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  1. The New XML Plug-in
  2. Modifying the UI Test App
  3. Summary
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Modifying the UI Test App

The second task is to modify the application so it loads the various plug-ins upon startup. I wanted to keep the ability to load a single plug-in file by filename; but I also want to lay the groundwork for the configuration that will be included by Part 4 of this series.

If you'll use the method of using plug-ins in a non-interactive environment, such as a Windows service or Web service, the UI file selector has to go. Instead, the application needs a method of loading one or more plug-ins when it's started.

To accomplish this task, I created a new configuration handler class that implements the IConfigurationHandler interface found in the System.Configuration namespace that comes with .NET. This interface lets me build a class that will be used to get configuration information from the application's configuration file and process it however I see fit. I added a new configuration file to the application using the Visual Studio IDE (called App.config by default). At the top of the file, I added a declaration that tells the application to use my configuration class to handle the new section I'm adding:

 <configSections>
 <section name="authenticationPlugins" 
        type="AuthenticationPlugin.UI.UITester.PluginConfigurationHandler, 
        AuthenticationPlugin.UI.UITester" />
</configSections>

This says the new section key will be named authenticationPlugins and it will be processed by the AuthenticationPlugin.UI.UITester.PluginConfigurationHandler class in the AuthenticationPlugin.UI.UITester assembly.

The new configuration section will look like this:

<authenticationPlugins>
<authenticationPlugin 
         fileName="C:\\Temp\\Plugins\\AuthenticationPlugin.Plugins.MyTest.dll"/>
<authenticationPlugin 
         fileName="C:\\Temp\\Plugins\\AuthenticationPlugin.Plugins.XmlAuthenticator.dll"/>
</authenticationPlugins>

Note the multiple use of the key authenticationPlugin within the authenticationPlugins section. One of these is used for every file that will be loaded as a plug-in when the test application starts up. This type of configuration would be difficult to pull off using the standard configuration method, which is why I need to build a new class specifically to handle the authenticationPlugins section.

Since the new class implements the IConfigurationHandler interface, it must declare the following method:

public object Create( object parent, object input, XmlNode section )

This method returns an object that holds the configuration information. The object is used by the application to retrieve the configuration information after it is loaded.

Listing 1-2 shows the new class AuthenticationPluginSettings at the top of the file. The listing also shows the configuration handler class, which I'll explain in further detail in a bit. In the meantime, the AuthenticationPluginSettings class has an internal ArrayList that holds the file names of the plug-ins. This list will be populated by the Create method of the handler, and read later by the application when it the application type-casts the configuration into the AuthenticationPluginSettings object.

Listing 1-2

using System;
using System.Collections;
using System.Configuration;
using System.Diagnostics;
using System.Xml;

namespace AuthenticationPlugin.UI.UITester
{

    /// <summary>
    /// An object that contains configuration settings.
    /// </summary>
    public class AuthenticationPluginSettings
    {
        private ArrayList plugins = new ArrayList();
 
        /// <summary>
        /// The list of plugins found in the configuration settings
        /// </summary>
        public ArrayList Plugins
        {
            get { return plugins; }
        }

        /// <summary>
        /// Adds a filename to the plugin file list.
        /// </summary>
        /// <param name="filename">Name of the plugin file</param>
        public void AddPlugin( string filename )
        {
            plugins.Add( filename );
        }
    }


	/// <summary>
	/// Summary description for ConfigurationHandler.
	/// </summary>
	public class PluginConfigurationHandler : IConfigurationSectionHandler
	{ 
        #region Configuration key names
        public const string PluginConfigSectionName = "authenticationPlugins";
        public const string PluginConfigNodeName = "authenticationPlugin";
        #endregion
        
        #region Attribute names 
        public const string PluginFileNameAttribute = "fileName";
        #endregion

        /// <summary>
        /// Necessary method for the implementation of the IConfigurationSectionHandler 
        /// interface
        /// </summary>
        /// <param name="parent"></param>
        /// <param name="input"></param>
        /// <param name="section"></param>
        /// An object that holds the configuration as processed by this method.</returns>
        public object Create( object parent, object input, XmlNode section )
        {
            AuthenticationPluginSettings settings = new AuthenticationPluginSettings();
            /* Don't try to process anything if there's nothing to process */
            if ( null != section )
            {
               /* Declare this here so it doesn't have to be created and 
                * destroyed in the loop */
               XmlNode attribute;
               /* Go through the child nodes of the configuration section and 
                * process them.
                */
               foreach ( XmlNode node in section.ChildNodes )
               {
                   /* Looking for plugin nodes here */
                   if ( PluginConfigNodeName == node.Name )
                   {
                       /* Get the fileName attribute */
                       attribute = node.Attributes.GetNamedItem( PluginFileNameAttribute );

                       if ( null != attribute ) 
                       {
                           /* Add it to the settings object */
                           Debug.WriteLine( string.Format
                             ( "Adding plugin file \"{0}\"", attribute.Value ) );
                           settings.AddPlugin( attribute.Value );
                       }
                       else
                       {
                          /* This just ain't right, son!  The PluginFileNameAttribute
                          /* needs to be defined for this to work, so the app will throw
                          /* a ConfigurationException */
                          throw new ConfigurationException( 
                          string.Format
                            ( "Error in configuration file.  \"{0}\" is missing from \"{1}\".",
                          PluginFileNameAttribute, PluginConfigNodeName ) );
                       }
                   }
                }
            }
            return settings;
        }
	}
}

The configuration handler class, PluginConfigurationHandler, implements the IConfigurationSectionHandler interface. Since the IConfigurationSectionHandler interface is found in the System.Configuration namespace and I'll be using XML in this class, I added the following lines to the top of the file ConfigurationHandler.cs file:

using System.Configuration;
using System.Xml;

Now the PluginConfigurationHandler class is modified to implement the interface:

public class PluginConfigurationHandler : IConfigurationSectionHandler

The Create method contains the code that intercepts the custom configuration section and uses classes in the System.Xml namespace to parse the configuration file. In Listing 1-2, the code is making sure the child node name is authenticationPlugin and extracting the value of the fileName. The value is added to the Plugins ArrayList in the AuthenticationPluginSettings object.


The configuration file is being handled at this point, so it's time to modify the original testing UI application to load up the configuration during load up time. The application will also keep its original ability to load a plug-in based off the file name in the txtFileName text box.

Listing 1-3 shows the testing application code in its entirety. If you downloaded and compiled the source in Part 1, you will notice a couple major differences between this code and the original version. For starters, the code to load an object from a file was moved out of the btnLogin_Click method and into its own method named GetPluginFromFile. This method takes the file name as a string argument and returns the plug-in class if it's found in the file.

Next, a new method called LoadPlugins uses the AuthenticationPluginsSettings class to load the plug-in file name list from the configuration file. This LoadPlugins method is called in the Form_Load method. Now, any plug-in that is listed in the configuration file is loaded when the application starts and used as a fall-back if the txtFileName text block is left blank.

Listing 1-3

using System;
using System.Diagnostics;
using System.Drawing;
using System.Collections;
using System.Configuration;
using System.ComponentModel;
using System.Windows.Forms;
using System.Data;
using System.IO;

using System.Reflection;

using AuthenticationPlugin.Common;

namespace AuthenticationPlugin.UI.UITester
{
	/// <summary>
	/// Summary description for Form1.
	/// </summary>
	public class Form1 : System.Windows.Forms.Form
	{
        private System.Windows.Forms.OpenFileDialog ofdPluginFile;
        private System.Windows.Forms.TextBox txtUserName;
        private System.Windows.Forms.TextBox txtPassword;
        private System.Windows.Forms.Button btnLogin;
		/// <summary>
		/// Required designer variable.
		/// </summary>
		private System.ComponentModel.Container components = null;
        private System.Windows.Forms.Button btnBrowse;
        private System.Windows.Forms.TextBox txtFileName;

        private ArrayList plugins = new ArrayList();

		public Form1()
		{
			//
			// Required for Windows Form Designer support
			//
			InitializeComponent();

			//
			// TODO: Add any constructor code after
                        // InitializeComponent call
			//
            LoadPlugins();
		}

		/// <summary>
		/// Clean up any resources being used.
		/// </summary>
		protected override void Dispose( bool disposing )
		{
			if( disposing )
			{
				if (components != null) 
				{
					components.Dispose();
				}
			}
			base.Dispose( disposing );
		}

		#region Windows Form Designer generated code
		/// <summary>
		/// Required method for Designer support - do not modify
		/// the contents of this method with the code editor.
		/// </summary>
		private void InitializeComponent()
		{
            this.btnLogin = new System.Windows.Forms.Button();
            this.ofdPluginFile = new System.Windows.Forms.OpenFileDialog();
            this.txtUserName = new System.Windows.Forms.TextBox();
            this.txtPassword = new System.Windows.Forms.TextBox();
            this.btnBrowse = new System.Windows.Forms.Button();
            this.txtFileName = new System.Windows.Forms.TextBox();
            this.SuspendLayout();
            // 
            // btnLogin
            // 
            this.btnLogin.Location = new System.Drawing.Point(368, 88);
            this.btnLogin.Name = "btnLogin";
            this.btnLogin.TabIndex = 0;
            this.btnLogin.Text = "Login";
            this.btnLogin.Click += new System.EventHandler(this.btnLogin_Click);
            // 
            // txtUserName
            // 
            this.txtUserName.Location = new System.Drawing.Point(8, 8);
            this.txtUserName.Name = "txtUserName";
            this.txtUserName.TabIndex = 4;
            this.txtUserName.Text = "Enter Username";
            // 
            // txtPassword
            // 
            this.txtPassword.Location = new System.Drawing.Point(8, 32);
            this.txtPassword.Name = "txtPassword";
            this.txtPassword.TabIndex = 5;
            this.txtPassword.Text = "Enter Password";
            // 
            // btnBrowse
            // 
            this.btnBrowse.Location = new System.Drawing.Point(368, 56);
            this.btnBrowse.Name = "btnBrowse";
            this.btnBrowse.TabIndex = 2;
            this.btnBrowse.Text = "Browse...";
            this.btnBrowse.Click += new System.EventHandler(this.btnBrowse_Click);
            // 
            // txtFileName
            // 
            this.txtFileName.Location = new System.Drawing.Point(8, 56);
            this.txtFileName.Name = "txtFileName";
            this.txtFileName.Size = new System.Drawing.Size(352, 20);
            this.txtFileName.TabIndex = 3;
            this.txtFileName.Text = "";
            // 
            // Form1
            // 
            this.AutoScaleBaseSize = new System.Drawing.Size(5, 13);
            this.ClientSize = new System.Drawing.Size(448, 117);
            this.Controls.Add(this.txtPassword);
            this.Controls.Add(this.txtUserName);
            this.Controls.Add(this.txtFileName);
            this.Controls.Add(this.btnBrowse);
            this.Controls.Add(this.btnLogin);
            this.Name = "Form1";
            this.Text = "Form1";
            this.ResumeLayout(false);

        }
		#endregion

		/// <summary>
		/// The main entry point for the application.
		/// </summary>
		[STAThread]
		static void Main() 
		{
			Application.Run(new Form1());
		}

        private void btnLogin_Click(object sender, System.EventArgs e)
        {
            bool isValidUser = false;
            string username = txtUserName.Text;
            string password = txtPassword.Text;
            try
            {
                if ( ! txtFileName.Text.Equals( string.Empty ) )
                {
                    /* Use the file name, since one was specified */
                    isValidUser = ((IAuthenticationPlugin)GetPluginFromFile
                      ( txtFileName.Text )).IsLoginValid( username, password );
                }
                else
                {
                    /* Loop through the plug-ins that were loaded during startup */
                    foreach ( object authPlugin in plugins )
                    {
                        isValidUser = 
                         ((IAuthenticationPlugin)authPlugin).IsLoginValid
                         ( username, password );
                        if ( isValidUser )
                        {
                            break;
                        }
                    }
                }
            }
            catch ( System.InvalidCastException )
            {
                /* The object didn't implement the IAuthenticationPlugin interface,
                /* and could not be cast correctly */
                ShowError
                 ( "The authentication object doesn't implement the correct interface." );
            }
            catch ( System.Exception ex )
            {
                ShowError( ex.ToString() );
            }

            MessageBox.Show( ( isValidUser ) ? "Login successful!" : "Login failed!" );
        }

        /// <summary>
        /// Loads up each plugin defined in the configuration file.
        /// </summary>
        private void LoadPlugins()
        {
            /* Use the new configuration settings object to get the plugins */
            AuthenticationPluginSettings configSettings = 
             (AuthenticationPluginSettings)ConfigurationSettings.GetConfig
             ( PluginConfigurationHandler.PluginConfigSectionName );
            
            /* Loop through the settings and add the plugins */
            foreach ( string fileName in configSettings.Plugins )
            {
                plugins.Add( GetPluginFromFile( fileName ) );
            }

        }

        /// <summary>
        /// Gets the object that is labeled with the AuthenticationPlugin
        /// attribute from the given filename.
        /// </summary>
        /// <param name="fileName"></param>
        /// <returns></returns>
        private object GetPluginFromFile( string fileName )
        {      
            object pluginObj = null;

            try
            {
                /* Initialize some standard objects that will
                /* be used to load up the assembly.
                 */
                Assembly assembly = null;
                string typeName = string.Empty;
                Type pluginType = null;

                if ( File.Exists( fileName ) )
                {
                    assembly = Assembly.LoadFile( fileName );
                }
                else
                {
                    /* The file that was specified does not exist.  Checking
                     * here is much faster than waiting until the catch block
                     * to handle an FileDoesNotExist exception.  Substitute the
                     * error message code with logging code, etc.
                     */
                    ShowError( string.Format
                     ("The specified file '{0}' does not exist.", fileName ) );
                }

                if ( null != assembly )
                {
                    /* Now the Assembly has been loaded.  We should look to see if we
                     * can find a type that uses the AuthenticationPlugin attribute
                     */
                    foreach ( Type type in assembly.GetTypes() )
                    {
                        if ( type.IsAbstract ) continue;
   
                        if ( type.IsDefined( typeof
                          ( AuthenticationPlugin.Common.AuthenticationPluginAttribute ), 
                          true ) )
                        {
                            /* We've found the authentication plugin type, so we'll
                             * stop here.  This is the first instance.  More code 
                             * could be added to throw an exception if more than one
                             * is defined, etc.
                             */
                            pluginType = type;
                            break;
                        }
                    }
                     
                    /* Make sure before trying to act on the object that a type 
                     * was actually found.
                     */
                    if ( null != pluginType )
                    {
                        pluginObj = Activator.CreateInstance( pluginType );
                    }
                    else
                    {
                        throw new ApplicationException
                         ( "The plugin does not contain the correct type." );
                    }
                }

            }
            catch ( System.BadImageFormatException )
            {
                /* The Assembly is invalid--it is not a valid .NET Assembly file. */
                ShowError( string.Format( "Invalid plugin file '{0}'", fileName ) );

            }

            return pluginObj;
        }

        private void ShowError( string message )
        {
            MessageBox.Show( message, 
                "Error", 
                MessageBoxButtons.OK, 
                MessageBoxIcon.Error );
        }

        private void btnBrowse_Click(object sender, System.EventArgs e)
        {
            ofdPluginFile.ShowDialog();

            if ( ofdPluginFile.FileName != string.Empty ) 
            {
                txtFileName.Text = ofdPluginFile.FileName;
            }
        }
	}
}

Testing the Settings

To test the new code, I moved the new plug-in and the simple test plug-in I created in Part 1 to the C:\Temp\Plugins directory, along with the users.xml file. When the test application is started, the application loads the two plug-ins as I defined them in the new configuration section. When the Login button is pushed, the test application uses each plug-in to see if it can find the user.

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