Create Instance of type using base class with generics

Question

I've created a plugin system within my code, which loads types from DLLS. I grab the type I want from the loaded DLL using this code;

var type = Assembly.LoadFrom(filePath).GetTypes()
                    .FirstOrDefault(t =>
                        t.IsClass && t.IsSubclassOfRawGeneric(typeof(DespatchBasePlugin<>)));

IsSubClassOfRawGeneric hunts down the base type as it is buried several classes down, the code works and the correct type is returned.

I then create an instance of this class using Activator;

DespatchBasePlugin<XMLSettingBase> obj = Activator.CreateInstance(type, new object[] { logger }) as DespatchBasePlugin<XMLSettingBase>;

Unfortunately the cast on this line creates a null reference. Removing the cast returns an instance of the class in question, but I need to store is as its base type.

This is the class being loaded(Shortened for brevity);

public class DHLPlugin : DespatchBasePlugin<UserSetting>
{
    public DHLPlugin(BaseForm logger) : base("DHL", logger)
    {
        this.order = 10;
    }
}

And this is the base class I want it to use(Note the class itself has a base class, it goes several layers deep);

public abstract class DespatchBasePlugin<TSettings> : DespatchBase<TSettings> where TSettings : XMLSettingBase, new()

The previous code used a base class with no generic assigned to it and worked absolutely fine. It looked like this;

DespatchBasePlugin obj = Activator.CreateInstance(type, new object[] { logger }) as DespatchBasePlugin;

I'm sure I'm doing something dumb, please tell me what it is.

Edit - Not marked this as duplicate as I believe this is a better question/answer than the other which consists of a generic link to MSDN as the answer. If this is not a correct way to use the duplicate system please let me know.

Can you show us the class declaration for the class it instantiates, and also the definition of DespatchBasePlugin<>? — ProgrammingLlama
– ProgrammingLlama, Commented Feb 2, 2018 at 12:04
Updated, thanks John. I missed out the base classes of the base class, it goes about 4 levels deep, not sure if you need all of them? — Tony Cheetham
– Tony Cheetham, Commented Feb 2, 2018 at 12:07
I think that's enough. Let me find the correct SO post to help you. — ProgrammingLlama
– ProgrammingLlama, Commented Feb 2, 2018 at 12:07
@tonyenkiducx Here's an older article from codeproject codeproject.com/Articles/1052356/… maybe that helps. I do not write the whole stuff again :) Btw. what John wrote looks similar. — user743414
– user743414, Commented Feb 2, 2018 at 12:49

ProgrammingLlama · Accepted Answer · 2018-02-02 12:34:47Z

3

You can use contravariance to define your plugin:

public class Program
{
    public static void Main()
    {
        var settings = new DerivedSettings()
        {Name = "John"};
        DerivedPlugin a = new DerivedPlugin(settings);
        IPlugin<BaseSettings> sample = (IPlugin<BaseSettings>)a;
        Console.WriteLine(sample.GetName());
    }
}

public abstract class BaseSettings
{
    public abstract string Name
    {
        get;
        set;
    }
}

public interface IPlugin<out TSettings>
    where TSettings : BaseSettings
{
    string GetName();
}

public abstract class BasePlugin<TSettings> : IPlugin<TSettings> where TSettings : BaseSettings
{
    protected readonly TSettings _settings;
    public BasePlugin(TSettings settings)
    {
        _settings = settings;
    }

    public virtual string GetName()
    {
        return _settings.Name;
    }
}

public class DerivedSettings : BaseSettings
{
    public override string Name
    {
        get;
        set;
    }
}

public class DerivedPlugin : BasePlugin<DerivedSettings>
{
    public DerivedPlugin(DerivedSettings settings): base (settings)
    {
    }
}

I've included a BasePlugin class, but this is optional and you can just directly use the interface.

answered Feb 2, 2018 at 12:34

ProgrammingLlama

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2 Comments

Tony Cheetham Over a year ago

I realise this is terrible, but I'm thinking I'll have to add an interface "wrapper", and then I'd have to cast the plugin to the BasePlugin type when I use it. It's legacy code....54 projects in a solution....Can't rewrite all of it.

Tony Cheetham Over a year ago

Marking as answer.. Not marking it as duplicate because I believe this question and answer are better than the other one. Your answer is much more explicit and shows a solution rather than just pointing to an MSDN article.

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