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Dynamic Loading Made Simple

, 24 Mar 2008 CPOL
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An article demonstrating quite how easy dynamic loading it (and why you should use it).

Introduction

Dynamic Loading is the process of loading code and using it at run time rather than at compile time. Why use it? Simple, dynamic loading makes the process of modifying a program and adding extra functionality later on easier (especially by third-party vendors). Furthermore it makes larger projects far more managable since each section of code functionality is seperate from each other. Many applications use this technique for implementation of plugins, e.g. Visual Studio and CIRIP (see demo link).

The included source shows how to load classes from dll files (dynamical link libraries) at runtime based upon a particular interface.

The Code

public static List<object> LoadAssemblies(String path, String interfaceName, bool recursive)
{
    List<object> output = new List<object>();

    String[] files = new String[0];
    if (recursive)
    {
        files = Directory.GetFiles(path, "*.dll", SearchOption.AllDirectories);
    }
    else
    {
        files = Directory.GetFiles(path, "*.dll", SearchOption.TopDirectoryOnly);
    }

    foreach (String filename in files)
    {
        Console.WriteLine("Loading " + filename);
        try
        {
            Assembly assembly = Assembly.LoadFrom(filename);

            foreach (Type t in assembly.GetTypes())
            {
                if (t.IsClass && !t.IsAbstract && t.GetInterface(interfaceName) != null)
                {
                    output.Add(Activator.CreateInstance(t));
                }
            }
        }
        catch (Exception err)
        {
            Console.WriteLine("Loading fail:" + err.ToString());
        }
        finally
        {
            Console.WriteLine("Loading complete");
        }
    }

    return output;
}

The LoadAssemblies method loads in a single instance of each class implementing interfaceName in dll files found at the path (or in deeper folders if recursive = true is specified).

Using the Code

To use the code the following steps must be taken;

  1. Create an application where dynamic loading support is required
  2. Create a new class library (dll) with a copy of the interface all plugins will support
  3. Create an explit link from the application to the interface library
  4. Create new class libraries (dlls) containing classes which implement the interface
  5. Add the included code into the application
  6. During the load in period of the application call LoadAssemblies; using the path to the dlls & the fully qualified name of the interface (e.g. DNBSoft.InterfaceA).
  7. Foreach of the returned objects (cast to the interface once returned as the List<object>) call the relevant methods to link the plugin to the application.

Points of Interest

Did you learn anything interesting/fun/annoying while writing the code? Did you do anything particularly clever or wild or zany? No and no, however plugin support is VERY useful, and once you get the hang of it you'll use it alot.

It's the way forward!!!

History

Version 1.0.0.0 - The one and only release

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

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About the Author

Derek Bartram
Software Developer Rail Research UK
United Kingdom United Kingdom
I originally studied for a masters in engineering of software engineering at The University of Birmingham during 2000-2004, of which I received a 2:1. I continued at Birmingham University working with Civil Engineering and Rail Research UK where I am currently in my final year of a 3 year PhD project developing a Computational Intelligent Approach to Railway Intervention Planning. Although my work has a significant focus on railway engineering and associated practices much of my work is with data mining (on SQL Server 2008) and computational intelligence (CI) techniques. My key areas of expertise in CI are clustering algorithms (including Rival Penalised Competitive Learning) and evolutionary algorithms.
 
Outside of my formal work I enjoy testing the latest technologies such as .NET 3.5 and the many frameworks of which it comprises (mainly WPF). I have several projects on the go including a .NET and DirectX port of Quake 3 and many utility libraries. I also maintain an extensive website coded in Cold Fusion which is regularly updated; more information is available about me there.

Comments and Discussions

 
GeneralMy vote of 5 PinmembereduMolaMazo21-Oct-11 1:55 
GeneralUse AppDomain PinmemberYitzhak Gootvilig10-Apr-08 4:25 
NewsAll DLL's get locked when loaded and it's clasess get instantiated, so no DLL can be dropped while the system is running! PinmemberAndyHo7-Apr-08 15:55 
QuestionWhat about security? Pinmembermbghtri7-Apr-08 5:47 
AnswerRe: What about security? PinmemberDerek Bartram7-Apr-08 9:28 
Generaldirections to #5 PinmemberBillWoodruff25-Mar-08 7:53 
GeneralRe: directions to #5 PinmemberDerek Bartram25-Mar-08 10:29 
GeneralRe: directions to #5 PinmemberBillWoodruff25-Mar-08 15:24 
Hi Derek,
 
Your compass now points to #5 with that thoughtful reply Smile | :)
 
The "beauty" of loading each plug-in into its own appdomain is that that all their resources can be reclaimed during your app's lifetime without flushing all of them and then re-loading the ones you need (in the case you load all your plugins into one separate appdomain).
 
The "ugliness" of separate appdomains in general is the complex marshalling that may be required for the main app to interact with them ... and the things you can't do period across appdomains ...
 
I do find your article valuable in itself; it's my own hangover from knocked myself out trying to get tree nodes back and forth from an appdomain to the main app that has left me with this "itch" Smile | :) The end of that story was to shift to passing only data back to the main app where new sets of treenodes were created based on the passed data. As you may have guessed from this statement : I am not from the planet "COM."
 
best, Bill
 
"The greater the social and cultural distances between people, the more magical the light that can spring from their contact." Milan Kundera in Testaments Trahis

GeneralVote of less than 5 PinmemberDerek Bartram24-Mar-08 23:40 
QuestionWhat about generics??? Pinmemberdatenkabel24-Mar-08 20:36 
AnswerRe: What about generics??? PinmemberDerek Bartram24-Mar-08 23:31 

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