Command.cs

using Autodesk.Revit.DB;
using Autodesk.Revit.UI;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using Autodesk.Revit.Attributes; // It's good practice to have all using statements at the top

namespace ReinMaker
{
    [Autodesk.Revit.Attributes.Transaction(Autodesk.Revit.Attributes.TransactionMode.Manual)]
    [Autodesk.Revit.Attributes.Regeneration(Autodesk.Revit.Attributes.RegenerationOption.Manual)]
    [Autodesk.Revit.Attributes.Journaling(Autodesk.Revit.Attributes.JournalingMode.NoCommandData)]
    public class Command : IExternalCommand
    {
        /// <summary>
        /// Implement this method as an external command for Revit.
        /// </summary>
        /// <param name="commandData">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(ExternalCommandData commandData, ref string message, Autodesk.Revit.DB.ElementSet elements)
        {
            Transaction transaction = new Transaction(commandData.Application.ActiveUIDocument.Document, "External Tool");
            try
            {
                transaction.Start();
                // create a factory to create the corresponding FrameReinMaker
                FrameReinMakerFactory factory = new FrameReinMakerFactory(commandData);

                // Do some data checks, such whether the user select concrete beam or column
                if (!factory.AssertData())
                {
                    message = "Please select a concrete beam or column without reinforcement.";
                    return Autodesk.Revit.UI.Result.Failed;
                }

                // Invoke work() method to create corresponding FrameReinMaker,
                // and create the reinforcement rebars
                factory.work();

                // if everything goes well, return succeeded.
                return Autodesk.Revit.UI.Result.Succeeded;
            }
            catch (Exception ex)
            {
                message = ex.Message;
                return Autodesk.Revit.UI.Result.Failed;
            }
            finally
            {
                transaction.Commit();
            }
        }
    }
}