C# Language Tutorial => Instantiating classes that implement an...

Example

If you want your application to support a plug-in system, for example to load plug-ins from assemblies located in plugins folder:

interface IPlugin
{
    string PluginDescription { get; }
    void DoWork();
}

This class would be located in a separate dll

class HelloPlugin : IPlugin
{
    public string PluginDescription => "A plugin that says Hello";
    public void DoWork()
    {
        Console.WriteLine("Hello");
    }
}

Your application's plugin loader would find the dll files, get all types in those assemblies that implement IPlugin, and create instances of those.

    public IEnumerable<IPlugin> InstantiatePlugins(string directory)
    {
        var pluginAssemblyNames = Directory.GetFiles(directory, "*.addin.dll").Select(name => new FileInfo(name).FullName).ToArray();
        //load the assemblies into the current AppDomain, so we can instantiate the types later
        foreach (var fileName in pluginAssemblyNames)
            AppDomain.CurrentDomain.Load(File.ReadAllBytes(fileName));
        var assemblies = pluginAssemblyNames.Select(System.Reflection.Assembly.LoadFile);
        var typesInAssembly = assemblies.SelectMany(asm => asm.GetTypes());
        var pluginTypes = typesInAssembly.Where(type => typeof (IPlugin).IsAssignableFrom(type));
        return pluginTypes.Select(Activator.CreateInstance).Cast<IPlugin>(); 
    }

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C# Language Reflection Instantiating classes that implement an interface (e.g. plugin activation)

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