AutoReload.cs

using System;

namespace Mine
{
    //  helper class
    public class PluginData
    {
        public DateTime _creation_time;
        public Autodesk.Revit.UI.IExternalCommand _instance;

        public PluginData(Autodesk.Revit.UI.IExternalCommand instance)
        {
            _instance = instance;
        }
    }

    //
    //   Base class for auto-reloading external commands that reside in other dll's
    //   (that Revit never knows about, and therefore cannot lock)
    //
    public class AutoReload : Autodesk.Revit.UI.IExternalCommand
    {
        // keep a static dictionary of loaded modules (so the data persists between calls to Execute)
        static System.Collections.Generic.Dictionary<string, PluginData> _dictionary;
        
        String _path;   // to the dll
        String _class_full_name;

        public AutoReload(String path, String class_full_name)
        {
            if (_dictionary == null)
            {
                _dictionary = new System.Collections.Generic.Dictionary<string, PluginData>();
            }
            if (!_dictionary.ContainsKey(class_full_name))
            {
                PluginData data = new PluginData(null);
                _dictionary.Add(class_full_name, data);
            }
            _path = path;
            _class_full_name = class_full_name;
        }

        public Autodesk.Revit.UI.Result Execute(
            Autodesk.Revit.UI.ExternalCommandData commandData, 
            ref string message, 
            Autodesk.Revit.DB.ElementSet elements)
        {
            PluginData data = _dictionary[_class_full_name];
            DateTime creation_time = new System.IO.FileInfo(_path).LastWriteTime;
            if (creation_time.CompareTo(data._creation_time) > 0)
            {
                //  dll file has been modified, or this is the first time we execute this command.
                data._creation_time = creation_time;
                byte[] assembly_bytes = System.IO.File.ReadAllBytes(_path);
                System.Reflection.Assembly assembly = System.Reflection.Assembly.Load(assembly_bytes);
                foreach (Type type in assembly.GetTypes())
                {
                    if (type.IsClass && type.FullName == _class_full_name)
                    {
                        data._instance = Activator.CreateInstance(type) as Autodesk.Revit.UI.IExternalCommand;
                        break;
                    }
                }
            }
            // now actually call the command
            return data._instance.Execute(commandData, ref message, elements);
        }
    }

    //
    //   Derive a class from AutoReload for every auto-reloadable command. Hardcode the path 
    //   to the dll and the full name of the IExternalCommand class in the constructor of the base class.
    //
    [Autodesk.Revit.Attributes.Transaction(Autodesk.Revit.Attributes.TransactionMode.Manual)]
    [Autodesk.Revit.Attributes.Regeneration(Autodesk.Revit.Attributes.RegenerationOption.Manual)]
    public class AutoReloadExample : AutoReload
    {
        public AutoReloadExample()
            : base("C:\\revit2014plugins\\ExampleCommand.dll", "Mine.ExampleCommand")
        {
        }
    }

}