C#

• Create a boundary interface that defines what the plugin should do. This interface should be in an assembly available to both the plugin developer (plugins need a compile time reference to the assembly) and the application. In the case of a releasable app or something where the same team develops both sides it can go in the main application assembly.
• Create plugin classes which implement the interface and do the work.
• Write some reflection code in your main application that finds instances of the plugins in relevant assemblies, and registers them.

public interface IReportGenerator {
 void Validate();
 void Execute();
}

public void RegisterPlugins(Assembly a) {
  foreach(Module mod in a.GetLoadedModules()){
    foreach(Type ty in mod.GetTypes()){
      foreach(Type intf in ty.GetInterfaces()){
        if(intf == typeof(IReportGenerator)){
          RegisterPlugin((IReportGenerator)Activator.CreateInstance(ty));
        }
      }
    }
  }
}

Assembly assembly = Assembly.LoadFile(fileToLoad);

private bool syncConfigFiles(string serverConfigFile, string localConfigFile)
        {
            XmlDocument _localDoc = new XmlDocument();
            XmlDocument _serverDoc = new XmlDocument();

            //Load the two config files for comparison
            _localDoc.Load(localConfigFile);
            _serverDoc.Load(serverConfigFile);

            //Get the various child nodes of the documents
            XmlNodeList _localNodes = _localDoc.DocumentElement.ChildNodes;
            XmlNodeList _serverNodes = _serverDoc.DocumentElement.ChildNodes;

            //Loop through all nodes of the server config file
            foreach (XmlNode _serverNode in _serverNodes)
            {
                //Loop through all nodes of the local config file
                foreach (XmlNode _localNode in _localNodes)
                {
                    if (_serverNode.Name == _localNode.Name)
                    {
                        //Get all the attributes of the current node of the server file
                        XmlAttributeCollection _serverAttributes = _serverNode.Attributes;

                        //Check whether attributes exist, like that, only the tester family parts 
                        if (_serverAttributes != null)
                        {
                            for (int i = 0; i < _serverAttributes.Count; i++)
                            {
                                //Check whether the current server attribute exists within local attributes list or not (= equals null)
                                if (_localNode.Attributes[_serverAttributes[i].Name] == null)
                                {
                                    //Create new node for import of the attributes (otherwise exception as it is not part of the current document)
                                    XmlNode _newNode = _localNode.OwnerDocument.ImportNode(_serverNode, true);
                                    if (i > 0)
                                    {
                                        //if it is not the first attribute, insert it after the last checked and present attribute
                                        _localNode.Attributes.InsertAfter(_newNode.Attributes[_serverAttributes[i].Name], _localNode.Attributes[_serverAttributes[i - 1].Name]);
                                    }
                                    else
                                    {
                                        //if it is the first attribute, insert it before the next present attribute
                                        int _tempI = i;
                                        while (_localNode.Attributes[_serverAttributes[_tempI + 1].Name] == null && _tempI < _serverAttributes.Count)
                                        {
                                            _tempI++;
                                        }
                                        //needed if only one element is present
                                        if (_tempI < _serverAttributes.Count)
                                        {
                                            _localNode.Attributes.InsertBefore(_newNode.Attributes[_serverAttributes[i].Name], _localNode.Attributes[_serverAttributes[_tempI + 1].Name]);
                                        }
                                        else
                                        {
                                            _localNode.Attributes.Append(_newNode.Attributes[_serverAttributes[i].Name]);
                                        }
                                    }
                                }
                            }
                        }
                    }
                }
            }
            /*
             * Declare XmlWriterSettings:
             * - each attribute gets a new line
             * - indent is needed to activate the previous setting
             * - CloseOutput frees the underlying stream writer, so that the file is not locked after writing
             */

            XmlWriterSettings _settings = new XmlWriterSettings();
            _settings.NewLineOnAttributes = true;
            _settings.Indent = true;
            _settings.CloseOutput = true;

            //Create file and close file handling
            XmlWriter _writer = XmlWriter.Create(localConfigFile, _settings);
            _localDoc.Save(_writer);
            _writer.Close();
            return true;
        }