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);
            }
        }
        cbArticle.CheckBoxCheckedChanged += cbArticle_CheckBoxCheckedChanged;
    }
    else
    {
        cbMethod.ForeColor = SystemColors.GrayText;
        FillGroupCodes();

        if (finished)
        {
            GetQueryType(gbTimespan.Controls.OfType<RadioButton>().FirstOrDefault(n => n.Checked));
        }
    }
}

public partial class CheckBoxComboBox : PopupComboBox
{
    #region EVENTS & EVENT HANDLERS

    public event EventHandler CheckBoxCheckedChanged;

    private void Items_CheckBoxCheckedChanged(object sender, EventArgs e)
    {
        OnCheckBoxCheckedChanged(sender, e);
    }

    #endregion

    #region EVENT CALLERS and OVERRIDES e.g. OnResize()

    protected void OnCheckBoxCheckedChanged(object sender, EventArgs e)
    {
        string ListText = GetCSVText(true);
        // The DropDownList style seems to require that the text
        // part of the "textbox" should match a single item.
        if (DropDownStyle != ComboBoxStyle.DropDownList)
            Text = ListText;
        // This refreshes the Text of the first item (which is not visible)
        else if (DataSource == null)
        {
            Items[0] = ListText;
            // Keep the hidden item and first checkbox item in
            // sync in order to ensure the Synchronise process
            // can match the items.
            CheckBoxItems[0].ComboBoxItem = ListText;
        }

        EventHandler handler = CheckBoxCheckedChanged;
        if (handler != null)
            handler(sender, e);
    }
...
}

/// <summary>
/// The CheckBox items displayed in the Popup of the ComboBox.
/// </summary>
[ToolboxItem(false)]
public class CheckBoxComboBoxItem : CheckBox
{
    #region PROTECTED MEMBERS

    protected override void OnCheckedChanged(EventArgs e)
    {
        // Found that when this event is raised, the bool value of the binded item is not yet updated.
        if (_CheckBoxComboBox.DataSource != null)
        {
            PropertyInfo PI = ComboBoxItem.GetType().GetProperty(_CheckBoxComboBox.ValueMember);
            PI.SetValue(ComboBoxItem, Checked, null);
        }
        base.OnCheckedChanged(e);
        // Forces a refresh of the Text displayed in the main TextBox of the ComboBox,
        // since that Text will most probably represent a concatenation of selected values.
        // Also see DisplayMemberSingleItem on the CheckBoxComboBox for more information.
        if (_CheckBoxComboBox.DataSource != null)
        {
            string OldDisplayMember = _CheckBoxComboBox.DisplayMember;
            _CheckBoxComboBox.DisplayMember = null;
            _CheckBoxComboBox.DisplayMember = OldDisplayMember;
        }
    }

    #endregion

public class User : DBObject<User>
{

}

DBObject<User>