Use #region and #endregion to define a collapsible code region.

#region Event Handlers

public void Button_Click(object s, EventArgs e)
{
    // ...
}

public void DropDown_SelectedIndexChanged(object s, EventArgs e)
{
    // ...
}

#endregion

These directives are only beneficial when an IDE that supports collapsible regions (such as Visual Studio) is used to edit the code.