public virtual int ButtonWidth {get; set;}

// Returns a SideButton instance for gcComboFrame1.
private SideButtonBase CreateSideButton()
{
    SideButton sideButton = new SideButton();

    // Specify a unique name for this button.
    sideButton.Name = "SideButton";

    // Set button text.
    sideButton.Text = "Add a item";

    // Set button's width.
    sideButton.ButtonWidth = 70;

    // Set this side button not to show in GcComboFrame.
    sideButton.Visible = ButtonVisibility.NotShown;

    // Set this side button to position at outside of GcComboFrame's right border.
    sideButton.Position = ButtonPosition.RightOutside;

    // Set the fat appearance for this button, and these settings will take effect when 
    // its associated GcComboFrame's FlatStyle is Flat.
    sideButton.FlatAppearance.BorderColor = Color.Red;
    sideButton.FlatAppearance.BorderSize = 2;
    sideButton.FlatAppearance.CheckedBackColor = Color.Blue;
    sideButton.FlatAppearance.MouseDownBackColor = Color.Yellow;
    sideButton.FlatAppearance.MouseOverBackColor = Color.Green;

    sideButton.Click += new EventHandler(SideButton_Click);

    return sideButton;
}

// Add a item into listBox1 when SideButton is clicked.
private void SideButton_Click(object sender, EventArgs e)
{
    if (this.textBox1.Text != string.Empty)
    {
        this.listBox1.Items.Add(this.textBox1.Text);
        this.textBox1.Clear();
    }
    else
    {
        MessageBox.Show("You must input a string first !");
    }
}