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Writing Plugin-Based Applications

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By Tim Dawson, published on 01 Feb 2004
Page 2 of 4
  1. Introduction
  2. The Interfaces & First Plugin
  3. The Host Application
  4. Wrapping Up

The Interfaces & First Plugin

Writing the Interfaces

First we create a new project of type Class Library, and define the two interfaces. For the purposes of this tutorial they will be very simple. Each plugin will offer a property with the name of the plugin, and a function which accepts two integers and returns a double. The host interface will expose a method so that the plugin can cause the host to show a message box.

Public Interface IPlugin
    Sub Initialize(ByVal Host As IHost)
    ReadOnly Property Name() As String
    Function Calculate(ByVal int1 As Integer, ByVal int2 As Integer) As Double
End Interface

Public Interface IHost
    Sub ShowFeedback(ByVal strFeedback As String)
End Interface

Lastly we set the build output directory to a common directory, where we'll also put the host application and the plugins.

Writing the first plugin

Since we'll need at least one to test the host application with, now we can write a simple plugin. Again, we create a new project of type Class Library, set the build output directory, and make sure we reference the interface class library we just created. Next, we modify the class provided thus:

Public Class Class1
    Implements PluginSample.Interfaces.IPlugin
    Private objHost As PluginSample.Interfaces.IHost
    Public Sub Initialize(ByVal Host As PluginSample.Interfaces.IHost) _
    Implements PluginSample.Interfaces.IPlugin.Initialize
        objHost = Host
    End Sub
    Public ReadOnly Property Name() As String Implements _
    PluginSample.Interfaces.IPlugin.Name
        Get
            Return "Example Plugin 1 - Adds two numbers"
        End Get
    End Property
    Public Function Calculate(ByVal int1 As Integer, ByVal int2 As Integer) As Double _
    Implements PluginSample.Interfaces.IPlugin.Calculate
        Return int1 + int2
    End Function
End Class

We've just created a simple plugin, which adds the two numbers provided together. Although we are accepting and storing a reference to the host interface, we don't actually use it. We'll do that in the next plugin we write.

Comments

  1. haneen ahmad
    27 Sep 2009 at 00:23

    hi

    when i run the host i found this error : Warning 1 Could not resolve this reference. Could not locate the assembly "Interfaces". Check to make sure the assembly exists on disk. If this reference is required by your code, you may get compilation errors. Plugin 1

    "PluginSample.Interfaces.IPlugin" is not defined

    can u help me to splve this problem

    thanks

  2. aroni san
    03 Jun 2008 at 00:13

    Just add event handlers to the menu items before returning from InitializeMenu, e.g.

    [quote]menu1.Click += new EventHandler (nameOfCallbackFunction);[/quote]

  3. JazzyXun
    22 Aug 2007 at 15:02
    Hi There,

    I found a very easy way to add controls to the host.

    Public Function InitializeMenu() As System.Windows.Forms.ToolStrip Implements Interfaces.IPlugin.InitializeMenu
            Dim Menu As New ToolStrip
            Dim menu1 As New ToolStripMenuItem("Plugin")
            menu1.DropDown = CreateCheckImageContextMenuStrip("Calculate")
            Menu.Items.Add(menu1)
            Menu.Dock = DockStyle.Top
            Return Menu
     End Function










    The interface should have this:    
    Function InitializeMenu() As ToolStrip



    Now you are able to add an control to the host like this:
    Me.ToolStripContainer1.TopToolStripPanel.Controls.Add(objPlugin.InitializeMenu)

    The only thing i did not figure out yet is how to handle when you click on the control.













  4. Jon Frost
    19 Jun 2007 at 08:23
    I liked this article, all seven times that I read it. But I felt that it lacked a bit of security. I came across this article on msdn. It shows quite nicely how to wrap untrusted code in a sort of security blanket.
  5. novy
    15 Apr 2007 at 11:20

    I rewrote your application in C# and everthing works except one thing.

    After loading  assembly and creating plugin instance I am casting plugin instance to plugin interface and I get excepion "Specified cast is not valid"

    I have no idea what is wrong.

  6. Christian Rygg
    16 Oct 2006 at 13:35
    This artice was just what I was looking for - however it turned out to lead me to the conclusion that the feature I wanted wasn't achievable by .Net - at least not without some dirty coding... You code describes plugging in, but not plugging out. To my shock I realized that .Net doesn't allow unloading of assemblies!

    I needed the plug in functionality to swap and update plug-ins on a regular basis, for an ASP.Net application. However, it seems I must shut down the site completely to replace an existing plug-in with a new version...
  7. borvik
    09 Dec 2005 at 17:11

    Great article!


    This will come in very handy with an application I have in mind .


    Just a note about using this in VB2005 (I'm using Express edition right now, hopefully I'll upgrade):


    The CreateInstance method of the PluginServices works fine, and does return an instance of the class that can be used but only when put into an Object type.


    You don't have the full list of functions and subroutines available to you using the object.


    When using the DirectCast, to cast the instance to an variable declared as the Interface - it doesn't like that cast and throws an exception.


    To fix this do what Prozac mentiond - change the references of [Assembly].LoadFrom() to [Assembly].LoadFile() and it works fine.


    Thanks for the great article!

  8. alchol100
    30 Nov 2005 at 01:59
    Hi Tim,

    I think your article is great, i'm looking for it for months, and finally...

    I just have a question:

    If I want that a plugin add a button ont the host form, is it possible? can you help me please?

    thanks in advance,
    Ivan
  9. Jimmy Kelly Jr
    17 Jun 2005 at 15:31

    The complete code is available in the source code, however I'll post it for you anyway.


    Code:

       Public Structure AvailablePlugin
           Public AssemblyPath As String
           Public ClassName As String
       End Structure


    Enjoy!

  10. Jimmy Kelly Jr
    17 Jun 2005 at 15:30

    I noticed that when I used [Assembly].LoadFrom(), it kept on loading the first plugin that was initially loaded,
    to fix this; use [Assembly].LoadFile() insted.


    DotNet Framework 1.1, Visual Studio 2003, Visual Basic.NET.

  11. Ralph Mallamace
    30 Sep 2004 at 23:42

    I can't get AvailablePlugin to work. VB.NET reports that
    "Type 'AvailablePlugin' is not defined." Has anyone else had this problem?

  12. malloc
    27 Apr 2004 at 11:51

    Hi.


    I've been trying to find the way of doing an application with pluggable components over vb6, but have not found any help. I read the subject of this article and was very excited but when I realized it was about .net I got frustrated. Could you please explain me how it is possible to make a plugin-based app in vb6.
    Thanx



    malloc

  13. Developer Fusion Bot
    01 Jan 1999 at 00:00

    This thread is for discussions of Writing Plugin-Based Applications.

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