CommandFormWallCount.cs

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using Autodesk.Revit.DB;
using Autodesk.Revit.UI;
using Autodesk.Revit.Attributes;

namespace TestWorks
{
    [Transaction(TransactionMode.Manual)]
    public class CommandFormWallCount : IExternalCommand
    {
        public Result Execute(ExternalCommandData commandData, ref string message, ElementSet elements)
        {
            UIApplication uiApp = commandData.Application;
            Document doc = uiApp.ActiveUIDocument.Document;

            using (System.Windows.Forms.Form form = new MySampleForm(doc))
            {
                if(form.ShowDialog() == DialogResult.OK)
                {
                    return Result.Succeeded;
                }
                else
                {
                    return Result.Cancelled;
                }
            }
        }
    }
}