User Interface.
Inheritance: System.Windows.Forms.Form
        ElementSet m_elements;                   // correspond to elements parameter in Execute(...)

        /// <summary>
        /// Implement this method as an external command for Revit.
        /// </summary>
        /// <param name="revit">An object that is passed to the external application
        /// which contains data related to the command,
        /// such as the application object and active view.</param>
        /// <param name="message">A message that can be set by the external application
        /// which will be displayed if a failure or cancellation is returned by
        /// the external command.</param>
        /// <param name="elements">A set of elements to which the external application
        /// can add elements that are to be highlighted in case of failure or cancellation.</param>
        /// <returns>Return the status of the external command.
        /// A result of Succeeded means that the API external method functioned as expected.
        /// Cancelled can be used to signify that the user cancelled the external operation
        /// at some point. Failure should be returned if the application is unable to proceed with
        /// the operation.</returns>
        public Autodesk.Revit.UI.Result Execute(Autodesk.Revit.UI.ExternalCommandData revit,
                                                ref string message,
                                                ElementSet elements)
        {
            // Set currently executable application to private variable m_revit
            m_revit    = revit.Application;
            m_elements = elements;

            Transaction tran = new Transaction(m_revit.ActiveUIDocument.Document, "BeamAndSlabNewParameter");

            tran.Start();

            // Show UI
            using (BeamAndSlabParametersForm displayForm = new BeamAndSlabParametersForm(this))
            {
                displayForm.ShowDialog();
            }

            tran.Commit();
            return(Autodesk.Revit.UI.Result.Succeeded);
        }
        Autodesk.Revit.UI.UIApplication m_revit; // application of Revit

        #endregion Fields

        #region Methods

        /// <summary>
        /// Implement this method as an external command for Revit.
        /// </summary>
        /// <param name="revit">An object that is passed to the external application 
        /// which contains data related to the command, 
        /// such as the application object and active view.</param>
        /// <param name="message">A message that can be set by the external application 
        /// which will be displayed if a failure or cancellation is returned by 
        /// the external command.</param>
        /// <param name="elements">A set of elements to which the external application 
        /// can add elements that are to be highlighted in case of failure or cancellation.</param>
        /// <returns>Return the status of the external command. 
        /// A result of Succeeded means that the API external method functioned as expected. 
        /// Cancelled can be used to signify that the user cancelled the external operation 
        /// at some point. Failure should be returned if the application is unable to proceed with 
        /// the operation.</returns>
        public Autodesk.Revit.UI.Result Execute(Autodesk.Revit.UI.ExternalCommandData revit, 
                                               ref string message, 
                                               ElementSet elements)
        {
            // Set currently executable application to private variable m_revit
            m_revit = revit.Application;
            m_elements = elements;

            Transaction tran = new Transaction(m_revit.ActiveUIDocument.Document, "BeamAndSlabNewParameter");
            tran.Start();

            // Show UI
            using (BeamAndSlabParametersForm displayForm = new BeamAndSlabParametersForm(this))
            {
                displayForm.ShowDialog();
            }

            tran.Commit();
            return Autodesk.Revit.UI.Result.Succeeded;
        }